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User's Guide to NOVAS Version C3.1

1. INPUT ARGUMENTS jd_tt double TT Julian date for apparent place ss_body struct object Pointer to structure containing the body designation for the solar system body defined in novas h accuracy short int Code specifying the relative accuracy of the output position 0 full accuracy 1 reduced accuracy OUTPUT ARGUMENTS ra double Apparent right ascension in hours referred to true equator and equinox of date dec double Apparent declination in degrees referred to true equator and equinox of date dis double True distance from Earth to the body at jd_tt in AU RETURNED VALUE short int 0 Everything OK 1 Invalid value of type in structure ss_body gt 10 Error code from function place Discussion This function computes the apparent place of a solar system body Your chosen version of function solarsystem accessed from function ephemeris determines the source of the body s barycentric rectangular coordinates used in the calculation Function app_planet works by calling place with Llocation gt where 0 and coord_sys 1 Return to Function List C 67 topo_planet short int topo_planet double jd_tt object ss_body double delta_t on_surface position short int accuracy double ra double dec double dis as URPOSE Computes the topocentric place of a solar system body REFERENCES Kaplan
2. RETURNED VALUE short int 0 everything OK 1 invalid value of coord_sys Discussion Function equ2ecl converts the equatorial position of an object into the equivalent ecliptic position equatorial coordinates ra and dec are converted to ecliptic coordinates elon and elat This function can be used for any kind of barycentric or geocentric coordinates the conversion involves a simple rotation and should be regarded as just a formalism As in C 76 function precession the input Julian date can be based on either the TDB or TT time scales with negligible resulting error ra and dec can be expressed with respect to either the mean equator and equinox of date jd_tt if coord_sys 0 or the true equator and equinox of date jd_tt if coord_sys 1 The representation of the ecliptic used for celestial coordinates is a smoothly moving mean plane described as part of the precession development However the mean and true equators intersect this ecliptic at different points Therefore the equinox which serves as the origin of ecliptic longitude as well as the origin of right ascension is different in the two cases elon will be expressed with respect to the same equinox as ra If jd_tt 0 0andcoord_sys 0 the function assumes ra and dec are expressed with respect to the ICRS and provides elon and elat with respect to the ecliptic and mean equinox of J2000 0 See functions equ2ecl_vec and ecl2equ_vec for the conversion
3. C 107 gcrs2equ transforms GCRS RA and dec to RA and dec on mean or true equator of date For true equator of date either the true equinox or the CIO can be specified as the origin of right ascension geo_posvel is called from place to compute the geocentric position and velocity vectors of an observer on or above the surface of the Earth grav_def replaces sun_field it supervises the evaluation of gravitational deflection of light due to the Sun Jupiter and other solar system bodies It calls a new function grav_vec to do the deflection calculation for each body grav_vec calculates the gravitational deflection of light due to a solar system body It is called by grav_def a new function that replaces sun_field iau2000a evaluates the AU 2000A nutation series nutation only based on IERS code Wallace 2003a iau2000b evaluates the IAU 2000B nutation series based on IERS code Wallace 2003b ira_equinox returns the value of the Equation of the Origins i e the right ascension of the equinox in the Celestial Intermediate Reference System from an analytical expression The Equation of the Origins is the arc on the true equator of date from the CIO to the equinox measured positively to the east light_time is called from place to antedate the position of a solar system body for light time limb_angle evaluates where an observed object is with respect to the Earth s limb horizon given the geocentric position vectors o
4. JPL Planetary and Lunar Ephemerides DE405 LE405 JPL IOM 312 F 98 048 Pasadena CA JPL http ssd jpl nasa gov iau comm4 relateds html Wallace P 2003a NU2000A Subroutine Subroutines for Chapter 5 in JERS Conventions ed D McCarthy amp G Petit IERS Tech Not 32 Frankfurt IERS http tai bipm org iers conv2003 conv2003_c5 html Wallace P 2003b NU2000B Subroutine Subroutines for Chapter 5 in JERS Conventions ed D McCarthy amp G Petit IERS Tech Not 32 Frankfurt IERS http tai bipm org iers conv2003 conv2003_c5 html Return to Table of Contents C 115 C 116 Appendix C How to Set Up the JPL Ephemerides C 1 Overview As described elsewhere in this document NOVAS requires access to a high accuracy solar system ephemeris in order to compute places of solar system bodies and the highest accuracy star places Groups in the U S France and Russia now construct high accuracy solar system ephemerides In NOVAS C solarsystem version I and solarsystem version 2 provide direct access to the developmental ephemerides designated as DEnnn which are produced by JPL in the U S The former is a front end to a USNO supplied C implementation of JPL s ephemeris access software while the latter is a front end to the JPL provided Fortran software both versions enable reading and interpolating a binary direct access ephemeris file The binary ephemeris file is created from ASCII data files and software s
5. ftp ssd jpl nasa gov in the address URL field When connected go to the pub eph planets directory Step 2 Download the JPL software ASCII ephemeris files and corresponding test data file Follow the CONTENTS TO BE RETRIEVED BY THE USER instructions in the file README txt which is also available as a webpage S We recommend following the instructions for non UNIX users regardless of your computer s operating system Our experience at USNO has been that the installation process is more a function of your computer s processor hardware than its operating system The test data file testpo xxx 48 http ssd jpl nasa gov planet_eph_export C 117 may be found in both the same directory as the ASCII ephemeris files and in the separate test data directory The characters used to terminate lines the end of line codes vary among operating systems Some systems use carriage returns CR some use line feeds LF and some use both The end of line codes may not be properly translated between operating systems when files are copied from one system to another depending on how the files are transferred and on what options are invoked If you have trouble reading or using any of the files downloaded from JPL check the encoding to ensure it is appropriate to your system Step 3 Configure the asc2eph f file provided by JPL For computer systems with Intel processors select uncomment the following statement at the beginni
6. including the prologs where the input and output arguments are described The most important terms are described in Chapter of this document and further information can be found in USNO Circular 179 A 8 References Kaplan G H 2005 The IAU Resolutions on Astronomical Reference Systems Time Scales and Earth Rotation Models USNO Circular 179 Washington DC USNO http www usno navy mil USNO astronomical applications publications circ 179 USNO Circular 179 Return to Table of Contents C 105 C 106 Appendix B List of Changes from NOVAS C2 0 1 to C3 1 This appendix lists in detail the changes between the current C3 1 version of NOVAS and earlier versions C3 0 of 2009 and C2 0 1 of 2000 Appendix A provides some perspective on these changes for people who are already familiar with NOVAS Most of the modifications are in response to resolutions passed by the IAU in 2000 and 2006 that recommended new models for fundamental astronomy within a new conceptual framework To the greatest extent possible the calling sequences for the highest level and most used functions from the previous versions of NOVAS have been preserved but a few important exceptions exist Many new calls are now available as well B 1 New Functions B 1 1 New Functions in NOVAS C3 0 cio_array is called from cio_location It reads and returns a set of values of the GCRS right ascension of the CIO near a given TDB Julian date from an external binary d
7. C 95 Nutation Models File nutation c The C version of NOVAS provides three implementations of the nutation model all of which are found in file nutation c Each model computes the nutation angles Ay and As which are the nutation in longitude and obliquity respectively These functions have the following input and output INPUT ARGUMENTS jd_high double High order part of TT Julian date jd_low double Low order part of TT Julian date OUTPUT ARGUMENTS dpsi double Nutation luni solar planetary in longitude in radians deps double Nutation luni solar planetary in obliquity in radians RETURNED VALUE None Discussion The nutation model is invoked via function nutation_angles in file novas c As previously mentioned the nutation model can be the most computationally intensive part of NOVAS It makes no sense to evaluate the full IAU 2000A nutation model 1 365 terms iau2000a unless the highest level of accuracy is needed Thus in addition to the full IAU 2000A model two reduced accuracy models truncated versions of IAU 2000A are also provided IAU 2000B iau2000b and 2000K nu2000k Section 1 4 discussed these models in greater detail Several higher level NOVAS functions have an input argument accuracy which specifies whether a full or reduced accuracy calculation is desired Whenever accuracy is set to O full accuracy IAU 2000A is used By default whenever
8. G H et al 1989 Astron Journ 97 1197 1210 Explanatory Supplement to the Astronomical Almanac 1992 Chapter 3 INPUT ARGUMENTS jd_tt double TT Julian date for topocentric place ss_body struct object Pointer to structure containing the body designation for the solar system body defined in novas h delta_t double Difference TT UT1 at jd_tt in seconds of time position struct on_surface Specifies the position of the observer structure defined in novas h accuracy short int Code specifying the relative accuracy of the output position 0 full accuracy 1 reduced accuracy OUTPUT ARGUMENTS ra double Topocentric right ascension in hours referred to true equator and equinox of date dec double Topocentric declination in degrees referred to true equator and equinox of date dis double True distance from Earth to the body at jd_tt in AU RETURNED VALUE short int QQ Everything OK 1 Invalid value of where in structure location gt 10 Error code from function place Discussion Function topo_planet computes the topocentric place of a solar system body neglecting atmospheric refraction for the location specified by the argument location at the time specified by the argument jd_tt The argument jd_tt is the TT time at which the topocentric place is to be computed The difference TT UT1 often called AT is passe
9. To be used only for Hipparcos or Tycho stars with linear space motion Both input and output data is in the ICRS REFERENCES one INPUT ARGUMENTS hipparcos struct cat_entry An entry from the Hipparcos catalog at epoch J1991 25 with all members having Hipparcos catalog units See Note 1 below struct defined in novas h OUTPUT ARGUMENTS hip_2000 struct cat_entry The transformed input entry at epoch J2000 0 See Note 2 below struct defined in novas h RETURNED lt D E G ira 1 Input Hipparcos catalog epoch and units Epoch J1991 25 Right ascension RA degrees Declination Dec degrees Proper motion in RA milliarcseconds per year Proper motion in Dec milliarcseconds per year Parallax milliarcseconds Radial velocity kilometers per second not in catalog 2 Output modified Hipparcos epoch and units Epoch J2000 0 Right ascension hours Declination degrees Proper motion in RA milliarcseconds per year Proper motion in Dec milliarcseconds per year Parallax milliarcseconds Radial velocity kilometers per second Discussion Function transform_hip takes Hipparcos catalog data which is published for epoch J1991 25 and transforms it to epoch J2000 0 for use in NOVAS functions such as place app_star topo_star virtual_star etc The Hipparcos input right ascension is expressed in degrees as in the catalog while the J2000 0 output
10. about 0 1 milliarcseconds for Ay and about 0 04 milliarcseconds for Ae and Ay sin over a longer interval of time than IAU 2000B place is a new general purpose function for computing apparent topocentric virtual astrometric etc places of stars and planets All substantive code for performing these calculations has been moved from app_star topo_star app_planet etc into place In the call to place the object requested is specified by the input parameter cel_object The type of place requested is specified by two input parameters one indicating the location of the observer and the other indicating the coordinate system of the output positions app_star topo_star app_planet etc now are just front ends to place rad_vel is called from place to compute the radial velocity of observed object with respect to the observer B 1 2 New Function in NOVAS C3 1 cel2ter transforms a vector from the celestial system GCRS to the terrestrial system ITRS B 2 Changes to NOVAS C Structures and Calling Sequences B 2 1 Changes in Structures and Calling Sequences from C2 0 1 to C3 0 All of the high level functions place app_star app_planet etc now assume that they are working with ICRS data including input RA dec and proper motion components for the stellar functions and position and velocity vectors obtained from solarsystem for both stellar and planetary functions virtual_star local_star virtual_planet local_planet astro_
11. for a topocentric place calculation this change supports satellite observations e_tilt now evaluates a more complete series for the complementary terms in the equation of the equinoxes formerly it just evaluated the two largest terms It uses the expression for the mean obliquity from the P03 precession formulation Internally the function works in either full or reduced accuracy mode as set by the accuracy input parameter Full accuracy mode obtains the sum of the terms from IERS function ee_ct C 111 Reduced accuracy obtains the sum of the terms from a nine term internal series equ2hor implements an improved algorithm for refraction geometry fund_args now evaluates expressions for the fundamental solar and lunar arguments from Simon et al 1994 nutation_angles now just calls either of the nutation functions iau2000a from the IERS or nu2000k a reduced accuracy version of iau2000a to do the hard work it does not contain a nutation series itself The nutation function called depends on the value of the accuracy input parameter In reduced accuracy mode iau2000b may be called instead of nu2000k by making a small code modification as explained in the prolog to the nutation_angles function precession now evaluates the precession angle polynomials for the Capitaine Wallace and Chapront 2003 P03 model which was recommended by an IAU resolution in 2006 Some code changes were made to ensure reversibility of transformat
12. if so refraction is applied to both sets of coordinates which can be useful for telescope pointing Refraction when requested is computed by function refract ra and dec the input topocentric right ascension and declination can be obtained from place with location gt where 1 and coord_sys 1 topo_star or topo_planet jd_ut1 is the UT1 time at which the topocentric place was computed The difference TT UT1 often called AT is passed to the function via argument delta_t Values of AT are pubiisheg annually in The Astronomical Almanac or can be obtained from the EO web site The coordinates of the pole xp and yp can be obtained from IERS Bulletins A and a although xp and yp can be set to zero 0 0 if sub arcsecond accuracy is not needed If refraction is requested sub arcsecond accuracy is unlikely The height of the observer and meteorological conditions at the observer s location contained in structure location are used only for refraction i e if ref_option is not equal to zero In this function the directions zd 0 0 the zenith and az 0 0 north are considered fixed in the terrestrial frame Specifically the zenith is along the geodetic normal and north is toward the ITRS reference pole If ref_option 0 for no refraction then rar ra and decr dec Return to Function List 6 http www usno navy mil USNO earth orientation eo products long term 7 http www iers org IERS EN Publications Bullet
13. local_star virtual_planet local_planet astro_star and astro_planet are now simply front ends to specific calls to place All substantive apparent place calculations of the various kinds are now done only in place The following changes in the basic algorithms were made 1 Calls to frame_tie were added in appropriate places to transform between the ICRS and the dynamical system 2 In updating a star s position for proper motion a correction to the epoch of interest for the difference in light time between the solar system barycenter the reference point for the input catalog data and the Earth itself is now included This change affects only stars with the greatest proper motions and then only at the 0 1 milliarcsecond level The new function d_light is used to compute the epoch offset 3 The Doppler factor k is included in the computation of stellar space motion vectors as discussed in the note on starvectors below 4 Modifications were made related to the change in gravitational deflection algorithms from sun_field to the more general function grav_def as discussed in the note above 5 Code has been introduced that allows a place to be expressed in the Celestial Intermediate Reference System equator of date with CIO as the right ascension origin 6 Code has been added that allows the input of an observer s instantaneous geocentric position and velocity vectors with respect to the true equator and equinox of date
14. met hod is set to either zero 0 for the CIO based approach or one 1 for the equinox based approach New NOVAS 3 1 function ce 2ter works similarly 4 http www usno navy mil USNO astronomical applications publications cire 179 C 104 A 6 New Features The functions in this section have been added to NOVAS to increase functionality and convenience place is new general purpose apparent place function Section A 2 describes this function in greater detail equ2ecl converts RA and dec to ecliptic longitude and latitude In addition egu2ecl_vec and ecl2equ_vec convert vectors from an equatorial to an ecliptic basis and vice versa respectively equ2gal converts ICRS RA and dec to galactic longitude and latitude gcrs2equ converts GCRS geocentric ICRS RA and dec to one of the equatorial systems of date make_object and make_observer are examples of a new set of functions which have been added to facilitate the construction of important data structures used in NOVAS cel2ter NOVAS 3 1 transforms a vector from the celestial system GCRS to the terrestrial system ITRS A 7 New Terminology Not surprisingly the IAU resolutions on reference systems and Earth rotation have required some new terminology and an IAU Working Group on Nomenclature for Fundamental Astronomy was established for the 2003 2006 triennium to systematize the usage New terms and abbreviations now appear in the comment statements of many NOVAS functions
15. you may notice an increase C 103 in execution time for some NOVAS applications However that extra computation time can be reduced Earth rotation calculations can be performed in either full or reduced accuracy mode as described in section 2 6 Many functions include an input argument accuracy that determines which mode will be used by that function and by any function it calls In full accuracy mode the various models are evaluated at the few microarcsecond level For nutation using this mode means that a 1 365 term trigonometric series is evaluated for each unique date However neither the models nor current observations are accurate at this level so much of the increased computational burden is unproductive Reduced accuracy mode can be used when the accuracy requirements are not better than 0 1 milliarcsecond for stars or 3 5 milliarcseconds for solar system bodies The computation time for these calculations is thereby reduced by about two thirds A 5 New Model for the Rotation of the Earth about its Axis IAU resolutions passed in 2000 established a new geometric paradigm for how we describe the Earth s spin around its axis Both the old and new paradigms are based on the instantaneous true equator of date but they use different fiducial points on the equator as the origin of right ascension and different time like quantities actually time dependent angles to describe the rotation of the Earth As described in section 1 4
16. 2000 REFERENCES IAU Resolution B1 8 adopted at the 2000 IAU General Assembly Manchester UK Kaplan G 2005 US Naval Observatory Circular 179 INPUT ARGUMENTS jd_high double High order part of UT1 Julian date jd_low double Low order part of UT1 Julian date OUTPUT ARGUMENTS None RETURNED VALUE double The Earth Rotation Angle theta in degrees Discussion This function supplies the Earth Rotation Angle 0 which is the geocentric angle in the instantaneous equatorial plane true equator between the directions toward the Terrestrial Intermediate Origin TIO and the Celestial Intermediate Origin CIO This function allows for the input UT1 time to be represented as a split Julian date See the discussion in the description of function sidereal_time Return to Function List C 79 cel_pole short int cel_pole double tjd short int type double dpolel double dpole2 PURPOSE This function allows for the specification of celestial pole offsets for high precision applications Each set of offsets is a correction to the modeled position of the pole for a specific date derived from observations and published by the IERS REFERENCES Kaplan G 2005 US Naval Observatory Circular 179 Kaplan G 2003 USNO AA Technical Note 2003 03 INPUT ARGUMENTS tjd double TDB or TT Julian date for pole offsets type short in
17. 2454580 9441875 UTC Consequently we will use a AAT value of 33 s and a UTI UTC value of 0 387845 s which are appropriate for this date const short int year 2008 const short int month 4 const short int day 24 const short int leap_secs 33 const double hour 10 605 const double utl_utc 0 387845 const double x_pole 0 002 const double y_pole 0 529 http maia usno navy mil ser7 mark3 out 31 A UTC Julian date is something of a non sequitur because UTC is not continuous due to leap seconds Here we are simply using julian_date with UTC input to obtain a value that allows us to compute Julian dates in better behaved time scales C 34 where x_pole and y_pole are the coordinates of the CIO with respect to the ITRS pole for 2008 April 24 So if we use jd_utc julian_date year month day hour the output argument jd_utc will have a value of 2454580 9441875 The next few lines of code should be something like jd_tt jd_utc double leap_secs 32 184 86400 0 jd_utl jd_ute utl_utc 86400 0 delta_t 32 184 leap_secs utl_utc where 86400 0 is the number of seconds in a day and the value of delta_t would be computed to be 65 571845 seconds If we had known the value of AT delt a_t at the start rather than the value of ut 1_utc UT1 UTC then the lines immediately above could have been replaced by jd_tt jd_utc double leap_secs 32 184 86400 0 j
18. B D Smith C A amp Sinclair A T 1992 in The Explanatory Supplement to the Astronomical Almanac ed P K Seidelmann Mill Valley CA Univ Sci Books 95 Kaplan G 1990 Bull AAS 22 930 Kaplan G H 2005 The IAU Resolutions on Astronomical Reference Systems Time Scales and Earth Rotation Models USNO Circular 179 Washington DC USNO http www usno navy mil USNO astronomical applications publications circ 179 USNO Circular 179 Kaplan G Bangert J Bartlett J Puatua W amp Monet A 2009 User s Guide to NOVAS 3 0 USNO Circular 180 Washington DC USNO http www usno navy mil USNO astronomical applications publications circ 180 USNO Circular 180 Kaplan G H Hughes J A Seidelmann P K Smith C A amp Yallop B D 1989 AJ 97 1197 Klioner S A 2003 AJ 125 1580 Seidelmann K ed 1992 The Explanatory Supplement to the Astronomical Almanac Mill Valley CA Univ Sci Books The Explanatory Supplement C 9 Abbreviations and Symbols Frequently Used AAT AT 6 A amp A AA AJ ASCII AU BCRS Bull AAS c CIO CIP dec ECEF EO ERA GCRS IAU ICRS IERS ITRS J2000 0 JD JPL NOVAS NRL RA SI SOFA TAI TDB TIO TT U S USNO UTI UTC TAI UTC an integral number of seconds TT UTI1 Earth Rotation Angle also ERA Astronomy and Astrophysics journal Astronomical Applications a USNO department The Astronomical Journal journal Americ
19. ICRS wobble now includes a very tiny inconsequential for most applications rotation about the z axis in the matrix to correct the ITRS longitude origin to TIO This rotation uses the recently published approximation to TIO longitude as a function of time which required that the new time argument also be added to this function Essentially this modification changes a W rotation to a W rotation Also the matrix element expressions changed from first order approximations to exact expressions for increased precision The following changes were made to constants e renamed MAU to AU and updated value e renamed KMAU to AU_KM and updated value e added AU_SEC from DE405 e added C_AUDAY e changed units of c from AU day to meters second e updated value of Gs from DE405 e added GE from DE405 e renamed EARTHRAD to ERAD changed units from kilometers to meters and updated value e updated value of F e renamed OMEGA to ANGVEL e added RMASS 12 e updated value of TWOPT e added ASEC360 e removed RAD2SEC e added ASEC2RAD B 3 2 Significant Internal Changes to Code from C3 0 to C3 1 app_star astro_star local_star topo_star and virtual_star all include a bug fix in the setup of the local variable obj_name make_cat_entry includes a bug fix in the setup of the output structure members star gt starname and star gt catalog make_object implements S1ZE_OF_OBJ_NAME and SITZE_OF_CAT_NAME and checks the length
20. Journ 97 1197 1210 Kaplan G H 2003 Another Look at Non Rotating Origins Proceedings of IAU XXV Joint Discussion 16 INPUT ARGUMENTS jd_ut_high double High order part of UT1 Julian date jd_ut_low double Low order part of UT1 Julian date delta_t double Value of Delta T TT UT1 at the input UT1 Julian date method short int Selection for method 0 CIO based method 1 equinox based method accuracy short int Selection for accuracy 0 full accuracy 1 reduced accuracy option short int 0 The output vector is referred to GCRS axes 1 The output vector is produced with respect to the equator and equinox of date xp double Conventionally defined x coordinate of celestial intermediate pole with respect to ITRS pole in arcseconds yp double Conventionally defined y coordinate of celestial intermediate pole with respect to ITRS pole in arcseconds vecl 3 double Position vector geocentric equatorial rectangular coordinates referred to ITRS axes terrestrial system in the normal case where option 0 OUTPUT ARGUMENTS vec2 3 double Position vector geocentric equatorial rectangular coordinates referred to GCRS axes celestial system or with respect to the equator and equinox of date depending on option C 53 RETURNED VALUE 0 everything is ok 1 invalid value of accuracy 2 inva
21. Purpose Place Function New Reference Systems New Models for Precession and Nutation New Model for the Rotation of the Earth about its Axis New Features New Terminology References List of Changes from NOVAS C2 0 1 to C3 1 New Functions B 1 1 New Functions in NOVAS C3 0 B 1 2 New Functions in NOVAS C3 1 Changes to NOVAS C Structures and Calling Sequences B 2 1 Changes from C2 0 1 to C3 0 B22 Changes from C3 0 to C3 1 Significant Internal Changes to Code B 3 1 Significant Internal Changes to Code from C2 0 1 to C3 0 B 3 2 Significant Internal Changes to Code from C3 0 to C3 1 Other Internal Code Changes B 4 1 Other Internal Code Changes from C2 0 1 to C3 0 B 4 2 Other Internal Code Changes from C3 0 to C3 1 References How to Set Up the JPL Ephemerides Overview Step by Step Guide A Comparison of SOFA and NOVAS C3 0 Goal Procedure Results References Addendum NOVAS C3 0 Code C 5 C 101 C 101 C 102 C 102 C 103 C 104 C 105 C 105 C 105 C 107 C 107 C 107 C 109 C 109 C 109 C 110 C 111 C 111 C 113 C 114 C 114 C 114 C 114 C 117 C 117 C 117 C 121 C 121 C 121 C 122 C 123 C 124 C 6 Introduction The Naval Observatory Vector Astrometry Software NOVAS is a source code library in Fortran C and Python that provides common astrometric quantities and transformations It can supply in one or two function calls the instantaneous celestial position of any star or planet in a variety of coordinate sys
22. Supplement to the Astronomical Almanac http www usno navy mil USNO astronomical applications publications circ 180 http www usno navy mil USNO astronomical applications publications circ 179 http www usno navy mil help astronomy help The current version of NOVAS may be obtained at http www usno navy mil USNO astronomical applications software products novas C 8 The official reference for NOVAS 3 0 is USNO Circular 180 which may be cited as Kaplan G Bangert J Bartlett J Puatua W amp Monet A 2009 User s Guide to NOVAS 3 0 USNO Circular 180 Washington DC USNO The official reference for all versions of NOVAS prior to 3 0 is the 1990 software report announcing its release which is Kaplan G 1990 NOVAS U S Naval Observatory Bull AAS 22 930 Acknowledgements Current version William T Harris of the USNO AA Department wrote most of the C code that accesses the JPL binary solar system ephemerides files eph_manager c and solsys1 c Previous versions Thomas K Buchanan working as part of the USNO NRL Optical Interferometer team did the initial conversion of many of the NOVAS Fortran subroutines to C William T Harris was largely responsible for completing the conversion and for NOVAS Version C1 0 David Buscher James Hilton Christian Hummel and Sandra Martinka users of early C versions of NOVAS provided valuable comments and suggestions References Hohenkerk C Y Yallop
23. accuracy is set to 1 reduced accuracy 2000K is used However a small coding change can be made to nutation_angles to replace 2000K with IAU 2000B as the reduced accuracy model The prolog and comments in nutation_angles explain how to make this change A summary of the models follows Nutation Function Accuracy Terms Accuracy Compared to Time Span Model Name Selection TAU 2000A mas P IAU 2000A iau2000a full 1 365 sts ies 2000K nu2000k reduced default 488 0 1 1700 2300 IAU 2000B iau2000b reduced alternate 77 1 1995 2050 Return to Function List C 96 Chapter 5 Equinox and CIO Based Paradigms Compared 5 1 Computing Hour Angles The equinox and ClO based celestial reference systems are part of two computational schemes for accounting for the Earth s instantaneous orientation with respect to the stars These two methods represent the same phenomena as they obviously must but in slightly different order The overall matrix that embodies for a given instant the terrestrial to celestial ITRS ICRS transformation is the same for both schemes Therefore the value of observable quantities will not be affected by the choice of which paradigm is used for the computations In NOVAS quantities such as declination and hour angle which are in principle measurable angles should have the same values regardless of the way in which they are computed Because both the equinox based and CIO based paradi
24. also be set to zero The difference TT UT1 often called AT is passed to the function via argument delta_t Values of AT are published annually in The Astronomical Almanac or can be obtained from the EO web site Atmospheric refraction can be subsequently applied to ra and dec by function equ2hor Function topo_star works by calling place with location gt where 1 and coord_sys 1 Loose catalog data can be assembled into a structure of type cat_entry by using function make_cat_entry Return to Function List 38 http www usno navy mil USNO earth orientation eo products long term C 63 virtual_star short int virtual_star double jd_tt cat_entry star short int accuracy double ra double dec PURPOSE Computes the virtual place of a star at date jd_tt given its catalog mean place proper motion parallax and radial velocity REFERENCES Kaplan G H et al 1989 Astron Journ 97 1197 1210 Explanatory Supplement to the Astronomical Almanac 1992 Chapter 3 INPUT ARGUMENTS jd_tt double TT Julian date for virtual place star struct cat_entry Pointer to catalog entry structure containing catalog data for the object in the ICRS defined in novas h accuracy short int Code specifying the relative accuracy of the output position 0 full accuracy 1 reduced accuracy OUTPUT ARGUMENTS ra double Virtual right ascension in hours referred to
25. an observer s location on a near Earth spacecraft typedef struct double sc_pos 3 double sc_vel 3 in_space where sc_pos 3 geocentric position vector x y Zz components in km sc_vel 3 geocentric velocity vector x_dot y_dot z_dot components in km s Both vectors with respect to true equator and equinox of date Structure observer Structure observer is a general container for information specifying the location of the observer typedef struct short int where on_surface on_surf in_space near_earth observer where where integer code specifying location of observer 0 observer at geocenter 1 observer on surface of earth 2 observer on near earth spacecraft on_surface structure containing data for an observer s location on the surface of the Earth where 1 near_earth data for an observer s location on a near Earth spacecraft where 2 Structure sky_pos Structure sky_pos contains data specifying a celestial object s place on the sky specifically the output from function place typedef struct double r_hat 3 double ra double dec double dis double rv sky_pos where r_hat 3 unit vector toward object dimensionless C 43 ra apparent topocentric or astrometric right ascension hours dec apparent topocentric or astrometric declination degrees dis tru geometric Euclidian distance to solar system body or 0 0
26. and with a new accuracy input parameter added precession can no longer process two arbitrary epochs because the new precession expressions are not as flexible as those of Lieske et al 1977 1979 Therefore one of the input epochs must be 2451545 0 J2000 0 In addition this function now returns an error code set_body was renamed make_object sidereal_time has new delta_t gst_type method and accuracy input parameters The former ee input parameter was removed In addition this function now returns an error code spin is no longer specifically associated with sidereal time It now applies a rotation about the current z axis with the angle expressed in degrees struct body was renamed object with the length of name member shortened to 50 characters and structure cat_ent ry member added struct site_info was renamed on_surface sun_field was replaced by grav_def a more general function that evaluates the gravitational deflection of light due to several solar system bodies tdb2tdt was renamed tdb2tt transform_cat has two modified and two new transformation options Existing opt ion 2 and existing opt ion 3 can no longer process two arbitrary epochs because the new precession expressions are not as flexible as those of Lieske et al 1977 1979 Therefore one of the input epochs for these options must be 2451545 0 J2000 0 New option 4 rotates data from the dynamical equator and equinox of J2000 0 to the ICRS while new
27. approaches represent independent algorithms for determining the location of the CIO on the equator and the tiny differences for dates that are several centuries from now are of no practical consequence Small differences in execution time may occur for the two approaches but those timing differences are likely to vary with the specific application depending for example on the order and spread of the dates that are processed by NOVAS For some applications using such an external file may be undesirable Because in many simple NOVAS applications the location of the CIO is never needed the best scheme is probably to start without using the CIO file If your application s execution time is critical you might want to experiment to see whether using a CIO file affects its performance one way or the other 2 6 Reduced accuracy Mode NOVAS has full accuracy and reduced accuracy modes that must be selected when calling about half of the functions in novas c see the prolog to each function and the corresponding description in Chapter 4 for specific functions Each of those functions has a short integer input parameter accuracy If accuracy is set to zero 0 the function and any lower level functions that it calls will proceed with full accuracy Alternatively if accuracy is Set to one 1 the function and any lower level functions that it calls will proceed with reduced accuracy In full accuracy mode e nutation calculations use
28. are referred to below double pos 3 vel 3 pose 3 vter 3 vcel 3 To obtain the barycentric or heliocentric coordinates BCRS vectors of a solar system body for example Mars first create an appropriate structure of type object rror make_object 0 4 Mars amp dummy_star amp mars 0 which uses the dummy_star structure created in Example 2 Then call ephemeris jd_tdb jd_tt Approximation good to 0 0017 seconds Jato Jd tab jd 1 0 0 rror ephemeris jd amp mars 1 accuracy pos vel where short integer origin has been set to one 1 to obtain a heliocentric position we have approximated jd_tdb jda tt and the output position and velocity vectors are pos and vel components in AU and AU day respectively For barycentric positions set origin to zero 0 If pos is a heliocentric vector it can be transformed to the ecliptic system fixed ecliptic of J2000 0 by calling equ2ecl_vec rror equ2ecl_vec T0 2 accuracy pos pose where pose is the output vector in the ecliptic system same units as pos pose could then easily be converted to heliocentric spherical coordinates ecliptic longitude ecliptic latitude and radius vector error vector2radec pose amp elon elat elon 15 0 r sqrt pose 0 pose 0 pose 1 pose 1 pose 2 pose 2 Finally transforming a vector from the terrestrial reference system to the celestial reference system can be useful Th
29. body at jd_tdb equatorial rectangular coordinates in AU referred to the ICRS vel 3 double Velocity vector of the body at jd_tdb equatorial rectangular coordinates in AU day referred to the ICRS RETURNED VALUE short int 0 Everything OK 1 Invalid value of origin 2 Invalid value of type in cel_obj 3 Unable to allocate memory 10 n where n is the error code from solarsystem 20 n where n is the error code from readeph Discussion Function ephemeris serves as the single interface between NOVAS and ephemerides of solar system bodies The version of ephemeris distributed with NOVAS provides direct support for the JPL ephemerides of major solar system bodies such as JPL s DE405 DE406 and C 83 DE421 and the USNO minor planet ephemerides USNO AE98 The function ephemeris calls an appropriate version of function solarsystem or solarsystem_hp in order to access an ephemeris of the major bodies in this context Sun Moon and Mercury through Pluto It accesses the minor planet ephemerides by calling readeph Neither the USNO minor planet ephemerides nor the JPL ephemerides are part of NOVAS and must be obtained elsewhere as discussed in sections 2 4 and 2 3 respectively Modifying function ephemeris to support ephemerides other than the two mentioned above is relatively easy In function ephemeris a switch structure is controlled by the value of type in a
30. calling program for example if the time of day has been stored in its own variable s from which jd_low is constructed For many applications the position of the split is not critical as long as the sum jd_high jd_low is correct in particular when used with computers providing 16 decimal digits of precision in double variables this function will yield values of gst precise to about 0 1 millisecond even if jd_high contains the entire Julian date and jd_low 0 0 Values of AT delta_t are published annually in The Astronomical Almanac or can be obtained from the EO web site If gst_type 1 for apparent sidereal time the output value of gst will correctly reflect the celestial pole offset in longitude if function cel_pole has previously been called Return to Function List http www usno navy mil USNO earth orientation eo products long term C 52 ter2cel short int ter2cel double jd_ut_high double jd_ut_low double delta_t short int method short int accuracy short int option double xp double yp double vecl double vec2 PURPOSE This function rotates a vector from the terrestrial to the celestial system Specifically it transforms a vector in the ITRS rotating earth fixed system to the GCRS a local space fixed system by applying rotations for polar motion Earth rotation nutation precession and the dynamical to GCRS frame tie REFERENCES Kaplan G H et al 1989 Astron
31. code and to extend its capabilities In response to user requests NOVAS 3 1 introduces a Python edition The initial Python release is a set of wrappers around the current C code In full accuracy calculations using the same data sources differences in the results from Fortran and C 7 C editions of NOVAS for the same calculation should not exceed 6 x 10 arcseconds 3 x lo radians for solar system bodies and 7 x 10 arcseconds 3 x 10 radians for stars Previous versions of NOVAS were based on the algorithms described in Kaplan et al 1989 Although the phenomena that are considered and the overall sequence of calculations remains much the same in the current release many of the models have been improved substantially over the last two decades in response to increased accuracy in observing techniques Specifically version 3 0 of NOVAS released in 2009 and described in USNO Circular 180 implements the resolutions on astronomical reference systems and Earth rotation models passed at the IAU General Assemblies in 1997 2000 and 2006 An explanation of the recent IAU resolutions can be found in The IAU Resolutions on Astronomical Reference Systems Time Scales and Earth Rotation Models Explanation and Implementation Kaplan 2005 hereafter USNO Circular 179 which contains significantly more information on topics only briefly mentioned in this document NOVAS 3 0 also improves the accuracy of its star and planet position c
32. convenient to consider only precession and to neglect nutation in defining celestial coordinate systems so that we can have for any given date and time both a mean system in which only precession is considered and a true system in which both precession and nutation are taken into account http www usno navy mil USNO astronomical applications publications circ 179 7 http asa usno navy mil SecM Section_M html 8 http syrte obspm fr iauW Gnfa NFA_Glossary html C 13 Thus we have the mean system of some date the true system of some date the mean system of 2000 0 etc precession nutation gcrs2equ Sidereal time is closely tied to the equatorial coordinate systems one day of sidereal time is marked by successive transits of the equinox across a specific geographic meridian and local sidereal time is just the apparent right ascension of stars transiting the observer s meridian Like the equatorial coordinate systems sidereal time comes in two flavors mean and apparent based on respectively the mean and true equinox and the system of right ascension that each defines sidereal_time In the last few decades of the 20 century several IAU working groups led a general re examination of astronomical coordinate systems at a very basic level Part of the motivation was quite practical most large ground based telescopes were no longer built on equatorial mounts and the equatorial coordinate systems were irrelevant to inc
33. data structure of type object Currently two cases within the switch are defined type 0 for major bodies to be handled via a call to solarsystem and type 1 for minor planets to be handled via a call to readeph To support ephemerides of other bodies simply define a new value of type and add another case block containing code that accesses the new ephemeris Alternate versions of solarsystem can be written to support alternate ephemerides i e non JPL of major solar system bodies Additional information concerning function solarsystem is provided in the following sections Return to Function List 4 http www usno navy mil USNO astronomical applications software products usnoae98 C 84 solarsystem short int solarsystem double tjd short int body short int origin double position double velocity PURPOSE Provides the position and velocity vectors of a planet or other solar system body at a specific time The origin of coordinates may b ither the barycenter of the solar system or the center of mass of the Sun REFERENCES JPL 2007 JPL Planetary and Lunar Ephemerides Export Information Pasadena CA JPL http ssd jpl nasa gov planet_eph_export Kaplan G H NOVAS Naval Observatory Vector Astrometry Subroutines USNO internal document dated 20 Oct 1988 revised 15 Mar 1990 INPUT ARGUMENTS tjd double Julian date of the desired time on the TDB time scale body short
34. done with and without corrections for polar motion precession and nutation and the P03 correction in SOFA The output of the C functions was compared with output from the corresponding Fortran programs Both the Fortran and C computations were executed on a 32 bit Intel Apple Macintosh system running the Leopard Mac OS X 10 5 operating system D 3 Results Table D1 shows that the latest C releases of NOVAS and SOFA agree at the sub microarcsecond level in the transformation between the celestial and terrestrial reference systems when the same Earth orientation parameters and conventions are used In this case including the P03 corrections in the SOFA nutation adds a discrepancy on the order of 1 4 uas Inclusion of the CIP offsets and polar motion does not significantly add to the differences in the two formulations as long as the parameters used are identical in the two cases Use of the external CIO_RA file in the NOVAS calculation adds about 0 05 uas to the difference for the above case while using the equinox method for the NOVAS computations does not have a significant effect on the results The results presented in Table D1 were obtained by computing the GCRS to ITRS transformations for the single time discussed in the SOFA Cookbook Therefore the values should be typical C 122 Table D1 Comparison of NOVAS C3 0 and SOFA C Corrections Applied Other Options Polar CIP P03 External Equinox Difference
35. ephemeris software package The user must set up the binary random access ephemeris file see Appendix C Important When using this version of solarsystem the user s program that calls the NOVAS functions must make a call to function ephem_open prior to calling the NOVAS functions ephem_open opens the binary JPL ephemeris file DE200 DE403 DE404 DE405 DE406 and DE421 are supported Similarly the user s program should call ephem_close to close the binary ephemeris file once ephemeris access is no longer required The functions ephem_open and ephem_close are located in eph_manager c The body identification numbers to be used with this version are listed below Name un Mercury Venus Earth Jupiter Saturn Uranus Neptune Pluto Moon Mar Return to Function List solarsystem version 2 File solsys2 c RETURNED VALUE short int O0 Everything OK 1 Invalid value of body or origin 2 Error detected by JPL software Discussion This version of solarsystem serves as the interface between the JPL s own Fortran based lunar and planetary ephemeris software and NOVAS The function contains a single call to Fortran subroutine jplint_ which in turn calls PLEPH and other Fortran subroutines in the JPL ephemeris software package The user is responsible for obtaining the Fortran ephemeris code and data setting up the binary random access ephemeris file and linking the
36. for star AU rv radial velocity km s Structure ra_of_cio Structure ra_of_cio contains the right ascension of the Celestial Intermediate Origin CIO with respect to the GCRS typedef struct double jd_tdb double ra_cio ra_of_cio where jd_tdb TDB Julian date ra_cio right ascension of the CIO with respect to the GCRS arcseconds 4 2 Function List The following functions are contained in file novas c Function name Level Purpose place Computes apparent direction of a star or solar system body given the time and observer s location Direction is expressed in one of several selectable coordinate systems sidereal_time Computes Greenwich sidereal time either mean or apparent Transforms arbitrary vector in rotating Earth fixed ITRS system to space fixed GCRS system terrestrial to celestial transformation Transforms arbitrary vector in space fixed GCRS system to rotating Earth fixed ITRS system celestial to terrestrial transformation Transforms topocentric RA and dec to zenith distance and azimuth optionally accounts for atmospheric refraction transform_cat Transforms star s catalog quantities for a change of epoch and or equator and equinox transform_hip Converts Hipparcos catalog data at epoch J1991 25 to epoch J2000 0 app_star Computes apparent place of a star given its catalog data topo_star Computes topocentric place of a star given geodetic location of observer virtual_star Computes v
37. general differ from one solarsystem version to another consult the documentation for the specific version in use Usually body 1 refers to Mercury body 2 refers to Venus body 3 refers to the Earth etc but the identification numbers for bodies such as the Sun or Moon differ across implementations Note The data structure of type object input to function ephemeris specifies the way that NOVAS identifies solar system bodies Code within ephemeris does the translation between the body numbers required by ephemeris and the body numbers required by solarsystem Furthermore some versions of solarsystem support only a subset of the major solar system bodies The minimum requirement is support for the Sun and Earth Here Earth refers to the geocenter and not the Earth Moon barycenter For highest precision applications using split Julian dates call solarsystem_hp Return to Function List C 86 solarsystem version 1 File solsys1 c RETURNED VALUE None Discussion This version of solarsystem provides an all C interface between NOVAS and the JPL lunar and planetary ephemerides Specifically it serves as the interface between USNO s C version of the JPL ephemeris software contained in file eph_manager c and the main set of NOVAS functions This version of solarsystem calls planet_ephemeris the C version of JPL s Fortran subroutine PLEPH which in turn calls other functions in the
38. given TDB Julian date Computes true right ascension of the CIO given TT Julian date Returns complementary terms for equation of the equinoxes Retrieves position and velocity of a solar system body from a fundamental ephemeris Provides information on orientation of Earth s axis obliquity nutation parameters etc Computes fundamental arguments mean elements of the Sun and Moon Computes intermediate RA of the equinox given TDB Julian date Computes Julian date given calendar date and time Computes the mean obliquity of the ecliptic Normalize angle into the range 0 lt angle lt 2 pi Supervises calculation of nutation parameters and provides nutation in longitude and obliquity Computes atmospheric refraction in zenith distance tdb2tt ic JConverts Barycentric Dynamical Time TDB to Terrestrial Time TT New function in NOVAS C3 0 Name change from NOVAS C2 0 1 New function in NOVAS C3 1 The following functions are contained in file nutation c iau2000a Evaluates high accuracy nutation series IAU 2000A model iau2000b Evaluates low accuracy nutation series TAU 2000B model nu2000k Evaluates low accuracy nutation series 2000K truncated version of IAU 2000A model New function in NOVAS C3 0 C 46 The following functions are contained in the various solsysn c files where n is an integer solarsystem Provides position and velocity vector
39. iauNut00a and iauEect00 C The document SOFA Tools for Earth Attitude also known as the SOFA Cookbook contains several Fortran examples of the transformation between terrestrial and celestial coordinate systems This appendix examines how one of those examples plays out in the C editions of both NOVAS and SOFA D 1 Goal These tests were designed to compare the transformation from GCRS to ITRS using the IAU 2000A 2006 models for precession and nutation We compared the CIO based method in NOVAS C3 0 at full accuracy with a C translation of the example in section 5 5 of the SOFA Cookbook titled IAU 2006 2000A CIO based using classical angles The goal was to verify that the NOVAS libraries which are mostly independent of SOFA produced results that agree with their SOFA counterparts at a level that is at least an order of magnitude better than the best observational results D 2 Procedure The comparison of the C editions of NOVAS and SOFA was based on an earlier comparison of the Fortran editions which is described in Appendix D of the User s Guide to NOVAS Version F3 0 Fortran section of this circular The C test functions were basically a line for line transliteration of the Fortran terceltest f and SOFA TEST f using the NOVAS and SOFA C functions respectively User s Guide to NOVAS Version F3 1 includes complete code for the full text of the NOVAS test program terceltest f and SOFA TEST f http www
40. int Body identification number for the solar system object of interest Mercury 1 Pluto 9 Sun 10 Moon 11 origin short int Origin code 0 solar system barycenter 1 center of mass of the Sun OUTPUT ARGUMENTS position 3 double Position vector of body at tjd equatorial rectangular coordinates in AU referred to the ICRS velocity 3 double Velocity vector of body at tjd equatorial rectangular system referred to the ICRS in AU day RETURNED VALUE short int 0 Everything OK Other values depend upon version in use Discussion Another NOVAS function ephemeris calls this function solarsystem to provide the position vector pos and velocity vector vel for body at time t jd The vectors computed by solarsystem are expressed with respect to ICRS axes in the BCRS metric The vectors are barycentric if origin 0 and heliocentric if origin 1 Three different versions of solarsystem are supplied in NOVAS each with its own internal logic One uses internally stored data or series expansions while the other two use the JPL ephemerides which exist as external data files Additional documentation see below is C 85 usually required for the proper use of each version You are of course free to supply your own version s of solarsystem providing that the arguments conform to the above specifications The values of the body identification number body will in
41. meters and is specified by the parameters x and yp The observational determinations are designated simply as x and y and current values are available from IERS Bulletin A Past values can be obtained at the EO web site For most purposes we can set Xp X and yp y see USNO Circular 179 section 6 5 2 or the IERS Conventions 2010 section 5 5 1 Several NOVAS functions require as input the x and y values for the date of interest although if the final accuracy requirements are no better than one arcsecond these values can be set to zero Because the difference AT TT UTI is used in only a few internal computations where the conversion from one time scale to another is not critical the value of AT needs to be accurate to only about one second Therefore one AT value will typically apply to all dates for a given year Finally the new IAU precession and nutation models are neither perfect nor complete and for very high accuracy applications observational corrections are sometimes needed These corrections now amount to less than one milliarcsecond 5 nrad These corrections are available from the same sources as the polar motion determinations and are designated as dX 2 http maia usno navy mil ser7 ser7 dat http www usno navy mil USNO earth orientation eo products daily http www usno navy mil USNO astronomical applications publications circ 179 C 23 and dY note that the units are milliarcseconds In the r
42. of UT1 through the addition or subtraction of leap seconds C 10 VLBI Very Long Baseline Interferometry WGS 84 World Geodetic System 1984 Xp Yp Polar motion components coordinates of the CIP with respect to the ITRS Return to Table of Contents Chapter 1 Astronomical Background At its highest level NOVAS computes the precise positions of selected celestial objects at specified dates and times as seen from a given location on or near the Earth Such positions can then be expressed in a number of coordinate systems Dates and times are specified in several astronomical time scales depending on the application Users of NOVAS should have a basic knowledge of astronomical coordinate systems and time terms like right ascension RA declination dec hour angle ecliptic equinox precession and sidereal time should be familiar Any number of texts on fundamental astronomy for example Spherical Astronomy Green 1985 lt an provide the essential concepts For more technical descriptions of the latest international standards on reference systems USNO Circular 179 cited in the Introduction can provide the background USNO Circular 179 documents the algorithms for many important calculations in NOVAS Others are described in the Kaplan et al 1989 paper mentioned in the Introduction or in the Explanatory Supplement In addition two glossaries may be useful to NOVAS users one published in The Astronomical Almanac and one com
43. of name C 113 precession includes the static variable first_time to fix a bug that occurs when jd_tdb2 is TO on the first call to this function Additionally in nutation c iau2000a iau2000b and nu2000k now use st atic storage class for the large const arrays In eph_manager c and eph_manager h some variables have been changed from long int to int to fix a problem on 64 bit Linux and Mac OS X 10 6 Snow Leopard systems B 4 Other Internal Code Changes B 4 1 Other Internal Code Changes from C2 0 1 to C3 0 Many minor changes have been made in the code Obviously many of the comment statements had to be revised and others had to be added such changes are too numerous to try to list Some variable names were changed For example the output RA and dec for the mean_star function was named mra and mdec the m indicating mean these are now ira and idec the i indicating ICRS Many similar trivial changes have been made B 4 2 Other Internal Code Changes from C3 0 to C3 1 For consistency throughout NOVAS and with IERS Conventions the variables representing the coordinates of the celestial intermediate pole with respect to the ITRS pole were renamed to xp and yp This change is most notable in the input arguments of ter2cel wobble and equ2hor ee_ct has additional references and notes mean_star uses make_cat_entry to set up the local tempstar structure sidereal_time has additional notes on
44. precision Generally the split would be at the Julian date s decimal point with the day count in the first argument and the fraction of a day in the second However using two arguments for the Julian date provides more precision only if the fractional part of the Julian date has been handled separately all along We have not done that so here the entire Julian date is just placed in the first argument and the second argument is set to zero 0 0 To compute local sidereal time either mean or apparent add the longitude east positive expressed in hours last gast geo_loc longitude 15 0 The result may have to be reduced to the range 0 to 24 hours by statements similar to the following if last gt 24 0 last 24 0 if last lt 0 0 last 24 0 The quantity that is analogous to Greenwich apparent sidereal time in the CIO based paradigm is 0 the ERA It can be computed theta era jd_ut1 0 0 where theta is the ERA degrees era like sidereal_time allows for a split input UT1 Julian date See section 5 1 for information on the difference between Greenwich apparent sidereal time and the ERA and how hour angles are computed in the two paradigms 3 6 Example 4 Other Frequently Requested Quantities In the following function calls vectors are used Vectors are simply double precision arrays with a dimension of 3 Most NOVAS internal calculations are performed with vectors and matrices The following vectors
45. right ascension is given in hours compatible with C 59 other NOVAS functions Function transform_cat with option 1 1s called to perform the epoch transformation The reference frame for both input and output is the ICRS This function should be used only for Hipparcos stars with linear space motion Radial velocity hipparcos gt radialvelocity is not given in the Hipparcos catalog If a value is not known set hipparcos gt radialvelocity 0 0 The radial velocity is important for only a small number of nearby high proper motion stars Return to Function List C 60 app_star short int app_star double jd_tt cat_entry star short int accuracy double ra double dec PURPOSE Computes the apparent place of a star at date jd_tt given its catalog mean place proper motion parallax and radial velocity REFERENCES Kaplan G H et al 1989 Astron Journ 97 1197 1210 Explanatory Supplement to the Astronomical Almanac 1992 Ch 3 INPUT ARGUMENTS jd_tt double TT Julian date for apparent place star struct cat_entry Pointer to catalog entry structure containing catalog data for the object in the ICRS defined in novas h accuracy short int Code specifying the relative accuracy of the output position 0 ves full accuracy 1 reduced accuracy OUTPUT ARGUMENTS ra double Apparent right ascension in hours referred to true equator an
46. the IAU 2000A model iau2000a nutation_angles e gravitational deflection is calculated using three bodies Sun Jupiter and Saturn grav_def e the equation of the equinoxes includes the entire series when computing the complementary terms ee_ct e geocentric positions of solar system bodies are adjusted for light travel time using split or two part Julian dates in calls to ephemeris and iterate with a convergence tolerance of 10 days light_time ephemeris e ephemeris calls the appropriate solar system ephemeris using split or two part Julian dates primarily to support light time calculations ephemeris solarsystem_hp light_time In reduced accuracy mode e nutation calculations use the 2000K model which is the default for this mode or the IAU 2000B model which may be set manually by user nu2000k iau2000b nutation_angles e gravitational deflection is calculated using only one body the Sun grav_def C 31 e the equation of the equinoxes excludes terms smaller than 2 microarcseconds when computing the complementary terms ee_ct e geocentric positions of solar system bodies are adjusted for light travel time using single value Julian dates in calls to ephemeris and iterate with a convergence tolerance of 10 days light time ephemeris solarsystem e ephemeris calls the appropriate solar system ephemeris using single value Julian dates ephemeris solarsystem In full accuracy mode the IAU 2000A
47. the ICRS The extragalactic radio sources that define the ICRS orientation are assumed to have no observable intrinsic angular motions Thus the ICRS is a space fixed system more precisely a kinematically non rotating system and as such it has no associated epoch its axes always point in the same directions with respect to distant galaxies However the ICRS was set up to approximate the conventional system defined by the Earth s mean equator and equinox of epoch J2000 0 the alignment difference is at the 0 02 arcsecond level which is negligible for many applications frame_tie Reference data for positional astronomy such as the data in astrometric star catalogs e g Hipparcos UCAC or 2MASS or barycentric planetary ephemerides e g JPL s DE405 are now specified within the ICRS more precisely stated they are specified within the BCRS with respect to the ICRS axes In the near Earth environment celestial coordinates and related quantities are expressed with respect to the GCRS or reference systems that are derived from it Because the orientation of the GCRS is derived from that of the BCRS the GCRS can be thought of as the geocentric ICRS Besides the GCRS itself the two reference systems most commonly used for expressing the apparent directions of astronomical objects as seen from the Earth are the true equator and equinox of date and the Celestial Intermediate Reference System Both are obtained from simp
48. the position of the observer structure defined in novas h accuracy short int Code specifying the relative accuracy of the output position 0 full accuracy 1 reduced accuracy OUTPUT ARGUMENTS ra double Local right ascension in hours referred to the local GCRS dec double Local declination in degrees referred to the local GCRS RETURNED VALUE short int 0 Everything OK 1 Invalid value of where in structure location gt 10 Error code from function make_object gt 20 Error code from function place Discussion See the discussion for function topo_star Function local_star is identical to topo_star in input arguments and use Here however the output arguments provide the local place of the star The local place is essentially the topocentric place expressed in the local GCRS Function local_star works by calling place with Llocation gt where 1 and coord_sys 0 Return to Function List C 65 astro_star short int astro_star double jd_tt cat_entry star short int accuracy double ra double dec PURPOSE Computes the astrometric place of a star at date jd_tt given its catalog mean place proper motion parallax and radial velocity REFERENCES Kaplan G H et al 1989 Astron Journ 97 1197 1210 Explanatory Supplement to the Astronomical Almanac 1992 Chapter 3 INPUT ARGUMEN
49. time coord_sys short int Code specifying coordinate system of the output position 0 GCRS or local GCRS 1 true equator and equinox of date 2 true equator and CIO of date 3 astrometric coordinates i e without light deflection or aberration accuracy short int Code specifying the relative accuracy of the output position 0 full accuracy 1 reduced accuracy OUTPUT ARGUMENTS output struct sky_pos Output data specifying object s place on the sky at time jd_tt with respect to the specified output coordinate system struct defined in novas h RETURNED VALUE 0 o problems ae invalid value of coord_sys 2 invalid value of accuracy 3 Earth is the observed object and the observer is either at the geocenter or on the Earth s surface not permitted gt 10 lt 40 10 error from function ephemeris AQ lt lt 5Q 40 error from function geo_posvel C 48 gt 50 70 50 error from function light_time gt 70 lt 80 70 error from function grav_def gt 80 lt 90 80 error from function cio_location gt 90 lt 100 90 error from function cio_basis Discussion Function place computes the apparent direction of a star or solar system body at a specified time and in a specified coordinate system The word star as used here refers to any object outside the solar system The apparent direction of a star computed by this funct
50. with UTC Coordinated Universal Time UTC is the basis for civil time systems worldwide and because it is distributed quite accurately by GPS is often used as the time tag for observations The key relationship is TT UTC AAT 32 184s where AAT is an integer representing the total count of leap seconds in UTC for example AAT 34 s for 2011 Equivalently TT TAI 32 184s where TAI is International Atomic Time If you will only be dealing with the geocentric celestial coordinates of objects TT is all you will need If you will also be computing topocentric coordinates for a specific location on the surface of the Earth you will also need to obtain UT1 The key relationship is UTI UTC UTI UTC where UT1 UTC is interpolated from the daily values of this quantity published by the IERS Alternatively UT1 TT AT See section 1 3 for more information on time scales including sources of data that can be used for the values of AAT UT1 UTC and AT To convert a date and time in the common format year month day hour minute second to a Julian date which is used by NOVAS for time arguments call julian_date Dates and times based on UTC TT or UT1 or any other time scale can be converted using julian_date the output Julian date simply has the same basis as the input date and time In the examples below we will be using 2008 April 24 10 36 18 0 UTC as the time of interest this corresponds to a Julian date of
51. EG2RAD is the degrees to radians conversion factor 0 01745329 Another important change to the high level functions is that the argument lists of the traditional place functions app_star topo_star app_planet topo_planet virtual_star etc have changed The earth data structure type body has been removed This structure is now created within the new place function described in the following section A new input parameter accuracy has been added to select either full accuracy or reduced accuracy calculations Finally function pnsw has been renamed to ter2cel with a change to its time argument this function carries out the terrestrial to celestial transformation All other changes to existing NOVAS calls involve lower level functions not frequently invoked by most users Appendix B details these changes C 101 A 2 place A New General Purpose Place Function All computational code to compute apparent topocentric virtual astrometric etc places of stars or planets has now been consolidated into a single new function called place The familiar calls to the traditional place functions from earlier versions of NOVAS still work essentially as before except for the important changes described in A 1 above but are now just front ends to place app_star topo_star app_planet topo_planet virtual_star local_star virtual_planet local_planet astro_star astro_planet This change eliminated much duplica
52. JPL Fortran code with NOVAS See the implementation notes below The body identification numbers to be used with this version are listed below Name Sun Mercury Venus art a gt Jupiter Saturn Uranus Neptune Pluto Moon Mars Implementation Notes In order to use NOVAS with solarsystem version 2 you must first obtain the planetary ephemeris export package from JPL as discussed in Appendix C If the verification process is successful the ephemeris file is ready to use The ephemeris data is obtained from the binary file by calling the access subroutines provided in the JPL export package Version 2 of solarsystem obtains ephemeris data from the binary file by calling Fortran subroutine jplint_ which is contained in NOVAS file jplint f Subroutine jplint_ in turn calls JPL Fortran subroutine PLEPH and all other supporting Fortran subroutines The C function solarsystem has a few features that make it possible for it to exchange data with the Fortran subroutine jplint_ First all of the C arguments of the call to jplint_ are addresses because Fortran uses call by address instead of call by value for arguments of subroutines Second all of the integer arguments in the call are designated as type long int in the C C 88 function to match the Fortran INTEGER default The DOUBLE PRECISION arguments in the subroutine are designated as type double in the C function Probably the biggest hurdle i
53. LL FSIZER3 NRECL KSIZE NRFILE NAMFIL leaving the lines containing CALL FSIZER1 and CALL FSIZER2 as comments Step 6 Compile and create executables a Create executable application asc2eph by compiling and linking file asc2eph f b Create executable application testeph by compiling and linking files testeph f and jplsubs f C 118 NOTE for Windows systems Some Fortran compilers may produce files with an exe extension i e asc2eph exe and testeph exe If so ignore the exe extension when using those applications in the following steps Step 7 Concatenate the ASCII data files and convert the concatenated file to binary form Follow the instructions in the ASCII to BINARY for non UNIX users section of the usrguide file which is located in the pub eph planets directory The resulting binary ephemeris file will be named JPLEPH or whatever name you specified in Step 5b Step 8 Test the binary ephemeris file Follow the instructions in the TESTING THE BINARY FILE section of the usrguide file which is located in pub eph planets You should have already downloaded the appropriate test data file in Step 2 NOTE In testing jplsubs f returned different values for some dates when compiled with Lahey Fujitsu Fortran 95 and GNU Fortran 4 1 2 under Red Hat Enterprise Linux ES release 4 Nahant Update 9 Step 9 Use the binary ephemeris fil
54. Motion Offsets Terms CIO_RA method uas No No No No No 0 25814 No No Yes No No 1 6752 No Yes Yes No No 1 6728 Yes Yes Yes No No 1 6735 Yes Yes No No No 0 28679 Yes Yes No Yes No 0 34369 Yes Yes No No Yes 0 28644 D 4 References Capitaine N Wallace P T Chapront J 2003 A amp A 412 567 P03 Kaplan G Bartlett J Monet A Bangert J amp Puatua W 2009 User s Guide to NOVAS F3 0 Washington DC USNO http www usno navy mil USNO astronomical applications publications circ 180 IAU 2009a Standards of Fundamental Astronomy SOFA http www iausofa org IAU 2009b SOFA Tools for Earth Attitude Software version 4 Document revision 1 1 SOFA Cookbook http www iausofa org sofa_pn pdf IERS 2003 Conventions 2003 Chapter 5 Transformation Between the Celestial and Terrestrial Systems Frankfurt Germany BKG http tai bipm org iers conv2003 conv2003_c5 html Monet A Kaplan G amp Harris W Testing Coordinate Frame Transformations NOVAS vs SOFA USNO AA Technical Note 2010 04 Washington DC USNO C 123 D 5 Addendum NOVAS C3 0 Code void terceltest Transform vectors from ITRS to GCRS double tjdh tjdl xp yp delt 65 25607389 vecl 3 vec2 3 tjd dx dy mobl tobl ee short num 0 FILE In_Data NULL dx 0 1750 dy 0 2259 Open the input file of Julian dates CIO coords ITRS vector In_Data fopen tercel test inpu
55. Position vector of body at tjd equatorial rectangular coordinates in AU referred to the ICRS velocity 3 double Velocity vector of body at tjd equatorial rectangular system referred to the ICRS in AU day RETURNED VALUE Value depends upon version in use Discussion Function solarsystem_hp provides positions and velocities for the major bodies of the solar system when the highest precision is required This function supports the split Julian date feature for highest precision The two parts of the input Julian date are stored in the two element array t jd Return to Function List C 91 solarsystem_hp version 1 File solsys1 c RETURNED VALUE None Discussion This version of solarsystem_hp provides an all C interface between NOVAS and the JPL lunar and planetary ephemerides when the highest precision is required Specifically it serves as the interface between USNO s C version of the JPL ephemeris software contained in file eph_manager c and the main set of NOVAS functions This version of solarsystem_hp calls planet_ephemeris the C version of JPL s Fortran subroutine PLEPH which in turn calls other functions in the ephemeris software package The user must set up the binary random access ephemeris file see Appendix C Important When using this version of solarsystem_hp the user s program that calls the NOVAS functions must make a call to function ephem_
56. S The components of vter can be in any units vcel will be in the same units vcel will sweep around the celestial sphere as the Earth rotates i e as jd_ut1 advances ter2cel like sidereal_time allows for a split input UT1 Julian date Use vector2radec to obtain vce1 s instantaneous spherical coordinates error vector2radec vcel amp ra amp dec where ra and dec are the GCRS right ascension and declination hours and degrees respectively of the point on the celestial sphere toward which vcel points At any jd_utl vcel can be compared to the directions of stars computed by place for the equivalent jd_tt with GCRS output coordinates selected Return to Table of Contents C 40 Chapter 4 Data Structures and Functions NOVAS can be used several different ways Some users will simply want to adopt a subset of the basic or utility functions for use in their own code systems For these users the function prologs should be sufficient for providing the information needed to use the functions Other users will want to implement the supervisory functions to compute for example apparent places of stars or topocentric places of solar system bodies This section of the user s guide is intended for these users 4 1 Important Data Structures Seven important data structures are used throughout NOVAS They are formally declared in file novas h Structure cat_entry Structure cat_entry contains the astrometric catalog data for a
57. TS jd_tt double TT Julian date for astrometric place star struct cat_entry Pointer to catalog entry structure containing catalog data for the object in the ICRS defined in novas h accuracy short int Code specifying the relative accuracy of the output position 0 full accuracy 1 reduced accuracy OUTPUT ARGUMENTS ra double Astrometric right ascension in hours referred to the ICRS without light deflection or aberration dec double Astrometric declination in degrees referred to the ICRS without light deflection or aberration RETURNED VALUE short int 0 Everything OK gt 10 Error code from function make_object gt 20 Error code from function place Discussion See the discussion for function app_star Function astro_star is identical to app_star in input arguments and use Here however the output arguments provide the astrometric place of the star in the ICRS Function astro_star works by calling place with location gt where 0 and coord_sys 3 Return to Function List C 66 app_planet short int app_planet double jd_tt object ss_body short int accuracy double ra double dec double dis J URPOSE Compute the apparent place of a solar system body REFERENCES Kaplan G H et al 1989 Astron Journ 97 1197 1210 Explanatory Supplement to the Astronomical Almanac 1992 Chapter 3
58. User s Guide to NOVAS Version C3 1 Naval Observatory Vector Astrometry Software C Edition John Bangert Wendy Puatua George Kaplan Jennifer Bartlett William Harris Amy Fredericks Alice Monet U S Naval Observatory March 2011 Approved for Public Release User s Guide to NOVAS Version C3 1 Naval Observatory Vector Astrometry Software C Edition In this document hyperlinks are in blue Table of Contents Introduction C 7 Citing NOVAS C 8 Acknowledgements C 9 References C 9 Abbreviations and Symbols Frequently Used C 10 Chapter 1 Astronomical Background C 13 1 1 Astronomical Coordinate Systems C 13 2 Computing Observable Quantities C 16 1 3 Time Scales for Astronomy C 19 1 4 Adopted Models for Precession and Nutation C 21 1 5 New Model for the Rotation of the Earth about its Axis C 22 1 6 Terrestrial Celestial Relationships C 23 1 7 References C 24 Chapter 2 Installing NOVAS C 25 2 1 List of Distribution Files C 25 2 2 Installation and Basic Validation C 27 23 Using External Solar System Ephemeris Files C 28 2 4 Using External Minor Planet Ephemeris Files C 29 2 0 Using an External CIO File C 30 2 6 Reduced accuracy Mode C 31 AT References C 32 Chapter 3 Sample Calculations C 33 3 1 Initialization C 33 3 2 Setting Time Arguments C 34 ee Example 1 Position of a Star C 35 3 4 Example 2 Position of the Moon C 36 a Example 3 Greenwich Sidereal Time C 38 3 6 Example 4 Other Frequen
59. VAS functions is the time of interest for example the time of an observation expressed as a Julian date JD Julian dates are simply a convenient format for representing a date and time in any time scale and are discussed below Two time scales are used as the basis for most of the Julian dates that are input arguments to the higher level NOVAS functions The first is Terrestrial Time TT which is effectively just a constant offset from TAI TT TAI 32 184 s Therefore TT UTC AAT 32 184 s Historically TT is considered continuous with the obsolete time scales Ephemeris Time ET and Terrestrial Dynamical Time TDT It is meant to be a smooth and continuous coordinate time scale independent of the rotation of the Earth The high level NOVAS functions that compute the apparent direction of an object at a specified time use Julian dates based on TT place app_star topo_star app_planet transform_cat 10 http www usno navy mil USNO astronomical applications publications cire 179 l http maia usno navy mil ser7 tai utc dat 1 http maia usno navy mil ser7 ser7 dat The second time scale is Universal Time UT1 which is based on the rotation of the Earth It is needed for computing sidereal time sidereal_time or the Earth Rotation Angle ERA or 0 era which in turn allows one to compute hour angles altitude and azimuth or other topocentric quantities UT1 is also obtained from UTC UT1 UTC UT1 UTC The va
60. a large number of observing stations in a way completely analogous to the definition of the ICRS by the coordinates of extragalactic radio sources The ITRS z axis is toward the north geodetic pole and its x axis is toward a point at longitude and latitude zero the y axis forms a right handed system with the other two axes Practical applications of astronomical data often require relating terrestrial coordinates to celestial coordinates and vice versa For example we may want the position of a celestial object expressed with respect to the local horizon system egu2hor Or we may have a vector expressed in an Earth fixed system that represents some instrumental axis and we would like to know where that vector is pointed on the celestial sphere NOVAS can perform the terrestrial to celestial transformation or its inverse specifically the transformation from the ITRS to the GCRS or the GCRS to the ITRS ter2cel cel2ter These transformations are a series of rotations that taken together represent the instantaneous orientation of the Earth in space precession nutation cio_basis sidereal_time era wobble Not all aspects of the Earth s orientation are predictable Polar motion represents the small shift of the geodetic north pole the ITRS z axis with respect to the rotational axis the CIP the largest part of which must be determined from observations Typically the total shift amounts to a few tenths of an arcsecond 1 2 urad 10
61. al_time and era require time arguments in the UT1 time scale The two procedures should yield the same value of GHA to within a micro arcsecond around the present time and identical values for DEC To obtain the local hour angle in either method simply add to the GHA the observer s longitude east positive in appropriate units Equinox Based Method CIO Based Method Use topo_star function or place with place with cel_object type 2 cel_object type 2 location 1 and location 1 and coord_sys 1 coord_sys 2 to obtain RA and DEC RA and DEC topocentric RA and dec of the star topocentric RA and dec of the star with respect to the true equator and with respect to the true equator and equinox of date CIO of date in hours and degrees respectively in hours and degrees respectively Then use sidereal_time with era function gst_type 1 to obtain GST THETA Greenwich apparent sidereal time Earth Rotation Angle 8 in hours in degrees Compute GHA GST RA GHA THETA 15 0 RA Greenwich in hours in hours hour angle The common notion of hour angle becomes somewhat problematic when polar motion is taken into account because what we usually regard as the Greenwich or observer s meridian a plane of constant geodetic longitude is not in general parallel to an hour circle on the celestial sphere when the geodetic pole and the CIP are not coin
62. alculations by including several small effects not previously implemented in the code see for example Klioner 2003 In addition some new convenience functions have been added in NOVAS 3 0 The current version NOVAS 3 1 provides some new capabilities and fixes some bugs Although Chapter provides some background material the intent of this user s guide is not to explain positional astronomy Many books have been written on this subject NOVAS users who desire additional information on the terminology and concepts used in the software should consult The Explanatory Supplement to the Astronomical Almanac Seidelmann 1992 hereafter The Explanatory Supplement and USNO Circular 179 Until the revision of the former publication is completed the latter serves as an important update providing an explanation of the recent IAU resolutions Citing NOVAS If you use NOVAS please send us an e mail that outlines your application This information helps justify further improvements to NOVAS Your comments and suggestions are also welcome This user s guide is the official reference for NOVAS C3 1 and may be cited as Bangert J Puatua W Kaplan G Bartlett J Harris W Fredericks A amp Monet A 2011 User s Guide to NOVAS Version C3 1 Washington DC USNO In addition we ask that you also direct your readers to the NOVAS website Hohenkerk et al 1992 reprinted parts of this article in Chapter 3 of The Explanatory
63. an Standard Code for Information Interchange astronomical unit Barycentric Celestial Reference System Bulletin of the American Astronomical Society journal speed of light Celestial Intermediate Origin Celestial Intermediate Pole declination Earth centered Earth fixed Earth Orientation a USNO department Earth Rotation Angle also 0 Geocentric Celestial Reference System International Astronomical Union International Celestial Reference System International Earth Rotation and Reference Systems Service International Terrestrial Reference System epoch 2000 January 1 12h TT JD 2451545 0 TT at the geocenter J2000 0 system is shorthand for the celestial reference system defined by the mean equator and equinox of J2000 0 Julian date Jet Propulsion Laboratory Naval Observatory Vector Astrometry Software Naval Research Laboratory right ascension Syst me International Standards of Fundamental Astronomy software International Atomic Time TAI UTC AAT Barycentric Dynamical Time Terrestrial Intermediate Origin Terrestrial Time TT TAI 32 184 s UT1 AT United States U S Naval Observatory Universal Time 1 universal time that is a measure of the Earth s rotational angle and subject to irregularities in the Earth s rotation UT1 TT AT Coordinated Universal Time an atomic time scale that is the basis for worldwide civil time replaces Greenwich Mean Time currently kept within 0 9 s
64. ance from Earth to the body in AU RETURNED VALUE short int 0 Everything OK 1 Invalid value of type in structure ss_body gt 10 Error code from function place Discussion See the discussion for function app_planet Function virtual_planet is identical to app_planet in input arguments and use Here however the output arguments provide the virtual place also called the proper place of the body The virtual place proper place is essentially the apparent place expressed in the GCRS Function virtual_planet works by calling place with location gt where 0 and coord_sys 0 Return to Function List C 70 local_planet short int local_planet double jd_tt object ss_body double delta_t on_surface position short int accuracy double ra double dec double dis as URPOSE Computes the local place of a solar system body REFERENCES Kaplan G H et al 1989 Astron Journ 97 1197 1210 Explanatory Supplement to the Astronomical Almanac 1992 Ch 3 INPUT ARGUMENTS jd_tt double TT Julian date for local place ss_body struct object Pointer to structure containing the body designation for the solar system body defined in novas h delta_t double Difference TT UT1 at jd_tt in seconds of time position struct on_surface Specifies the position of the observer structure defined in novas h accuracy short int Code
65. and Chapter 5 the conventional scheme is based on the equinox and sidereal time in the new paradigm the reference point is called the CIO and the time like quantity is called the ERA A more complete explanation of the new concepts along with the algorithms used with them can be found in Chapter 6 of USNO Circular 179 NOVAS 3 0 and later implement both the equinox and CIO based computational schemes Implementing the CIO based paradigm has required the addition of many new functions along with new code added to existing functions First the function place is coded to provide output right ascensions with respect to either the equinox or the CIO the choice is made through input argument coord_sys Function gcrs2equ can similarly convert GCRS right ascensions and declinations to their equatorial equivalents for a given date with output right ascensions measured with respect to either zero point cio_ra provides the angle between the two zero points that is the difference between the two right ascension systems Functions cio_location cio_basis cio_array ira_equinox provide lower level support to these computations Function era computes the ERA for any instant Two high level NOVAS functions that involve Earth rotation sidereal_time and ter2cel the latter replaces the old pnsw have been re coded to perform their internal calculations using either the equinox based or CIO based paradigm When each function is called an input argument
66. anuary 1 so that any date in recorded human history has a positive JD Over 2 4 million days have elapsed since JD 0 and thus for current dates seven digits of precision are taken up just by the day count if the JD is given by a standard double precision floating point number about 9 digits are left to represent the time of day Therefore we expect a double precision floating point JD can represent time to a precision of about 10 day 100 ps However after further examination Kaplan Bartlett and Harris 2011 determined that the actual precision is closer to 20 1 us In those NOVAS functions where more precision is appropriate the JD can be split between two input arguments one that carries the high order part of the JD e g the day count and the other that carries the low order part e g the fraction of a day Note that for Oh TT UT1 or TDB the fractional part of the Julian date is 0 5 An online calendar date to Julian date converter is available at the AA Department web site gt NOVAS has utility functions to convert between calendar date and Julian date and vice versa They work for any time scale that is their input and output arguments should be considered to be just different ways of expressing the same instant within the same time scale julian_date cal_date The epoch J2000 0 is considered to be an event at the geocenter at Julian date 2451545 0 TT which is 2000 January 1 12h TT 13 http maia usno navy mil
67. are cases where they are needed they are pre specified to NOVAS for use in subsequent calculations for a specific date cel_pole 1 7 References Capitaine N Wallace P T amp Chapront J 2003 A amp A 412 567 P03 Green R 1985 Spherical Astronomy New York Cambridge Univ Press Hilton J L et al 2006 Celest Mech amp Dynamical Astron 94 351 Kaplan G H 2005 The IAU Resolutions on Astronomical Reference Systems Time Scales and Earth Rotation Models USNO Circular 179 Washington DC USNO http www usno navy mil USNO astronomical applications publications circ 179 Kaplan G H 2009 Nutation Series Evaluation in NOVAS 3 0 USNO Circular 181 Washington DC USNO http www usno navy mil USNO astronomical applications publications circ 181 Kaplan G Bartlett J amp Harris W 2011 The Error in the Double Precision Representation of Julian Dates USNO AA Tech Note 2011 02 Kaplan G H Hughes J A Seidelmann P K Smith C A amp Yallop B D 1989 AJ 97 1197 Lieske J H Lederle T Fricke W amp Morando B 1977 A amp A 58 1 Mathews P M Herring T A amp Buffett B A 2003 J of Geophys Res 107 ETG 3 1 McCarthy D D amp Luzum B J 2003 Celest Mech amp Dynamical Astron 85 37 Petit G amp Luzum B eds 2010 Chapter 5 in JERS Conventions IERS Tech Not 36 Frankfurt IERS Seidelmann K ed 1992 The Explanatory Supplement to the Astr
68. ars the effects are Proper motion generalized the three dimensional space motion of the star relative to that of the solar system barycenter between the catalog epoch and the date of interest Assumed linear and computed from the catalog proper motion components radial velocity and parallax Projected onto the sky the motion amounts to less than 1 arcsecond per year usually much less except for a few nearby stars starvectors proper_motion Parallax the change in our perspective on stars in the solar neighborhood due to the position of the Earth in its orbit Its magnitude is distance in parsecs and hence is always less than 1 arcsecond bary2obs Gravitational light bending the apparent deflection of the light path in the gravitational field of the Sun and to a much lesser extent the other planets Although it reaches 1 8 arcsecond at the limb of the Sun it falls to 0 05 arcsecond 10 from the Sun and amounts to no more than a few milliarcseconds over the hemisphere of the sky opposite the Sun grav_def grav_vec Aberration the change in the apparent direction of light caused by the observer s velocity v with respect to the solar system barycenter Independent of distance it is approximately equal to v c expressed as an angle Therefore it can reach 21 arcseconds for observers on the surface of the Earth and somewhat more for instruments in orbit aberration Atmospheric refraction the total an
69. ates after 2560000 5 December 2296 This version of solarsystem calls NOVAS function precession and certain expressions in the solarsystem algorithm have been adjusted to conform to the IAU 2006 precession The body identification numbers to be used with this version are listed below Return to Function List C 90 solarsystem_hp short int solarsystem_hp double tjd 2 short int body short int origin double position double velocity PURPOSE Provides an interface between the JPL direct access solar system ephemerides and NOVAS C for highest precision applications REFERENCES JPL 2007 JPL Planetary and Lunar Ephemerides Export Information Pasadena CA JPL http ssd jpl nasa gov planet_eph_export Kaplan G H NOVAS Naval Observatory Vector Astrometry Subroutines USNO internal document dated 20 Oct 1988 revised 15 Mar 1990 INPUT ARGUMENTS tjd 2 double Two element array containing the Julian date which may be split any way although the first element is usually the integer part and the second element is the fractional part Julian date is on the TDB or T_eph time scale body short int Body identification number for the solar system object of interest Mercury 1 Pluto 9 Sun 10 Moon 11 origin short int Origin code 0 solar system barycenter 1 center of mass of the Sun OUTPUT ARGUMENTS position 3 double
70. be included as an option egu2hor Another function is available to transform right ascension and declination to ecliptic longitude and latitude egu2ecl Still another transforms right ascension and declination to galactic longitude and latitude egu2gal Reference System for the Location of the Observer The location of an Earth based observer is specified in NOVAS by longitude latitude and height with respect to the World Geodetic System 1984 WGS 84 reference ellipsoid Coordinates provided by GPS if uncorrected for the local datum are referred to WGS 84 which is also sometimes called the Earth centered Earth fixed ECEF system The International Terrestrial Reference System ITRS is a geocentric rectangular coordinate system used for high precision work WGS 84 coordinates are functionally equivalent within a few centimeters to ITRS coordinates Thus the geodetic positions used by NOVAS are consistent with the ITRS geo_posvel 1 2 Computing Observable Quantities NOVAS is mostly used for computing for a selected object the instantaneous angular coordinates or the equivalent unit vector components at which it might be observed within one of several user selected coordinate systems Obviously the values of the angular coordinates computed by NOVAS depend on the coordinate system requested but there are several phenomena that affect the observed position of a star or planet that are independent of the coordinate system For st
71. cal Almanac pp 103 104 Capitaine N et al 2003 Astronomy And Astrophysics 412 pp 567 586 Hilton J L et al 2006 IAU WG report Celest Mech 94 pp 351 367 INPUT ARGUMENTS jd_tdb1 double TDB Julian date of first epoch posl 3 double Position vector geocentric equatorial rectangular coordinates referred to mean dynamical equator and equinox of first epoch jd_tdb2 double TDB Julian date of second epoch OUTPUT ARGUMENTS pos2 3 double Position vector geocentric equatorial rectangular coordinates referred to mean dynamical equator and equinox of second epoch RETURNED VALUE short int 0 everything OK 1 Precession not to or from J2000 0 jd_tdb1 or Jjd_tdb2 not 2451545 0 Discussion This function precesses the input position vector pos1 from the equator and equinox of d_tdb1 to the equator and equinox of jd_t db2 the resulting vector is pos2 One of the two input Julian dates must be standard epoch J2000 0 either jd_tdb1 or jd_tdb2 must be 2451545 0 exactly To precess a vector from one arbitrary date to another call precession twice using J2000 0 as the middle date That is in the first call jd_tdb1 jd_tdb1 first Julian date and jd_tdb2 2451545 0 in the second call 2451545 0 and jd_tdb2 second Julian date Formally the current precession algorithm is a function of Barycentric Dynamical Time TDB but using TT as the basis for the input Julian da
72. cation checkout stars c Run the checkout stars application d Compare the results that you get from checkout stars with the data in file checkout stars usno txt If the results agree the installation was probably successful but see the important note below Important Note The checkout stars application exercises one supervisory function and most but not all of the low level functions in novas c The use of the checkout stars application is not a complete test of NOVAS Comparing the results from the NOVAS C supervisory functions with results from the analogous NOVAS Fortran supervisory functions will constitute a more complete check of your NOVAS implementation This setup of NOVAS using solarsystem version 3 will calculate the positions of stars or extragalactic objects with errors not exceeding 1 5 milliarcseconds if your input catalog data is that good or the Sun with an error not exceeding 2 arcseconds However it will not provide the positions of solar system objects other than the Sun For more general or demanding applications NOVAS must use another version of solarsystem 2 3 Using External Solar System Ephemeris Files NOVAS requires access to a solar system ephemeris which provides NOVAS with the heliocentric and barycentric positions and velocities of desired solar system objects referred to the ICRS A solar system ephemeris is required even when only precise star positions are needed in that ca
73. celestial object equator and equinox and units will depend on the catalog While this structure can be used as a generic container for catalog data all high level NOVAS functions require ICRS catalog data with the appropriate units which are shown in parentheses below typedef struct char starname SIZE_OF_OBJ_NAME char catalog SIZE_OF_CAT_NAME long int starnumber double ra double dec double promora double promodec double parallax double radialvelocity cat_entry where starname SIZE_OF_OBJ_NAME name of celestial object catalog SIZE_OF_CAT_NAME catalog designator e g HIP starnumber integer identifier assigned to object ra ICRS right ascension hours dec ICRS declination degrees promora ICRS proper motion in right ascension milliarcseconds year promodec ICRS proper motion in declination milliarcseconds year parallax parallax milliarcseconds radialvelocity radial velocity km s SIZE_OF_OBJ_NAME and SIZE_OF_CAT_NAME are constants also defined in novas h Each is the number of characters in the string the string length plus the null terminator C 41 Structure object Structure object designates a celestial object typedef struct short int type short int number char name SIZE_OF_OBJ_NAME cat_entry star object where type number name s
74. cident See the discussion in section 6 5 4 of USNO Circular 179 5 2 Other Computational Considerations Three high level NOVAS functions that involve Earth rotation sidereal_time ter2cel and cel2ter can perform their internal calculations using either the equinox based paradigm or the CIO based paradigm As previously mentioned sidereal_time computes sidereal time ter2cel performs the terrestrial to celestial transformation and cel2ter performs the celestial to terrestrial transformation egu2hor is indirectly involved because it calls ter2cel The method used within the three functions is selected when that function is called The integer input argument method must be set to either zero 0 to use the CIO based method or to one 1 to use the equinox based method Because no external difference occurs in how sidereal_time ter2cel or cel2ter are used and because the two computational paradigms yield answers that are consistent within a few microarcseconds over many centuries a practical basis for choosing seldom arises However the equinox method is much more efficient if mean sidereal time is to be computed http www usno navy mil USNO astronomical applications publications cire 179 C 98 Finally another of the new Earth rotation related functions is worth mentioning For a given TT Julian date cio_ra provides the right ascension of the CIO with respect to the true equinox of date With a sign reversal this quanti
75. conds for Venus at its closest Distance errors of up to 500 m 3 x 10 AU can also result independent of distance If errors of this magnitude are important care needs to be taken in specifying a more accurate value of AT An error in AT of 1 s will not result in a significant error in the topocentric places of stars Values of AT are published annually in The Astronomical Almanac or can be obtained from the EO web site output gt ryv the radial velocity is the predicted radial velocity measure z times the speed of light an inherently spectroscopic quantity For a star it includes all effects such as gravitational red shift contained in the catalog barycentric radial velocity measure which is assumed given in cel_object gt star radialvelocity For a solar system body it applies to a fictitious emitter at the center of the observed object assumed to be massless no gravitational red shift and does not in general apply to reflected light Loose catalog data can be assembled into a structure of type cat_ent ry by using function make_cat_entry Similarly we recommend using function make_object to create the input structure cel_object Return to Function List 3 http www usno navy mil USNO earth orientation eo products long term C 50 sidereal_time short int sidereal_time double jd_high double jd_low double delta_t short int gst_type short int method short int accuracy double gst PURPOSE Compu J
76. d app Return to Function List C 89 solarsystem version 3 File solsys3 c RETURNED VALUE short int O Everything OK 1 Input Julian date tjd out of range 2 Invalid value of body Discussion This version of solarsystem provides the position and velocity of the Earth or Sun only without reference to any external data file The heliocentric position and velocity of the Earth are computed by evaluating trigonometric series When barycentric positions and velocities are required a number of additional approximations are involved therefore barycentric positions and velocities computed by this version of solarsystem are less accurate than heliocentric positions and velocities This version of solarsystem produces data within the following error limits compared to the JPL DE405 ephemeris for dates within the interval 1800 2050 Earth heliocentric positions 840 km Earth heliocentric velocities 1 4 m s Earth barycentric positions 2500 km Earth barycentric velocities 1 4 m s When this version of solarsystem is used in the computation of the apparent place of the Sun it should contribute less than 2 arcseconds of error When used in the computation of apparent places of stars it should contribute less than 1 5 milliarcseconds of error This assessment applies to the interval 1800 2050 This version of solarsystem will return ierr 1 for Julian dates prior to 2340000 5 August 1694 or Julian d
77. d equinox of date jdtt dec double Apparent declination in degrees referred to true equator and equinox of date jdtt RETURNED VALUE short int 0 Everything OK gt 10 Error code from function make_object gt 20 Error code from function place Discussion Function app_star computes the apparent place of a star for time jd_tt As used here any object outside the solar system is a star The calling program should set promora promodec parallax or radialvelocity within structure star to 0 0 if their values are unknown if their values are zero within the associated error or if they describe an extragalactic object Function app_star works by calling place with location gt where 1 0 and coord_sys Function make_cat_entry can assemble loose catalog data into a structure of type cat entry Return to Function List C 61 topo_star short int topo_star double jd_tt double delta_t cat_entry star on_surface position short int accuracy double ra double dec PURPOSE Computes the topocentric place of a star at date jd_tt given its catalog mean place proper motion parallax and radial velocity REFERENCES Kaplan G H et al 1989 Astron Journ 97 1197 1210 Explanatory Supplement to the Astronomical Almanac 1992 Chapter 3 INPUT ARGUMENTS jd_tt double TT Julian date for topocentric place delta_t double Difference TT UT1 at j
78. d for data tabulated in astrometric catalogs and fundamental solar system ephemerides Geocentric Celestial Reference System GCRS replaces the geocentric system based on the mean equator and equinox of J2000 0 It is used for geocentric apparent positions of celestial objects measurements and coordinates in the near Earth environment and artificial Earth satellite ephemerides The BCRS and GCRS are nearly parallel systems related by a relativistic transformation International Celestial Reference System ICRS is the name applied to the orientation of the axes of the BCRS based on the adopted coordinates of several hundred extragalactic radio sources that are assumed to have no net systematic motion The resulting orientation is close to but not exactly aligned with the mean equator and equinox of J2000 0 The ICRS is a space fixed kinematically non rotating system 4 http www usno navy mil USNO astronomical applications publications circ 179 C 102 Because of their close relationship the abbreviations BCRS and ICRS are often used interchangeably Celestial Intermediate Reference System is a system for geocentric apparent positions of stars and planets based on the true instantaneous equator of date and a zero point of right ascension at the CIO see section A 5 The geocentric system based on the true equator and equinox of date is also still used for geocentric apparent positions of stars and planets T
79. d is provided by era Function cio_location obtains the location of the CIO for a given date in one of two ways and sets an output argument ref_sys that indicates which way was used If an external file of http www usno navy mil USNO astronomical applications publications circ 179 C 99 CIO right ascension values is available nominally called cio_ra bin and located in the current directory then cio_location will provide the GCRS right ascension of the CIO and will set ref_sys to one 1 If this file is not available then cio_location will provide the true right ascension of the CIO the arc on the instantaneous equator from the equinox to the CIO obtained from a series expansion and will set ref_sys to two 2 cio_basis can work with either coordinate of the CIO The two methods are equivalent within several microarcseconds over six centuries centered on the year 2000 it is not clear which method is more correct To do the hard work cio_location calls either cio_array for ref_sys 1 or ira_equinox for ref_sys 2 cio_location always initially checks to see if the external file of CIO right ascensions is present If it is cio_array reads the file which is the output from a numerical integration covering years 1700 to 2300 and returns an array to be interpolated the interpolation directly provides the right ascension of the CIO in the GCRS A copy of CIO_RA TXT is provided as part of the NOVAS distribution along with a util
80. d to C 68 the function via argument delta_t Values of AT are published annually in The Astronomical Almanac or can be obtained from the EO web site The user s choice of ephemerides determines the values to be used in structure ss_body which identifies the solar system object Atmospheric refraction can be subsequently applied to ra and dec by function equ2hor Function topo_planet works by calling place with location gt where 1 and coord_sys l Return to Function List http www usno navy mil USNO earth orientation eo products long term C 69 virtual_planet short int virtual_planet double jd_tt object ss_body short int accuracy double ra double dec double dis As URPOSE Compute the virtual place of a solar system body REFERENCES Kaplan G H et al 1989 Astron Journ 97 1197 1210 Explanatory Supplement to the Astronomical Almanac 1992 Chapter 3 INPUT ARGUMENTS jd_tt double TT Julian date for virtual place ss_body struct object Pointer to structure containing the body designation for the solar system body defined in novas h accuracy short int Code specifying the relative accuracy of the output position 0 full accuracy 1 reduced accuracy OUTPUT ARGUMENTS ra double Virtual right ascension in hours referred to the GCRS dec double Virtual declination in degrees referred to the GCRS dis double True dist
81. d_tt in seconds of time star struct cat_entry Pointer to catalog entry structure containing catalog data for the object in the ICRS defined in novas h position struct on_surface Specifies the position of the observer structure defined in novas h accuracy short int Code specifying the relative accuracy of the output position 0 full accuracy 1 reduced accuracy OUTPUT ARGUMENTS ra double Topocentric right ascension in hours referred to true equator and equinox of date jd_tt dec double Topocentric declination in degrees referred to true equator and equinox of date jd_tt RETURNED VALUE short int 0 Everything OK 1 Invalid value of where in structure location gt 10 Error code from function make_object gt 20 Error code from function place Discussion Function topo_star computes the topocentric place of a star neglecting atmospheric refraction for the location specified by the argument location at time jd_tt Note that jd_tt is the TT time at which the topocentric place is to be computed The word star as used here refers to any object outside the solar system If the values of promora promodec parallax or radialvelocity within structure star are unknown or zero C 62 within the errors of measurement the calling program should set them to zero For extragalactic objects these input values should
82. d_utl jd_tt delta_t 86400 0 3 3 Example 1 Position of a Star Suppose we have the catalog data from star FK6 1307 Groombridge 1830 for epoch J2000 0 expressed in the ICRS ICRS right ascension at J2000 0 hours 11 88299133 ICRS declination at J2000 0 degrees 37 71867646 Proper motion in RA milliarcseconds per year 4003 27 Proper motion in dec milliarcseconds per year 5815 07 Parallax milliarcseconds 109 21 Radial velocity kilometers per second 98 8 To obtain the apparent geocentric place of the star on our date of interest with respect to the equator and equinox of that date first make a structure of type cat_entry make_cat_entry GMB 1830 FK6 1307 11 88299133 37 71867646 4003 27 5815 07109 21 98 8 amp star Then simply call app_star supplying it with the catalog quantities error app_star jd_tt amp star accuracy amp ra amp dec In this example and other examples in this chapter the returned value from the function error ts an error indicator Non zero values of error indicate an error condition inside the function see the function prolog for details The calling function should take appropriate action in such cases The output coordinates ra and dec hours and degrees respectively represent the apparent geocentric coordinates of the star with respect to the true equator and equinox of date The computation takes into account all time dependent effects that shift the sta
83. double precision floating point values type double 3 1 Initialization You may choose to call many NOVAS functions in either full or reduced accuracy modes Usually all functions in a single application will have the same accuracy requirements Therefore you may wish to define a short integer constant for this purpose at the beginning of your main function example operates in full accuracy mode so accuracy is set to zero const short int accuracy 0 A value of one 1 would have set reduced accuracy For more information on the two modes see section 2 6 Three high level NOVAS functions sidereal_time ter2cel and cel2ter can perform their internal calculations using either the CIO based method or equinox based method If you will be calling these functions multiple times you may also wish to define a short integer constant for this purpose with a value of zero 0 for the CIO based method or one 1 for the equinox based method In example the appropriate method is chosen when these functions are called Because the equinox based method is more efficient for computing sidereal time example chooses this mode For more information about these two methods see section 5 2 3 2 Setting Time Arguments You should also consider how you will handle dates and times As described in section 1 3 NOVAS uses either of two time scales TT or UT1 as the basis for the time input argument to its higher level functions However you may be working
84. e and right ascension measured with respect to the CIO is called intermediate right ascension The complete table of nomenclature for apparent positions of various types updated for NOVAS 3 0 and later is below Final Coordinate System Observer at Geocenter oo a True equator and equinox of date apparent place topocentric place Celestial Intermediate Reference System intermediate place GCRS proper or virtual place local place GCRS astrometric place variant of proper or virtual place in which some calculations are omitted The only difference between a position expressed in the Celestial Intermediate Reference System and the same position expressed with respect to the true equator and equinox of date is an offset in right ascension the equation of the origins The equation of the origins is the angle between the equinox and the CIO both of which lie on the instantaneous equator ira_equinox NOVAS function place can be used to compute all of these types of positions the input argu ments allow you to select both the location of the observer and the coordinate system in which the computed position is to be expressed These selections make the nomenclature superfluous place does not apply atmospheric refraction but that can be added by a subsequent call to egu2hor 1 3 Time Scales for Astronomy As explained in USNO Circular 179 basically two kinds of time scales are used in astronomy those that are bas
85. e vector might represent a geographic position a geodetic reference line or direction or an instrumental axis For this transformation the vector starts out as an Earth fixed vector expressed with respect to the ITRS axes For example the vector toward the local vertical orthogonal to the ellipsoid at the place of interest is simply cos 9 cos A cos sin A sin d where is the geodetic latitude and A is the longitude A vector along a telescope s polar axis would nominally point toward 0 0 1 in this system lon_rad geo_loc longitude DEG2RAD lat_rad geo_loc latitude DEG2RAD sin_lon sin lon_rad cos_lon cos lon_rad j sin_lat sin lat_rad cos_lat cos lat_rad j vter 0 cos_lat cos_lon vter 1 cos_lat sin_lon vter 2 sin_lat Any such ITRS vector can be transformed into the equivalent GCRS vector with a single call to ter2cel rror ter2cel jd_ut1 0 0 delta_t 1 accuracy 0 x_pole y_pole vter vcel Strictly ephemeris requires a TDB based Julian date as input TDB differs from TT by at most 1 7 ms see section 1 3 If this time difference is important use function tbd2tt to determine the difference between the two time scales and adjust j d_tdb accordingly For the example here neglecting the difference can lead to an error in the position of Mars of about 50 meters C 39 where vter is the input vector terrestrial ITRS and vce1 is the equivalent output vector celestial GCR
86. e with NOVAS After successfully completing the test compile and link NOVAS function solarsystem version 2 solsys2 c and the JPL subroutines contained in jplsubs f with the remaining relevant parts of NOVAS and your application code Alternatively the binary ephemeris file can be used directly with solarsystem version 1 and the software in file eph_manager c Unless you specified otherwise in Step 5b the binary JPL ephemeris file or an alias to it should reside in the same directory as your executable application Return to Table of Contents C 119 C 120 Appendix D A Comparison of SOFA and NOVAS C3 0 The Standards of Fundamental Astronomy IAU 2009a SOFA library is the official collection of approved software for positional astronomy operating under the auspices of International Astronomical Union IAU Division 1 Fundamental Astronomy Both Fortran and C libraries are available An international SOFA Reviewing Board manages the collection Generally NOVAS is independent of SOFA although both software libraries include code that is similar to two IERS Fortran modules Function iau2000a which evaluates the full 1 365 term IAU 2000A nutation series in NOVAS 3 0 is based on the IERS subroutine NU2000A Function ee_ct which evaluates the complementary terms in the equation of the equinoxes is based on IERS function EECT2000 The corresponding modules in SOFA are respectively iau_NUTOOA and iau_EECTOO Fortran and
87. ed on the Syst me International SI second atomic time and those that are tied to the irregular rotation of the Earth in essence lab time and astronomical time respectively Theoretical time scales not kept by any real clocks are also used that are the time basis for that is the fourth dimension of the BCRS and GCRS We almost always start with Coordinated Universal Time UTC which is the worldwide basis for civil time UTC USNOJ is obtained from the Global Positioning System GPS In addition UTC can be obtained with varying accuracy from Network Time Protocol NTP services on the Internet and from radio time broadcasts e g WWV WWV WWVB and CHU cell phones TV etc UTC is based on SI seconds at sea level on the rotating Earth From UTC one adds an integral number of SI seconds to obtain International Atomic Time TAI TAI UTC AAT where AAT is a total count of leap seconds in UTC for example AAT 34 s for 2011 The USNO Earth Orientation EO Department provides a complete table of AAT values on line The current value may also be found at the beginning of Bulletin A published by the International Earth Rotation and Reference System Service IERS The addition of a leap second to UTC which increases AAT by 1 second is usually done when needed at 23 59 59 UTC on December 31 and is announced about six months in advance NOVAS Time Arguments Typically the first input argument to most NO
88. el functions perform computations corresponding to individual physical effects or transformations aberration light bending precession polar motion etc Supervisory level functions call the basic and utility subroutines in the proper order to compute the apparent coordinates of stars or solar system bodies for specific dates and times If desired users can interact exclusively with the supervisory level functions and not become concerned with the details of the geometry or physical models involved in the computation The Fortran version of NOVAS goes back to the late 1970s but has been updated periodically to use new more accurate models that represent the evolving standards of the international astronomical and geodetic communities The success of the initial Fortran package led to requests for a C language version In the early 1990s the U S Naval Observatory USNO Naval Research Laboratory NRL optical interferometer group converted parts of NOVAS to C for use in their project That work formed the basis for the first complete edition of NOVAS in C designated NOVAS Version C1 0 which the USNO Astronomical Applications AA Department completed and released in 1996 A major revision of the NOVAS Fortran code took place in 1998 with the primary goal of supporting data conforming to the International Celestial Reference System ICRS Shortly thereafter the C version of NOVAS was updated to version C2 0 to reflect the changes in the Fortran
89. ephemerides of solar system objects and astrometric reference data on galactic and extragalactic objects i e the data in astrometric star catalogs The GCRS is the system appropriate for describing the rotation of the Earth the orbits of Earth satellites and geodetic quantities such as instrument locations and baselines The directions of astronomical objects as seen from the geocenter can also be expressed in the GCRS The analysis of precise observations inevitably involves quantities expressed in both systems and the transformations between them Functions in NOVAS may work with BCRS vectors GCRS vectors or both with appropriate conversions If the orientation of the BCRS axes in space is specified the orientation of the GCRS axes then follows from the relativistic transformation between the two systems The orientation of the BCRS is given by what is called the International Celestial Reference System ICRS http www usno navy mil USNO astronomical applications publications circ 179 C 14 The ICRS is a triad of coordinate axes with their origin at the solar system barycenter and with axis directions effectively defined by the adopted coordinates of about 200 extragalactic radio sources observed by VLBI listed in Section H of The Astronomical Almanac The abbreviations BCRS and ICRS are often used interchangeably because the two concepts are so closely related the ICRS is the orientation of the BCRS while the BCRS is the metric for
90. er than directions that is internally it places all objects at their computed distance from the solar system barycenter Objects of unknown distance are placed on the NOVAS celestial sphere at a radius of 1 Gigaparsec 2 x 10 astronomical units starvectors These vectors are all defined within the BCRS until the relativistic aberration calculation is applied which effectively takes an input vector in the BCRS and produces an output vector in the GCRS Nomenclature When all these effects are included we obtain star or planet coordinates in the GCRS that reflect where the star or planet actually appears in the sky The coordinates can then be transformed to other reference systems if desired We will call the results of this process generically the apparent position or observed position of the object However some caution with the semantics is in order because the term apparent place has traditionally been reserved specifically for the star or planet position we obtain by applying all these effects except refraction for a geocentric observer with the coordinates expressed with respect to the true equator and equinox of date For an observer on the surface of the Earth the corresponding term is topocentric place If the apparent star or planet position is instead expressed with respect to the mean equator and equinox of J2000 0 the terms used in NOVAS have been virtual place and local place respectively Although
91. f the observer and the object place calls limb_angle for the topocentric cases in order to decide whether to include the gravitational deflection of light due to the Earth itself make_observer creates a structure of type observer specifying the location of the observer make_observer_at_geocenter creates a structure of type observer specifying an observer at the geocenter make_observer_in_space creates a structure of type observer specifying the position and velocity of an observer situated on a near Earth spacecraft make_observer_on_surface creates a structure of type observer specifying the location of and weather for an observer on the surface of the Earth make_in_space creates a structure of type in_space specifying the position and velocity of an observer situated on a near Earth spacecraft make_on_surface creates a structure of type on_surface specifying the location of and weather for an observer on the surface of the Earth mean_obliq is called from e_tilt and cel_pole to compute the mean obliquity of the ecliptic norm_ang is called by ee_ct to normalize angles into the range 0 lt angle lt 2 7 46 http tai bipm org iers conv2003 conv2003_c5 html C 108 nu2000k is a modification of iau2000a It evaluates a truncated version 2000K of the full IAU 2000A nutation series and uses a consistent set of expressions for the fundamental arguments those of Simon et al 1994 It is more accurate than the IAU 2000B series
92. fore you will have to consult your compiler manual for detailed instructions The instructions given for version 2 of solarsystem provide an example of how such files could be hand led Return to Function List C 94 solarsystem_hp version 3 File solsys3 c RETURNED VALUE short int O Everything OK 1 Input Julian date tjd out of range 2 Invalid value of body 3 This version of solarsystem not valid for use with NOVAS C Discussion NOVAS does not provide a high precision counterpart of solarsystem version 3 that works without requiring an external data file Thus this version of solarsystem_hp is essentially a dummy function that acts according to the value of variable act ion which is set within the function itself If action 1 the default this function returns an error code of 3 indicating appropriately that the function does not provide high precision position and velocity of the Earth and Sun If action 2 must be manually set this function simply calls function solarsystem version 3 and returns the low precision position and velocity An error code of 0 no error is also returned This action may be useful for code testing purposes but is neither appropriate nor recommended for normal use of NOVAS Use alternate versions of solarsystem_hp located in the various solsysn c files where n is an integer when the highest precision is needed Return to Function List
93. gms are constructed on the instantaneous true equator of date the plane orthogonal to the CIP declinations are in fact completely unaffected How are hour angles of celestial objects computed in the old and new paradigms Assume that we are considering Greenwich hour angles that is hour angles measured from the meridian of geodetic longitude zero the X Z plane of the ITRS without polar motion In the equinox based scheme we compute the topocentric place of the object of interest with respect to the true equator and equinox of date Then we compute Greenwich apparent sidereal time and subtract the object s apparent right ascension to form the hour angle In the CIO based scheme we compute the object s topocentric place with respect to the true equator and CIO of date To form the hour angle we compute the ERA and subtract the ClO based right ascension also called the intermediate right ascension The two results should be identical The computed GHA may have to be reduced to the range 12 to 12 The table below summarizes the two equivalent procedures for hour angle and the NOVAS functions that would be used for each assuming that polar motion is neglected The procedures outlined here provide the Greenwich hour angle GHA of a star only the first step would be different for a solar system body Functions app_star topo_star and place require time arguments in the TT time scale topo_star also requires AT while sidere
94. gular change in the direction of the light path through the Earth s atmosphere applies only to an observer on or near the surface of the Earth The direction of refraction is always assumed to be parallel to the local vertical and a function only of zenith distance although these assumptions may not be true in all cases At optical wavelengths its magnitude is zero at the zenith about 1 arcminute at a zenith distance of 45 and 0 5 at the horizon Refraction is roughly proportional to the atmospheric pressure at the observer but it also depends on other atmospheric parameters and the observational wavelength equ2hor refract The same effects are relevant to objects in the solar system except that the proper motion calculation is replaced by a function that retrieves the object s barycentric position from its ephemeris as part of an iterative light time calculation light_time ephemeris Extragalactic objects can be considered to be stars with zero parallax and proper motion The star or planet positions computed by considering all these effects obviously depend on the location of the observer so that an observer on the surface of the Earth will see a slightly different position than one at the geocenter the differences being greater for solar system objects especially nearby ones reaching about 1 for the Moon In computing these effects the same functions are used for stars and planets because NOVAS uses position vectors rath
95. he complementary terms in the equation of the equinoxes iau2000a nutation_angles grav_def ephemeris ee_ct If your accuracy requirements are no better than about 1 milliarcsecond you may wish to consider reduced accuracy mode which is described in Section 2 6 for your own applications 2 4 Using External Minor Planet Ephemeris Files The USNO AE98 minor planet ephemerides contain positions for fifteen of the largest asteroids and are used in the production of The Astronomical Almanac These ephemerides along with software to compact read and interpolate them are available on CD ROM from the USNO The software is written in C and designed to be used with NOVAS To include these ephemeris files in your NOVAS programs you will need to install a working copy of the USNO AE98 software and convert the relevant ASCII ephemerides to binary Chebyshev polynomial ephemerides You will still need one version of solarsystem solarsystem version with access to the JPL lunar and planetary ephemeris is recommended Instructions for installing and testing the USNO AE98 software and for converting ephemerides are provided with the software Section 2 3 and Appendix C discuss the use and installation of a JPL lunar and planetary ephemeris In order to access the USNO AE98 minor planet ephemerides function ephemeris calls function readeph which is part of the USNO AE98 package and is not part of or supplied with NOVAS A dummy versio
96. he basic solar system ephemerides are provided in the ICRS that is within the BCRS as aligned to the ICRS axes or at least are C 15 consistent with it to within the data s inherent accuracy place ephemeris The latter case will probably apply to most FK5 compatible data specified with respect to the mean equator and equinox of J2000 0 the J2000 0 system The distinction between the ICRS and the J2000 0 system becomes important only when an accuracy of 0 02 arcsecond or better is important frame_tie Nevertheless because NOVAS is designed for the highest accuracy applications the ICRS is mentioned as the reference system of choice for many input arguments to NOVAS functions Reference Systems for NOVAS Output Data For output coordinates e g the position of Mars on a certain date three options for the coordinate system are available The coordinates themselves can be right ascension and declination or the components of a unit vector You can request coordinates expressed in the GCRS the equator and equinox of date or the Celestial Intermediate Reference System equator and CIO of date These coordinate systems can be requested for either geocentric or topocentric output place gcrs2equ NOVAS can also convert topocentric right ascension and declination with respect to the equator and equinox of date to local horizon coordinates zenith distance and azimuth The angular shift due to atmospheric refraction can
97. hese reference systems are now used for the input and output arguments to NOVAS functions For example NOVAS 3 0 and later assume that input reference data such as catalog star positions and proper motions and the basic solar system ephemerides are provided in the ICRS that is within the BCRS as aligned to the ICRS axes or at least are consistent with it to within the data s inherent accuracy The distinction between the ICRS and the system defined by the mean equator and equinox of J2000 0 the J2000 0 system becomes important only when an accuracy of 0 02 arcseconds or better is needed Nevertheless because NOVAS is designed for the highest accuracy applications the ICRS is given as the reference system of choice for many input arguments to NOVAS functions Because the ICRS axes are not precisely aligned to those of the J2000 0 system a new function called frame_tie is available to transform vectors between the two systems This transformation is a very small fixed rotation frame_tie is used for both barycentric vectors BCRS to or from the barycentric J2000 0 system and geocentric vectors GCRS to or from the geocentric J2000 0 system frame_tie is called many times in both directions within the NOVAS code It is needed because precession and nutation can properly be applied only to vectors in a real equatorial system vectors in the GCRS geocentric ICRS must be transformed via frame_tie to the J2000 0 system before preces
98. iausofa org 50 http tai bipm org iers conv2003 conv2003_c5 html gt http www usno navy mil USNO astronomical applications publications circ 180 C 121 Function terceltest is provided in the addendum it was run using the following input parameters e Universal Time UT1 2400000 5 54195 4999991658 days Julian date The UT1 value is divided into two parts i e two separate arguments because of the large number of significant digits needed for precise results The best agreement between NOVAS and SOFA was obtained when UT 1 was split in the exactly the same place In the Fortran tests splitting the date differently produced differences of about 3 microarcseconds e Difference between TT and UT1 AT 65 25607389s SOFA does not use AT this value is the difference between the TT and UT1 Julian dates in the SOFA example expressed in seconds e Polar coordinates XP 0 0349282 YP 0 4833163 arcsec e CIP offsets DX 0 1725 DY 0 265 arcsec The SOFA function iauNut06a includes small corrections to the nutation series arising from the P03 precession that are not used in the NOVAS calculations The corrections amount to only a few microarcseconds for current dates NOVAS does not directly produce an overall GCRS to ITRS rotation matrix as SOFA does The NOVAS rotation matrix was constructed simply by passing the three vectors 1 0 0 0 1 0 and 0 0 1 in succession through function ter2cel A series of tests was
99. ifying the location of and weather for an observer on the surface of the Earth make_in_space Creates structure of type in_space specifying the position and velocity of an observer situated on a near Earth spacecraft make_on_surface Creates structure of type on_surface specifying the location of and weather for an observer on the surface of the Earth nutation Applies nutation to position vector precession Applies precession to position vector proper_motion Updates position vector of a star to allow for its space motion rad_vel Predicts radial velocity of observed object as would be measured by spectroscopy radec2vector Converts RA dec and distance to a position vector spin Rotates vector by specified angle about the z axis C 45 Function name Level Purpose starvectors i Converts a star s RA dec proper motion etc to position and velocity vectors terra i Converts geodetic coordinates to geocentric position vector vector2radec i Converts position vector to RA and dec Adjusts Earth fixed vector for polar motion Computes calendar date and time given Julian date Allows for the specification of celestial pole offsets for high precision applications Returns array of Julian dates and corresponding values of RA of CIO in GCRS given TDB Julian date and number of dates desired Returns location of CIO as RA with respect to either GCRS origin or true equinox of date
100. iles ending with txt extensions Compare the USNO generated output files with the results produced by your local installation as described in Section 2 2 basic installation Section 2 3 including a JPL solar system ephemeris and Section 2 4 including a USNO AE98 minor planet ephemeris checkout stars c main function that calls functions in novas c and solsys3 c to validate a basic local installation checkout stars usno txt USNO output from checkout stars application checkout stars full c main function that calls functions in novas c and solsys1 c with full accuracy to validate a local installation for use with solar system ephemerides requires binary JPL ephemeris file checkout mp c main function that calls functions in novas c solsys1 c and USNO AE98 minor planet software to validate a local installation for use with minor planet ephemerides requires minor planet ephemeris and binary JPL ephemeris files checkout mp usno txt USNO output from checkout mp application 4 http www usno navy mil USNO astronomical applications software products usnoae98 C 26 The following two files contain sample code to get you started developing your own applications using NOVAS Chapter 3 discusses each of the examples included and provides instructions for using the sample code The file with a c extension is the C source code that produced the associated ASCII text file txt example c main function that calls selected function
101. in seconds of time at jd_tdb dpsi double Nutation in longitude in arcseconds at jd_tdb deps double Nutation in obliquity in arcseconds at jd_tdb RETURNED VALUE None Discussion This function computes various quantities related to the orientation of the Earth s rotation axis vector toward Celestial Intermediate Pole with respect to the ecliptic plane at a specific time The computation involves a call to function nutation_angles to evaluate the nutation series The output values of the last four arguments will correctly reflect the celestial pole offsets if function cel_pole has previously been called Return to Function List C 82 ephemeris short int ephemeris double jd 2 object cel_obj short int origin short int accuracy double pos double vel PURPOSE Retrieves the position and velocity of a solar system body from a fundamental ephemeris REFERENCES one INPUT ARGUMENTS jd 2 double TDB Julian date split into two parts where the sum jd 0 jd 1 is the TDB Julian date cel_obj struct object Pointer to structure containing the designation of the body of interest defined in novas h origin short int Origin code 0 solar system barycenter 1 center of mass of the Sun accuracy short int Selection for accuracy 0 full accuracy 1 reduced accuracy OUTPUT ARGUMENTS pos 3 double Position vector of the
102. ins bulletins html C 56 transform_cat short int transform_cat short int option double date_incat cat_entry incat double date_newcat char newcat_id SIZE_OF_CAT_NAME cat_entry newcat PURPOSE To transform a star s catalog quantities for a change of epoch and or equator and equinox Also used to rotate catalog quantities on the dynamical equator and equinox of J2000 0 to the ICRS or vice versa REFERENCES one INPUT ARGUMENTS option short int Transformation option 1 change epoch same equator and equinox 2 change equator and equinox same epoch 3 change equator and equinox and epoch 4 change equator and equinox J2000 0 to ICRS D change ICRS to equator and equinox of J2000 0 date_incat double TT Julian date or year of input catalog data incat struct cat_entry An entry from the input catalog with units as given in the struct definition struct defined in novas h date_newcat double TT Julian date or year of transformed catalog data newcat_id SIZE_OF_CAT_NAME char Catalog identifier SIZE_OF_CAT_NAME 1 characters maximum e g HIP Hipparcos TY2 Tycho 2 OUTPUT ARGUMENTS newcat struct cat_entry The transformed catalog entry with units as given in the struct definition struct defined in novas h RETURNED VALUE 0 Everything OK 1 Invalid value of an input date for op
103. ion to from J2000 0 sidereal_time returns the value of sidereal time either mean or apparent Internally this function can work by either of two methods as set by the method input parameter Equinox based method evaluates the expression for mean sidereal time following the discussion in section 2 6 2 of USNO Circular 179 The ERA 9 is obtained from era For apparent sidereal time the equation of the equinoxes including the complementary terms is obtained from e _ tilt CIO based method obtains the sidereal time using the method of section 6 5 4 of USNO Circular 179 That equation is based on the position of the true equinox of date in the Celestial Intermediate Reference System the basis of which is obtained from cio_basis The ERA 9 is obtained from era Mean sidereal time when requested is calculated by subtracting the equation of the equinoxes obtained from e_tilt In either method sidereal_time era evaluates O using the input UT1 epoch but other components of sidereal time are evaluated using TDB set equal to TT with TT UTI1 AT where the delta_t input parameter sets the AT value ter2cel performs the terrestrial to celestial transformation on a given vector 1 e the total rotation from the ITRS to the GCRS it includes calls to frame_tie at appropriate places Internally this function can work by either of two methods as set by method input parameter Equinox based method evaluates the old style transformation a
104. ion takes into account the star s proper motion linear three dimensional space motion from the catalog epoch to the date requested parallax gravitational deflection of light by solar system bodies mostly the Sun and aberration The same effects are computed for solar system bodies except that the proper motion calculation is replaced by an algorithm that retrieves the object s barycentric position from its ephemeris as part of an iterative light time calculation Extragalactic objects are treated as stars with zero proper motion and parallax The result in all cases is an apparent direction expressed in the GCRS which is optionally transformed into either of two other output coordinate systems as specified by argument coord_sys Astrometric place is a variant of the above calculation that is appropriate for some types of differential measurements Light bending and aberration are ignored under the assumption that they are the same for all objects within a small area of the sky Astrometric places are expressed in the ICRS The observer s location may be at the geocenter on or near the surface of the Earth or in orbit around the Earth as specified by the where member of the argument location Function place does not take into account atmospheric refraction which would be appropriate only for observers on or near the surface of the Earth but its effect can be added by a subsequent call to function egu2hor For stars the
105. irect access file cio_basis returns orthonormal basis vectors for the Celestial Intermediate Reference System with respect to the GCRS It requires a prior call to cio_location cio_location returns the RA of the CIO at a given TDB Julian date either with respect to the GCRS or with respect to the true equator and equinox of date cio_ra returns the value of the true RA of the CIO for a given TDB Julian date d_light evaluates the difference in light time to a star between the solar system barycenter and the Earth ecl2equ_vec converts an ecliptic position vector to an equatorial position vector ee_ct evaluates a 34 term series for complementary terms in the equation of the equinoxes based on the work of Capitaine Wallace and McCarthy 2003 which is also discussed in Petit and Luzum eds 2010 equ2ecl converts equatorial RA and dec to ecliptic longitude and latitude equ2ecl_vec converts an equatorial position vector to an ecliptic position vector equ2gal converts ICRS RA and dec to galactic longitude and latitude era evaluates the ERA 0 frame_tie sets up the frame tie matrix and transforms a vector from the dynamical mean J2000 0 system to the ICRS or vice versa This function implements a first order matrix with second order corrections to the diagonal elements patterned after the work by Hilton and Hohenkerk 2004 Given the smallness of the angles involved and their uncertainties this approach is quite adequate
106. irtual place of a star given its catalog data local_star Computes local place of a star given geodetic location of observer astro_star Computes astrometric place of a star given its catalog data Computes ICRS J2000 0 place of a star given its apparent place Computes apparent place of a solar system body topo_planet Computes topocentric place of a solar system body given geodetic location of observer virtual_planet Computes virtual place of a solar system body C 44 Function name Level Purpose local_planet Computes local place of a solar system body given geodetic location of observer Computes astrometric place of a solar system body Adjusts position vector for aberration of light due to motion of Earth bary2obs Changes origin of coordinates from barycenter of solar system to center of mass of Earth cio_basis Computes orthonormal basis vectors with respect to GCRS of right handed system defined by CIP z direction and CIO x direction d_light For a star returns difference in light time between solar system barycenter and observer Or returns the light time from observer to point on light ray closest to a given solar system body Converts ecliptic position vector to an equatorial position vector Converts RA and dec to ecliptic longitude and latitude Converts equatorial position vector to an ecliptic position vector Conver
107. ith jplint f provide an interface between NOVAS and JPL s Fortran solar system ephemeris access code see detailed discussion in Chapter 4 solsys3 c versions of functions solarsystem and solarsystem_hp that provide the position and velocity of the Earth and Sun without reference to an external data file see detailed discussion in Chapter 4 solarsystem h header file for the solsysl c solsys2 c and solsys3 c files eph_manager c C implementation of JPL s solar system ephemeris access code for use with solsysl c requires a binary JPL ephemeris 2 http www usno navy mil USNO astronomical applications software products novas novas c C 25 eph_manager h Header file for eph_manager c readeph0 c dummy version of function readeph the highest level call to the USNO AE98 minor planet ephemerides software when positions of selected minor planets are desired replace this file with readeph c from the USNO AE98 package jplint f Fortran subroutines that when used with solsys2 c provide an interface between NOVAS and JPL s Fortran ephemeris access code see detailed discussion in Chapter 4 README txt basic information for using NOVAS C3 1 NOVAS_C3 1_Guide pdf User s Guide to NOVAS C3 1 this manual In addition the following six files are provided to assist you in validating the installation of NOVAS on your local system Files with a c extension are the C source code that created the associated ASCII text f
108. ity program called cio_file c to convert the text file to a binary direct access file section 2 5 describes creating cio_ra bin 2 9 Mbytes from CIO_RA TXT 7 5 Mbytes If cio_ra bin is not present then cio_location calls ira_equinox to evaluate the equation of the origins from a closed form expression that includes the evaluation of nutation in longitude a lengthy series of trigonometric terms The result locates the CIO with respect to the equinox on the instantaneous equator At no point does NOVAS use the CIO locator s which is described in IERS documents and The Astronomical Almanac 5 4 References Kaplan G H 2005 The IAU Resolutions on Astronomical Reference Systems Time Scales and Earth Rotation Models USNO Circular 179 Washington DC USNO http www usno navy mil USNO astronomical applications publications circ 179 USNO Circular 179 USNO amp HMNAO 2009 The Astronomical Almanac for the Year 2011 Washington DC GPO and subsequent editions Return to Table of Contents C 100 Appendix A Overview of How NOVAS Has Changed A detailed list of the changes in the NOVAS C code from the previous versions C2 0 1 of 2000 and C3 0 of 2009 is given in Appendix B This appendix provides some perspective on these changes for people who are already familiar with NOVAS Most of the modifications have been made in response to resolutions passed by the IAU in 2000 and 2006 that recommended new models for fundamental astr
109. ize It is plain text ASCII that can be modified by any text editor The first record is a header which should remain as such although its contents are not used by NOVAS but all the other records contain a Julian date and information for that date in ascending date order Simply trim from the beginning and ending of the file any records for dates that you are sure you will never need be sure to leave at least ten extra dates on each end to allow the internal interpolation scheme to work properly For example CIO_RA TXT contains data for years 1700 through 2300 but you may need only the range from 2010 to 2015 Leave the data within your overall anticipated date range as is that is do not attempt to make several groups of dates within the same file The final file that you create must be a fixed interval list running from the first date to the last date with no gaps after the header record When you have modified CIO_RA TXT C 30 appropriately follow the steps above to convert your custom version to the binary direct access format useable by NOVAS The output message and file will differ from that above based on the changes you made If a CIO file cio_ra bin is not present when NOVAS needs to determine the location of the CIO NOVAS will simply revert to using an internal computation for this information The results differ by a few microarcseconds at most and those differences are reached only for dates before 1900 or after 2200 The two
110. le rotations of the GCRS and in both cases the fundamental plane is the true equator of date gcrs2equ In the new terminology the true or instantaneous equator is the plane orthogonal to the Celestial Intermediate Pole CIP which is the celestial pole defined by the adopted precession and nutation algorithms see section 1 4 The only difference between these two systems is the origin of right ascension the points on the equator where RA 0 are respectively the true equinox and the Celestial Intermediate Origin CIO The CIO is a recently introduced fiducial point on the equator that rigorously defines one rotation of the Earth see section 1 5 cio_location The GCRS and the two equatorial systems obtained from it have their origin at the geocenter For topocentric coordinates or vectors referred to an observer at a specific location on or near the surface of the Earth analogous coordinate systems exist although no semantic distinction is usually made between them and their geocentric equivalents In NOVAS the topocentric equivalent of the GCRS is referred to as the local GCRS and its spatial axes are assumed to be obtained from the GCRS by a Galilean transformation simple shift of origin without a change in orientation The two topocentric equator of date systems are obtained similarly Reference System for NOVAS Input Data NOVAS now assumes that input reference data such as catalog star positions and proper motions and t
111. lid value of method gt 10 10 error from function cio_location gt 20 20 error from function cio_basis Discussion Function ter2cel rotates a vector from the terrestrial to the celestial system Specifically it transforms a vector in the ITRS a rotating Earth fixed system to the GCRS a local space fixed system by applying rotations for polar motion Earth rotation nutation precession and the dynamical to GCRS frame tie The input vector might represent a cardinal direction at the observer s position a geodetic baseline or some instrumental axis The units for the vector components are arbitrary and the output vector will have the same units as the input vector Geodetic coordinates in the WGS 84 system also sometimes called the Earth centered Earth fixed ECEF system can be considered to be compatible with the ITRS This function allows for the input UT1 time to be represented as a split Julian date See the discussion in the description of function sidereal_time Both jd_ut_high and jd_ut_low should be non negative for normal use jd_ut_low 0 0 is acceptable Values of AT delta_t are published annually in The Astronomical Almanac or can be obtained from the EO web site The option flag only works for the equinox based method Set xp yp 0 0 to omit the polar motion rotation Function cel2ter performs the reverse transformation rotating the vector from the celestial system GCRS to the terrestrial s
112. lue of UT1 UTC is available in a daily interval tabulation on the IERS web site data marked P are predictions IERS publishes historical values in Bulletin B The values of UT1 UTC often change at the millisecond level over one day In computing the topocentric direction of a celestial object with respect to Earth fixed axes e g altitude and azimuth l arcsecond accuracy in the final angles requires 67 ms accuracy in UT1 Because UTI1 UTC can have an absolute value up to 900 ms it is an important correction for all but the crudest applications that is in most cases it is not acceptable to approximate UT1 as being equal to UTC A few of the lower level NOVAS functions use time arguments based on Barycentric Dynamical Time TDB e_tilt precession ephemeris solarsystem TDB differs from TT only by periodic variations due mainly to the Earth s elliptical orbit and described by General Relativity the largest of which has an amplitude of 1 6 ms and a period of one year tdb2tt The difference between the two time scales can often be neglected in practice as noted in the function preambles where appropriate TDB is equivalent to Tepn the barycentric coordinate time argument of the Jet Propulsion Laboratory JPL planetary and lunar ephemerides As previously mentioned time is specified within NOVAS as Julian dates which can be used for any of the above time scales Julian dates are a simple count of days since noon on 4713 BC J
113. lving atmospheric refraction here the values are simply placeholders You should create a structure of type on_surface for that location make_on_surface latitude longitude height temperature pressure amp geo_loc Now call topo_star error topo_star jd_tt delta_t star amp geo_loc accuracy amp rat amp dect where rat and dect reflect the position of the star as it would be seen from that particular location the small differences from the geocentric coordinates ra and dec arise mainly from diurnal aberration topo_star uses the catalog data on the star created earlier 3 4 Example 2 Position of the Moon Obtaining the coordinates of solar system objects other than the Sun requires that either solarsystem version 1 or version 2 be used First you will need to create a structure of type cat_entry for a dummy star which is then used as a placeholder in a structure of type ob ject for the Moon make_cat_entry DUMMY xxx 0 0 0 0 0 0 0 0 0 0 0 0 0 amp dummy_star rror make_object 0 11 Moon amp dummy_star amp moon Then call function app_planet error app_planet jd_tt amp moon accuracy amp ra amp dec amp dis where again ra and dec are the apparent geocentric coordinates of the Moon hours and degrees respectively with respect to the true equator and equinox of date and dis is the true Euclidean geocentric distance astronomical units or AU at time jd_tt The comp
114. mponents of binary stars Also this function cannot be properly used to bring data from old star catalogs into the modern system because old catalogs were compiled using a set of constants in particular the rate of precession that are incompatible with modern values The option 2 and option 3 transformations involve the dynamical system that is the moving mean equator and equinox The mean equator and equinox of J2000 0 was the most common reference system for modern astrometric catalog data before the ICRS was introduced in 1998 Now catalog data is usually referred to the ICRS which is a reference system fixed with respect to distant extragalactic objects not defined by any Earth motions and with no associated date The option 4 transformation is used to convert catalog quantities from the mean equator and equinox of J2000 0 the J2000 0 system to the ICRS The option 5 transformation is the opposite The arguments date_incat and date_newcat are ignored for these transformations Function frame_tie can be used to transform vectors from the J2000 0 system to the ICRS or vice versa See function transform_hip for transforming Hipparcos catalog data to epoch J2000 0 SIZE_OF_CAT_NAME is defined in novas h Return to Function List transform_hip void transform_hip cat_entry hipparcos cat_entry hip_2000 PURPOSE To convert Hipparcos catalog data at epoch J1991 25 to epoch J2000 0 for use within NOVAS
115. n implementing version 2 of solarsystem involves the proper compiling and linking of files containing different languages The procedures will be specific to your computing platform therefore you will have to consult your compiler manual for detailed instructions The following instructions are offered only as a guideline they provide a specific example of how such files were successfully handled on a PC running Red Hat Enterprise Linux ES release 4 Nahant Update 8 1 Create a single file named jplsubs f with all of the JPL Fortran ephemeris access subroutines as discussed in Appendix C Compile the Fortran files jplsubs f and jplint f without invoking the linkage editor to create the object file jplsubs o and jplint o The Fortran compiler linker iS g77 g77 c jplsubs f jplint f Compile again without invoking the linkage editor the C files main c novas c novascon c nutation c solsys2 c and readephO c that create the object files main o novas o novascon o nutation o solsys2 0 and readephO o In this example main c is the name of the file containing the user s program The C compiler linker is gcc gcc c main c novas c novascon c nutation c solsys2 c readeph0O c 4 Finally link the object files using the Fortran linker object files g77 o app main o novas o novascon o nutation o solsys2 o readeph0 o jplsubs o jplint o n create executable named app In this example the resulting executable file is name
116. n of readeph is provided in file readeph0 c The dummy function enables NOVAS to be used without the minor planet package i e for computing positions of major solar system bodies and stars only To use USNO AE98 with NOVAS replace file readephO c provided with NOVAS with readeph c allocate c and chby c from the USNO minor planet ephemerides software when compiling and linking Once you have a DEnnn binary solar system ephemeris file the USNO AE98 software and a binary Chebyshev polynomial ephemeris of a minor planet you can test the combined set up with solarsystem version on your system as follows a Compile and link files checkout mp c novas c novascon c nutation c solsys1 c eph_manager c readeph c allocate c and chby c The last three code files are from the USNO AE98 software b Name the resulting application checkout mp c Run the checkout mp application 7 http www usno navy mil USNO astronomical applications software products usnoae98 38 http www usno navy mil USNO astronomical applications software products usnoae98 ae98 rm C 29 d Compare the results that you get from checkout mp with the data in file checkout mp usno txt If the results agree the installation was probably successful but see the important note above The USNO output file checkout mp usno txt was generated using DE405 from JPL and pallas chby from the minor planet ephemerides Like the check out stars full application checko
117. ng of the program PARAMETER NRECL 4 Step 4 Split the testeph f file provided by JPL into two files jplsubs f and testeph f The first new file jplsubs f should contain JPL subroutines FSIZER3 PLEPH including ENTRY DPLEPH INTERP SPLIT STATE and CONST extracted from the original JPL test program The second revised file testeph f should contain only the main program and no subroutines JPL subroutines FSIZER1 and FSIZER2 will not be needed and are not retained in either file The creation of the new file jplsubs f is advantageous for the long term use of the JPL subroutines and the binary ephemeris files with NOVAS The new file contains no extraneous material and is named descriptively Step 5 Configure the new jplsubs f file a For computer systems with Intel processors in subroutine FSIZER3 of jplsubs f uncomment and set NRECL 4 b In subroutine FS ZER3 of jplsubs f identify the binary ephemeris file to be used by setting NAMFIL JPLEPH You may wish to specify a more complete file name including the appropriate directories such as NAMFIL users mystuff ephem JPLEPH to avoid having to keep an alias to this binary file in all your working directories c In subroutine FSIZER3 of jplsubs f set KSI ZE to the correct value for the specific ephemeris being used see the comments in the code e g set KSIZE 2036 for DE405 and DE421 d Insubroutine STATE of jplsubs f select uncomment CA
118. ntaneous equatorial plane between the TIO and the CIO which is a linear function of universal time UT1 This angle is called the Earth Rotation Angle ERA and is designated by 9 era Some internal calculations in NOVAS can be performed using either the equinox or the CIO as the fundamental fiducial point on the moving astronomical equator The user can select the basis for these calculations via an input parameter method The results are identical to within a microarcsecond around the current time and the computational burden is about the same 8 http www usno navy mil USNO astronomical applications publications circ 18 1 The CIO and TIO are technically examples of non rotating origins although neither is fixed within its respective coordinate system However the slow drift of the TIO due to polar motion with respect to standard geodetic coordinates the ITRS or effectively WGS 84 amounts to only 1 5 millimeters per century and is completely negligible for astronomical purposes C 22 1 6 Terrestrial Celestial Relationships NOVAS uses the ITRS for specifying locations and directions on or near the surface of the Earth As mentioned at the end of section 1 1 the ITRS is consistent to within a few centimeters with WGS 84 coordinates provided by GPS and it is sometimes referred to as the Earth centered Earth fixed system ECEF The ITRS is a geocentric system with the directions of its axes defined by the coordinates of
119. nutation series 1 365 terms is used iau2000a Evaluating the series for nutation is usually the main computational burden in NOVAS so using reduced accuracy mode improves execution time often noticeably In reduced accuracy mode the NOVAS 2000K nutation series 488 terms is used by default nu2000k This mode can be used when the accuracy requirements are not better than 0 1 milliarcsecond for stars or 3 5 milliarcseconds for solar system bodies Selecting this approach can reduce the time required for Earth rotation computations by about two thirds However if your reduced accuracy application requires the AU approved truncated nutation series IAU 2000B 77 terms you can edit the function nutation_angles slightly so that it calls iau2000b in reduced accuracy mode instead of nu2000k the necessary changes are described in the prolog to that function Section 1 4 and USNO Circular 181 Kaplan 2009 provide some additional information about nutation series 2 7 References Hilton J L 1999 AJ 117 1077 http www usno navy mil USNO astronomical applications software products usnoae98 USNO AE98 Kaplan G H 2009 Nutation Series Evaluation in NOVAS 3 0 USNO Circular 181 Washington DC USNO http www usno navy mil USNO astronomical applications publications circ 181 USNO amp HMNAO 2010 The Astronomical Almanac for the Year 2011 Washington DC GPO and subsequent editions Return to Table of Contents ittp w
120. o angular motions the Earth s rotation and the precession of its axis In the case of apparent sidereal time nutation is also included One rotation of the Earth is about 8 ms longer than one mean sidereal day For about two decades people who routinely deal with the most precise measurements of the Earth s rotation have been advocating for a change in the way it is described Their ideas were introduced in resolutions passed by the IAU in 2000 In this new paradigm the reference point on the moving celestial equator for the description of Earth rotation is called the Celestial Intermediate Origin CIO Unlike the equinox this point has no motion along the equator at all as the orientation of the equator changes in space due to precession and nutation the CIO remains on the equator but its instantaneous motion is always at right angles to it cio_location cio_ra Thus loosely speaking two transits of the CIO across a terrestrial meridian define one rotation of the Earth The CIO is a point on the celestial equator near RA 0 in the Celestial Intermediate Reference System it defines RA 0 and there is a corresponding point on the terrestrial equator near longitude 0 called the Terrestrial Intermediate Origin TIO For all astronomical purposes the TIO can be considered a point fixed at geodetic longitude zero on the Earth s rotational equator In the new paradigm the rotation of the Earth is specified by the angle in the insta
121. of vectors between equatorial and ecliptic systems Return to Function List cio_ra short int cio_ra double jd_tt short int accuracy double ra_cio PURPOSE This function computes the true right ascension of the celestial intermediate origin CIO at a given TT Julian date This is equation of the origins REFERENCES Kaplan G 2005 US Naval Observatory Circular 179 INPUT ARGUMENTS jd_tt double TT Julian date accuracy short int Selection for accuracy 0 full accuracy 1 reduced accuracy OUTPUT ARGUMENTS ra_cio double Right ascension of the CIO with respect to the true equinox of date in hours or RETURNED VALUE short int 0 everything OK 1 invalid value of accuracy gt 10 10 the error code from function cio_location gt 20 20 the error code from function cio_basis Discussion This function supplies the true right ascension of the Celestial Intermediate Origin CIO ra_cio equation of the origins ra_cio Greenwich apparent sidereal time Earth Rotation Angle where all quantities are expressed in hours Return to Function List C 78 era double era double jd_high double jd_low PURPOSE This function returns the value of the Earth Rotation Angle theta for a given UT1 Julian date The expression used is taken from the note to IAU Resolution B1 8 of
122. ointer to structure containing observer s location defined in novas h ra double Topocentric right ascension of object of interest in hours referred to true equator and equinox of date dec double Topocentric declination of object of interest in degrees referred to true equator and equinox of date ref_option short int 0 no refraction 1 include refraction using standard atmospheric conditions 2 include refraction using atmospheric parameters input in the location structure OUTPUT ARGUMENTS zd double Topocentric zenith distance in degrees affected by refraction if ref_option is non zero xaz double Topocentric azimuth measured east from north in degrees C 55 rar double Topocentric right ascension of object of interest in hours referred to true equator and equinox of date affected by refraction if ref_option is non zero decr double Topocentric declination of object of interest in degrees referred to true equator and equinox of date affected by refraction if ref_option is non zero RETURNED VALUE None Discussion Function equ2hor takes the topocentric celestial coordinates of an object and computes the equivalent local horizon coordinates The function uses a method that properly accounts for polar motion which is significant at the sub arcsecond level Atmospheric refraction can be included in the transformation and
123. onomical Almanac Mill Valley CA Univ Science Books The Explanatory Supplement USNO amp HMNAO 2009 The Astronomical Almanac for the Year 2011 Washington DC GPO and subsequent editions Return to Table of Contents Chapter 2 Installing NOVAS 2 1 List of Distribution Files The table below lists the fifteen files that comprise the main distribution of NOVAS C3 1 Except for the user s guide which is in Portable Document Format PDF the files are all plain ASCII text Files with a c extension are C source code their associated header files are designated by a h extension The single file with a f extension contains Fortran source code novas c contains all supervisory and utility functions and most basic functions see Section 4 2 for a full listing novas h header file for novas c includes structure definitions and function prototypes novascon c contains most mathematical and physical constants used by NOVAS novascon h header file for novascon c nutation c contains functions implementing the IAU 2000A IAU 2000B and 2000K nutation models nutation h header file for nutation c solsys1 c versions of functions solarsystem and solarsystem_hp that when used with eph_manager c provide a complete all C interface between NOVAS and JPL s lunar and planetary ephemerides see detailed discussion in Chapter 4 solsys2 c versions of functions solarsystem and solarsystem_hp that when used w
124. onomy within a new conceptual framework To the greatest extent possible the calling sequences for the highest level and most used functions from the previous versions of NOVAS have been preserved but a few important exceptions exist Many new calls are now available as well A 1 Important Changes in Calls Probably the most important change to existing NOVAS calls is the change of proper motion and parallax units in the calls to app_star virtual_star astro_star transform_cat and transform_hip all of which deal with star positions The units have been changed as follows e proper motion in right ascension from seconds per century to milliarcseconds per year e proper motion in declination from arcseconds per century to milliarcseconds per year e parallax from arcseconds to milliarcseconds These changes have been made to conform to the units used in most modern star catalogs e g Hipparcos Tycho 2 or the FK6 which in turn follow from the observational techniques now used in the construction of such catalogs Obviously star data previously used with NOVAS must either be replaced or transformed The transformation equations from old to new units are as follows pmrnew pmrold 150 0 cos decO DEG2RAD proper motion in RA pmdnew pmdold 10 0 proper motion in declination paxnew paxold 1000 0 parallax where dec0 is the catalog declination J2000 0 or ICRS of the star in degrees and D
125. open prior to calling the NOVAS functions ephem_open opens the binary JPL ephemeris file DE200 DE403 DE404 DE405 DE406 and DE421 are supported Similarly the user s program should call ephem_close to close the binary ephemeris file once ephemeris access is no longer required The functions ephem_open and ephem_close are located in eph_manager c The body identification numbers to be used with this version are listed below Name un Mercury Venus Earth Z S Jupiter Saturn Uranus Neptune Pluto Moon Return to Function List solarsystem_hp version 2 File solsys2 c RETURNED VALUE short int O0 Everything OK 1 Invalid value of body or origin 2 Error detected by JPL software Discussion This version of solarsystem_hp serves as the interface between the JPL s own Fortran based lunar and planetary ephemeris software and NOVAS when the highest precision is required The function contains a single call to Fortran subroutine jplihp_ which in turn calls DPLEPH and other Fortran subroutines in the JPL ephemeris software package The user is responsible for obtaining the Fortran ephemeris code and data setting up the binary random access ephemeris file and linking the JPL Fortran code with NOVAS See the implementation notes below The body identification numbers to be used with this version are listed below Name Sun Mercury Venus Earth Mar Jupiter Saturn Uran
126. option 5 does the opposite rotation wobble has a new Julian date argument B 2 2 Changes in Structures and Calling Sequences from C3 0 to C3 1 To prevent possible array overflows two new preprocessor constants SIZE_OF_OBJ_NAME and SIZE_OF_CAT_NAME have been defined in novas h SIZE_OF_OBJ_NAME defines the length of the starname character array in the cat_entry structure and the length of the name character array in the object structure SIZE_OF_CAT_NAME defines the length of the catalog character array in the cat_entry structure Although the actual lengths of C 110 the arrays are unchanged the make_cat_entry and make_object function prototypes and definitions now use these new preprocessor constants in their argument lists wobble has anew direction input parameter Several changes were made to the ephemeris manager code in eph_manager c and eph_manager h e The function names Ephem_Open Ephem_Close Planet_Ephemeris State Interpolate and Split were all changed to lower case 1 e ephem_open ephem_close planet_ephemeris state interpolate and split to conform to internal coding standards e ephem_open also has an additional output argument de_number that indicates the DE number of the opened ephemeris file B 3 Significant Internal Changes to Code B 3 1 Significant Internal Changes to Code from C2 0 1 to C3 0 app_star topo_star app_planet topo_planet virtual_star
127. piled by the IAU Working Group on Nomenclature for Fundamental Astronomy A very cursory overview of some of the most important aspects of the astronomical calculations performed by NOVAS follows In this chapter names of functions relevant to the subject being discussed are shown in brackets In the background material special terms referred to in IAU resolutions are printed in bold when first mentioned other relevant terms with widely accepted meanings in astronomy are initially printed in italics In descriptions of specific C code bold text will be used to refer to file names and italic text will be used to refer to function names Variable names or code snippets will be presented in atypewriter like font 1 1 Astronomical Coordinate Systems Astronomical coordinate systems have traditionally been based on the extension of the Earth s equatorial plane to infinity along with a fiducial direction in that plane the equinox The direction of the equinox is along the line of nodes where the equatorial and ecliptic planes meet Because the Earth s equator and ecliptic are both in motion the equator s motion is described by precession and nutation the ecliptic s motion is due to the Earth s orbital variations an infinite number of such coordinate systems exist each one corresponding to the orientation of the two planes at a specific date and time The situation is complicated by the fact that for some purposes in the past it was
128. r s position from its catalog coordinates except atmospheric refraction which is location and weather dependent the star s space motion to the date of interest parallax due to the Earth s position in its orbit gravitational light bending in the solar system aberration due to the Earth s orbital velocity and the precession and nutation of the Earth s axis Important note Hipparcos catalog data although expressed with respect to the ICRS refer to epoch 1991 25 and must be converted to epoch J2000 0 before being used as input arguments to any NOVAS function Use function transform_hip to do this Most other C 35 modern catalogs including the FK6 used above Tycho 2 UCAC etc provide data for epoch J2000 0 that need no conversion If we want the apparent topocentric place of the star that is the star s position as it would be seen except for refraction from a particular location on Earth such as off the Atlantic coast near Truro Massachusetts const double latitude 42 0 const double longitude 70 const double height 0 0 const double temperature 10 0 const double pressure 1010 0 where longitude and latitude are the location s geodetic longitude and latitude degrees with east longitude and north latitude positive and height is the height of the location above sea level meters The temperature and pressure in degrees Celsius and millibars respectively are only used in calculations invo
129. reasingly important space observations Furthermore the most precise fundamental measurement those that by their nature are most closely related to the equatorial systems are now obtained from Very Long Baseline Interferometry VLBI a radio technique that has no direct sensitivity to the equinox Another important consideration was the need for astronomical coordinate systems that were part of a general relativistic framework All of these factors were folded into some very important resolutions passed by the IAU in 1997 and 2000 which form the basis for the coordinate systems used in the current version of NOVAS These resolutions have introduced concepts and terminology that may not be familiar to astronomers accustomed to the traditional systems and names which were used in the previous versions of NOVAS The new systems are outlined in the following paragraphs but see USNO Circular 179 for a much more complete description One of the resolutions passed by the IAU in 2000 defined two systems of space time coordinates one for the solar system and the other for the near Earth environment within the framework of General Relativity by specifying the form of the metric tensors for each and the four dimensional space time transformation between them The former is called the Barycentric Celestial Reference System BCRS and the latter is called the Geocentric Celestial Reference System GCRS The BCRS is the system appropriate for the basic
130. required input data stored in the cel_object gt star argument are the standard five astrometric quantities from a catalog together with radial velocity if known Any parameter should be set to 0 0 if its value is unknown All catalog data used as input to this function must apply to epoch J2000 0 and be expressed with respect to the ICRS For Hipparcos catalog data see function transform_hip Extragalactic objects should be treated as stars but with all input parameters set to 0 0 except for the catalog right ascension and declination For solar system bodies the argument cel_object gt number must contain the identification number from the list of objects supported by the ephemeris in use The values of location gt where and coord_sys for various kinds of place are listed below Type of Place virtual place proper place local place apparent place topocentric place location gt where Type of Place al topocentric astrometric place Place name not widely recognized outside of NOVAS NOVAS functions app_star topo_star app_planet topo_planet etc are now just special purpose front ends to place Important The input value of AT delta_t is used only when location gt where 1 or 2 observer is on surface of Earth or in a near Earth satellite An error in AT of 1 s can result in a topocentric place error of up to 0 3 arcsecond for the Moon but proportionally less for more distant bodies e g 3 milliarcse
131. s in novas c and solsys1 c example code discussed in Chapter 3 requires a binary JPL ephemeris example usno txt USNO output from example application Finally an ASCII text file txt containing the right ascensions of the CIO in the GCRS is supplied along with C source code c to convert it to a binary direct access file The set up and use of this file is discussed in Section 2 5 CIO_RA TXT right ascensions of the CIO in the GCRS from 1700 to 2300 cio _file c main function that converts CIO_RA TXT ASCII to cio_ra bin binary 2 2 Installation and Basic Validation To install NOVAS and perform a basic validation of the code on your local system follow the instructions given below These instructions assume that you know how to compile and link C source code on your computer system Details of the process are dependent on your particular computer system In particular some C compilers require an explicit link to the math library which is typically accomplished using the 1m option in the command line see your compiler s documentation for details NOVAS has been successfully implemented on Microsoft Windows Apple Macintosh and Red Hat Enterprise Linux systems The most basic validation of NOVAS requires no external ephemeris files a Copy all NOVAS files to a directory on your local system b Compile and link files checkout stars c novas c novascon c nutation c solsys3 c and readephO c Name the resulting appli
132. s of NOVAS but the C edition also includes the 77 term AU approved truncated nutation series IAU 2000B iau2000b which is accurate to about milliarcsecond in the interval 1995 2050 McCarthy amp Luzum 2003 16 http www usno navy mil USNO earth orientation eo products long term V http maia usno navy mil ser7 finals data See http maia usno navy mil ser7 readme finals for file format C 21 More information on the implementation of nutation in NOVAS can be found section 2 6 section 4 3 and in USNO Circular 181 18 Nutation Series Evaluation in NOVAS 3 0 Kaplan 2009 As mentioned in section 1 1 the celestial pole described by the new precession and nutation models with very small observational corrections is called the Celestial Intermediate Pole CIP The true equator of date also called the instantaneous equator or the intermediate equator is the plane orthogonal to the direction of the CIP The CIP is also the rotational pole on the surface of the Earth see section 1 6 1 5 New Model for the Rotation of the Earth about its Axis IAU resolutions passed in 2000 also dealt in a very fundamental way with how the Earth s spin around its axis is described The conventional treatment is based on the equinox and sidereal time Greenwich or local sidereal time is just the Greenwich or local hour angle of the equinox of date However the equinox is constantly moving due to precession so that sidereal time combines tw
133. s of a solar system body solarsystem_hp Provides position and velocity vectors of a solar system for applications requiring the highest precision sun_eph Computes equatorial spherical coordinates of Sun referred to the mean equator and equinox of date K New function in NOVAS C3 0 Only present in solsys3 c 4 3 Important Functions Descriptions of some of the most frequently used NOVAS functions follow The prologs in the source code at the beginning of each NOVAS function are intended to provide enough information for correct usage They are reproduced here followed by additional discussion and recommendations C 47 place short int place double jd_tt object cel_object observer location double delta_t short int coord_sys short int accuracy sky_pos output PURPOSE This function computes the apparent direction of a star or solar system body at a specified time and in a specified coordinate system REFERENCES Kaplan G et al 1989 Astronomical Journal 97 1197 1210 Klioner S 2003 Astronomical Journal 125 1580 1597 INPUT ARGUMENTS jd_tt double TT Julian date for place cel_object struct object Specifies the celestial object of interest structure defined in novas h location struct observer Specifies the location of the observer structure defined in novas h delta_t double Difference TT UT1 at jd_tt in seconds of
134. s per the previous function pnsw but with a call to frame_tie added at the end to put final vector in GCRS This method uses apparent sidereal time obtained from sidereal_time CIO based method performs the transformation of equations 3 and 4 given by Kaplan 2003 based on the Celestial Intermediate Reference System The orthonormal basis of the system is obtained from cio_basis and the ERA 9 is obtained from era In either method the fast angle rotation about z axis is evaluated using the input UT1 epoch but other components of sidereal time are evaluated using TDB set equal to TT with TT UT1 AT where the delt a_t input parameter sets the AT value 7 http www usno navy mil USNO astronomical applications publications circ 179 C 112 transform_cat transform_hip and starvectors were modified to accommodate the new proper motion and parallax units transform_cat and starvectors now use sin parallax to compute distance rather than parallax an inconsequential change that just makes the expression formally correct Also the Doppler Factor k mentioned in the Hipparcos documentation and other papers is now applied in computing the space motion vector The computational distance used for objects of zero parallax has been increased to 1 Gpe 2 06 x 10 AU transform_cat calls frame_tie for new transformation option 4 and for the new transformation opt ion 5 which rotate data between dynamical J2000 0 system and
135. se the desired solar system objects are the Earth and Sun Thus an ephemeris of the barycentric Earth and the barycentric Sun is the minimum requirement NOVAS C3 0 and later access such data for solar system objects through function ephemeris As provided ephemeris supports access to a JPL ephemeris of the major solar system bodies here defined as Sun Moon eight planets and Pluto and provides direct support for access to the USNO Ephemerides of the Largest Asteroids Hilton 1999 hereafter USNO AE98 Function ephemeris accesses the JPL ephemerides by calling the appropriate version of solarsystem or solarsystem_hp If you need to compute star or radio source positions to better than 1 5 milliarcseconds positions of the Sun to better than 2 arcseconds or positions of solar system bodies other than the Sun you will have to use function solarsystem version 1 or version 2 which require external ephemerides instead of solarsystem version 3 which is self contained solarsystem version 1 along with functions in eph_manager c provides a complete C language interface between NOVAS and one of the JPL development ephemerides DEnnn such as DE405 The planet_ephemeris function is the C version of the Fortran subroutine PLEPH and it in turn calls other functions in the JPL ephemeris software package Alternatively solarsystem version 2 along with subroutines in jplint f provides an interface to the JPL ephemeris access code
136. ser7 mark3 out 14 http www iers org IERS EN Publications Bulletins bulletins html 15 http www usno navy mil USNO astronomical applications data services cal to jd conv C 20 The difference AT TT UT1 expressed in seconds is required for certain NOVAS functions that use both TT and UT internally but require only one type of input time argument A table of historical values of AT is on pages K9 K10 of The Astronomical Almanac and more recent values and predictions can be found online at EO web site i Values of AT can also be computed from AT 32 184s AAT UT1 UTC For example on 2010 December 1 AT 66 3054 s which is based on a AAT value of 34 s and a UT1 UTC value of 0 1214 s obtained from finals data and rounded to the nearest ten thousandth of a second More information on AT is given in section 1 6 below 1 4 Adopted Models for Precession and Nutation Astronomers realized over a decade ago that the old standard models for the precession and nutation were in need of revision The value of the angular rate of precession in longitude adopted by the IAU in 1976 and incorporated into the widely used precession formulation by Lieske and collaborators 1977 is too large by about 0 3 arcsecond per century 3 milli arcseconds per year The amplitudes of a number of the largest nutation components specified in the 1980 IAU Theory of Nutation are also known to be in error by several milliarcseconds Both the prece
137. sion is used If your code only interacts with the highest level NOVAS functions all this is transparent to you However if you use precession within your own code you should precede it by a call to frame_tie with the middle argument direction gt 0 if your input vector is expressed in the GCRS that is if it is derived from an input source based on the ICRS Output data from many of the supervisory level NOVAS functions can be expressed in the GCRS or either of two equator of date systems the true equator and equinox of date or the Celestial Intermediate Reference System The latter two systems differ only in their right ascension origins and in the new paradigm they are understood to be derived from the GCRS by applying a few rotations A 4 New Models for Precession and Nutation As described in section 1 4 new models for both precession and nutation have been adopted by the IAU and have been incorporated into NOVAS Although the underlying developments for these effects are different than in NOVAS 2 0 from a programming point of view little has changed The functions that directly involve precession and nutation precession nutation nutation_angles sidereal_time essentially work the same as before but with slightly different results The new nutation model has more than ten times the number of trigonometric terms than the previous model Because evaluation of nutation has always been the most computationally intensive task in NOVAS
138. specifying the relative accuracy of the output position 0 full accuracy 1 reduced accuracy OUTPUT ARGUMENTS ra double Local right ascension in hours referred to the local GCRS dec double Local declination in degrees referred to the local GCRS dis double True distance from Earth to the body in AU RETURNED VALUE short int 0 Everything OK 1 Invalid value of where in structure location gt 10 Error code from function place Discussion See the discussion for function topo_planet Function local_planet is identical to topo_planet in input arguments and use Here however the output arguments provide the local place of the star The local place is essentially the topocentric place expressed in the local GCRS C 71 Function ocal_planet works by calling place with location gt where 1 and coord_sys 0 Return to Function List astro_planet short int astro_planet double jd_tt object ss_body short int accuracy double ra double dec double dis As URPOSE Compute the astrometric place of a solar system body REFERENCES Kaplan G H et al 1989 Astron Journ 97 1197 1210 Explanatory Supplement to the Astronomical Almanac 1992 Chap 3 INPUT ARGUMENTS jd_tt double TT Julian date for astrometric place ss_body struct object Pointer to structure containing the bod
139. ssion and nutation errors are significant relative to current observational capabilities Thus the resolutions passed by the IAU in 2000 mandated an improvement to the precession and nutation formulations NOVAS 3 0 and later incorporate the models adopted in response to these resolutions precession nutation The precession model is the P03 solution of Capitaine Wallace and Chapront 2003 as recommended by the IAU Working Group on Precession and the Ecliptic Hilton et al 2006 The P03 precession model was formally adopted by the IAU in 2006 The nutation model is taken from Mathews Herring and Buffett 2002 This model referred to as the IAU 2000A nutation consists of 1 365 trigonometric terms more than ten times the number in the previous model nutation_angles iau2000a Because evaluation of nutation has always been the most computationally intensive task in NOVAS you may notice an increase in execution time for some applications To reduce execution time NOVAS 3 0 and later provide an optional reduced accuracy mode in which a truncated nutation series is used This nutation series is specific to NOVAS and is referred to as 2000K nu2000k It consists of the largest 488 terms in the IAU 2000A series and provides an accuracy of about 0 1 milliarcsecond specifically 0 1 milliarcsecond for Ay and about 0 04 milliarcsecond for Ae and Ay sin 2000K is the default reduced accuracy nutation series in both Fortran and C edition
140. star astro_planet and mean_star produce output positions in the ICRS app_star virtual_star astro_star app_planet virtual_planet astro_planet mean_star topo_star topo_planet local_star and local_planet have a new accuracy input parameter The former earth input parameter was removed from each of these functions aberration no longer returns an error code bary_to_geo was renamed bary2obs cel_pole can now accept input corrections to the pole positions as either dy de or dX dY the choice of correction is specified by a new input parameter t ype In either case the units of the correction must be in milliarcseconds In addition this function now returns an error code earthtilt was renamed e_tilt with anew accuracy input parameter ephemeris now returns an error code get_earth was eliminated and its functionality moved to place C 109 make_cat_entry changed its proper motion units in both RA and dec to milliarcseconds year and changed its parallax units to milliarcseconds Proper motion in RA includes a cos dec factor nutate was renamed nutation It has anew accuracy input parameter and it no longer returns an error code nutation_angles has anew accuracy input parameter and it no longer returns an error code pnsw was renamed fer2cel terrestrial to celestial transformation with the former input argument t jd changed to a UT1 Julian date split into a pair of double precision words jd_ut_high and jd_ut_low
141. t Type of pole offset 1 for corrections to angular coordinates of modeled pole referred to mean ecliptic of date that is delta delta psi and delta delta epsilon 2 for corrections to components of modeled pole unit vector referred to GCRS axes that is dx and dy dpolel double Value of celestial pole offset in first coordinate delta delta psi or dx in milliarcseconds dpole2 double Value of celestial pole offset in second coordinate delta delta epsilon or dy in milliarcseconds OUTPUT ARGUMENTS None RETURNED VALUE short int 0 Everything OK 1 Invalid value of type Discussion This function allows for the specification of celestial pole offsets for high precision applications The offsets describe the observed position of the Celestial Intermediate Pole CIP with respect to the position computed from the standard precession and nutation models The offsets are subsequently applied as corrections to the nutation in longitude and nutation in obliquity within e_tilt Thus e_ti t output arguments tob1l ee dpsi and deps will be affected Because other NOVAS functions such as sidereal_time call e_tilt to obtain data related to the Earth s orientation in space the celestial pole offsets specified here are propagated through the data that the various NOVAS functions provide C 80 Daily values of the celestial pole offsets are published for example in IERS Bulle
142. t dat r while feof In_Data fscanf In_Data ShislfSlfSlfsl FSlFSlFSl Enum amp tjdh amp tjdl amp xp amp yp amp vecl 0 amp vecl 1 amp vecl 2 Set transformation method accuracy level and utl utc tijd tjdh tjdl celpol tjd 2 dx dy e_tilt tjd 0 amp mobl amp tobl amp ee amp dx amp dy Rotate vecl from ITRS to GCRS vec2 ter2cel tjdh tjdl delt 1 0 0 xp yp vecl vec2 printf Si 20 17 20 17 m 20 17 n num vec2 0 vec2 1 vec2 2 fclose In_Data Return to Table of Contents C 124
143. tar Structure on_surface type of object 0 major planet Pluto Sun or Moon 1 minor planet 2 object located outside the solar system star nebula galaxy etc object number For type 0 Mercury 1 Pluto 9 Sun 10 Moon 11 For type 1 minor planet number For type 2 set to 0 object is fully specified in name of the object 1 SIZE _OF_OBJ_NAME T basic astrometric dat located outside the s data for a star struct cat_entry imited to characters a for any celestial object olar system the catalog Structure on_surface contains data for the observer s location on the surface of the Earth The atmospheric parameters temperature and pressure are used only by the refraction function refract called from function equ2hor when ref_option 2 dummy values can be used otherwise Additional parameters can be added to this structure if a more sophisticated refraction model is employed typedef struct double double double latitude longitude height double temperature double pressure on_surface where latitude longitude height temperature pressure geodetic geodetic height of th ITRS latitude ITRS longitude observer north positive east positive meters degrees degrees temperature degrees atmospheric pressure C 42 Celsius millibars Structure in_space Structure in_space contains data for
144. tatic geometry but by its motion so its position at any time is given by the result of an integral that has been evaluated either analytically or numerically In NOVAS both results are available the position of the CIO can be taken from an external file that is the output of a numerical integration or it can be obtained from an analytical expression for the equation of the origins cio_location See sections 6 5 1 1 and 6 5 1 2 of USNO Circular 179 The NOVAS implementation of the CIO based Earth rotation paradigm for a given date is based on the construction of the Celestial Intermediate Reference System for that date using vectors toward the CIP and the CIO These two directions define respectively the z axis and x axis of the celestial intermediate system The direction toward the CIP in the GCRS can be computed by passing the vector 0 0 1 through functions nutation precession and frame_tie in succession Given the direction of the CIP the other piece of required information is the location of the CIO for the same date which is provided by cio_location described below The basis vectors of the intermediate system with respect to the GCRS are computed by cio_basis see section 6 5 1 of USNO Circular 179 for the algorithms Having these basis vectors available allows NOVAS to easily transform any vector in the GCRS to the intermediate system The only other quantity used in the CIO based paradigm is the ERA which is trivial to compute an
145. te code provides more flexibility and allows for possible future additions such as binary star orbits or nonlinear terms in proper motion place can produce star or planet positions within the Celestial Intermediate Reference System that is part of the new paradigm for Earth rotation calculations see below place provides its output position both in spherical coordinates right ascension declination and for solar system bodies geometric distance and as a unit vector In addition place furnishes radial velocity It also allows the observer to be located at the geocenter on or near the Earth s surface or in a near Earth spacecraft You may want to consider changing your calls to app_star app_planet etc to the equivalent calls to place the code for the new versions of the traditional place functions specifies the appropriate input parameters A 3 New Reference Systems The IAU resolutions of 2000 defined several new reference systems for fundamental astronomy The Barycentric Celestial Reference System BRCS Geocentric Celestial Reference System GCRS International Celestial Reference System ICRS and Celestial Intermediate Reference System are defined briefly below More detailed descriptions of these systems are in section 1 1 of this document and in Chapters 1 3 and 6 of USNO Circular 179 Barycentric Celestial Reference System BCRS replaces the barycentric system based on the mean equator and equinox of J2000 0 It is use
146. tems The library also provides access to all of the building blocks that go into such computations single purpose functions for common astrometric algorithms such as those for precession nutation aberration parallax etc NOVAS calculations are accurate at the sub milliarcsecond level The package is an easy to use facility that can be incorporated into data reduction programs telescope control systems and simulations The United States U S Nautical Almanac Office uses NOVAS in the production of its sections of The Astronomical Almanac The NOVAS algorithms are based on a vector and matrix formulation that is rigorous and consistent with recent recommendations of the International Astronomical Union IAU Objects inside and outside the solar system are treated similarly The position vectors formed and operated on by the NOVAS functions are defined within either the Barycentric Celestial Reference System BCRS or the Geocentric Celestial Reference System GCRS as appropriate Both of these systems are described in IAU resolutions passed in 2000 GCRS quantities are converted to more familiar coordinate systems such as the equator and equinox of date by applying standard rotations Three levels of functions are involved basic utility and supervisory Basic level functions supply the values of fundamental variables such as the nutation angles and the heliocentric positions of solar system bodies for specific epochs Utility lev
147. tes results in a maximum error of only about 3 x 10 arcseconds which is totally negligible Standard epoch J2000 0 although C 74 formally defined in the TT time scale is the same in the TT and TDB time scales to the precision given by double precision Julian dates at J2000 0 TT TDB 10 second 10 day Return to Function List C 75 equ2ecl short int equ2ecl double jd_tt short int coord_sys short int accuracy double ra double dec double elon double elat PURPOSE To convert right ascension and declination to ecliptic longitude and latitude REFERENCES one INPUT ARGUMENTS jd_tt double TT Julian date of equator equinox and ecliptic used for coordinates coord_sys short int Coordinate system selection 0 mean equator and equinox of date jd_tt 1 true equator and equinox of date jd_tt 9 2 ss TERS ecliptic is always the mean plane accuracy short int Selection for accuracy 0 full accuracy 1 reduced accuracy ra double Right ascension in hours referred to specified equator and equinox of date dec double Declination in degrees referred to specified equator and equinox of date OUTPUT ARGUMENTS x elon double Ecliptic longitude in degrees referred to specified ecliptic and equinox of date xelat double Ecliptic latitude in degrees referred to specified ecliptic and equinox of date
148. the GCRS dec double Virtual declination in degrees referred to the GCRS RETURNED VALUE short int 0 Everything OK gt 10 Error code from function make_object gt 20 Error code from function place Discussion See the discussion for function app_star Function virtual_star is identical to app_star in input arguments and use Here however the output arguments provide the virtual place also called the proper place of the star The virtual place proper place is essentially the apparent place expressed in the GCRS Function virtual_star works by calling place with location gt where 0 and coord_sys 0 Return to Function List C 64 local_star short int local_star double jd_tt double delta_t cat_entry star on_surface position short int accuracy double ra double dec PURPOSE Computes the local place of a star at date jd_tt given its catalog mean place proper motion parallax and radial velocity REFERENCES Kaplan G H et al 1989 Astron Journ 97 1197 1210 Explanatory Supplement to the Astronomical Almanac 1992 Ch 3 INPUT ARGUMENTS jd_tt double TT Julian date for local place delta_t double Difference TT UT1 at jd_tt in seconds of time star struct cat_entry Pointer to catalog entry structure containing catalog data for the object in the ICRS defined in novas h position struct on_surface Specifies
149. the appropriate pole coordinates for the date of interest The output coordinates zd az rar and decr are respectively the zenith distance degrees azimuth degrees right ascension hours and declination degrees The output values of zd rar and decr are affected by atmospheric refraction for refraction options 1 and 2 If ref_option equals 0 the output right ascension and declination values are the same as the input values zd and az are referred to the horizon C 37 system that is tangent to the Earth s reference ellipsoid at the observer s location that is the deflection of the vertical the local undulation of the geoid is not taken into account 3 5 Example 3 Greenwich Sidereal Time To obtain Greenwich sidereal time call sidereal_time rror sidereal_time jd_ut1 0 0 delta_t 1 1 accuracy gast where short integer gst_t ype and short integer met hod have each been set to one 1 to compute Greenwich apparent sidereal time using the equinox based method In this example the output sidereal time gast is Greenwich apparent sidereal time in hours If gst_type had been set to zero 0 the output sidereal time would have been Greenwich mean sidereal time in hours Choosing between equinox based and CIO based methods is discussed in section 3 1 and section 5 2 sidereal_time and several other functions allow for a split input UT1 Julian date high and low order parts in the first two arguments for increased
150. the output coordinates in t_place a structure of type sky_pos will be based on the true equator and equinox of date typedef struct double r_hat 3 double ra double dec double dis double rv sky_pos Here double r_hat 3 is a dimensionless unit vector in the apparent direction of the Moon in the same coordinate system as the right ascension and declination i e it is exactly equivalent to the spherical coordinates place has many options for both input and output refer to its description in Chapter 4 or look at its prolog Once you have the topocentric celestial coordinates of an object these can be transformed into local altitude and azimuth by a call to equ2hor If we have used place to obtain the topocentric coordinates then equ2hor jd_utl delta_t accuracy 0 0 0 0 amp geo_loc rat dect 1 amp zd amp az amp rar amp decr where the refraction option short integer ref_option is set here to one 1 and the x and y coordinates of the CIO with respect to the ITRS pole double xp and double yp have been set to zero 0 0 The refraction option selected here is for standard atmospheric conditions The other options for refraction are zero 0 for no refraction or two 2 for refraction based on the atmospheric conditions indicated by location a structure of type on_surface Setting the CIO coordinates to zero is appropriate when sub arcsecond accuracy is unnecessary otherwise these arguments should contain
151. these last two terms were suggested in the Kaplan et al 1989 paper previously cited they do not appear to have been widely used outside of the context of NOVAS The above terminology is reflected in the names of the high level functions that perform the computations where app stands for apparent place topo stands for topocentric place etc app_star app_planet topo_star topo_planet virtual_star virtual_planet local_star local_planet In addition an astrometric place calculation can be used for some differential measurements it is the same as virtual place except that light bending and aberration and refraction are not computed under the assumption that these effects are the same for all objects within a small field of view astro_star astro_planet In response to the introduction of the new AU recommended coordinate systems we must make some adjustments and additions to the nomenclature The mean equator and equinox of J2000 0 considered as a geocentric system has been replaced by the GCRS The IAU Working Group on Nomenclature 2003 2006 recommended that the term proper place be used for what is called virtual place in NOVAS With the introduction of the Celestial Intermediate Reference System with its right ascension origin at the CIO we now have two more possibilities for apparent positions one geocentric and one topocentric The geocentric coordinates are called the object s intermediate plac
152. tins A and B The celestial pole offsets effectively correct for errors or incompleteness in the standard precession or nutation models If you use cel_pole make sure it is called before any other functions for a given date Values of the pole offsets that you specify by a call to cel_pole will be used by e_tilt until you explicitly change them Important For compatibility with the precession and nutation models used in NOVAS 3 0 and later specify type 2 and use only IERS dX and dY values with respect to IAU 2000A sometimes labeled IAU 2000 These pole offset values will generally not exceed 0 5 milliarcsecond and therefore cel_pole would need to be called only when very high accuracy is required Return to Function List http www iers org IERS EN Publications Bulletins bulletins html C 81 e_tilt void e_tilt double jd_tdb short int accuracy double mobl double tobl double ee double dpsi double deps PURPOSE Computes quantities related to the orientation of the Earth s rotation axis at Julian date jd_tdb REFERENCES one INPUT ARGUMENTS jd_tdb double TDB Julian Date accuracy short int Selection for accuracy 0 full accuracy 1 reduced accuracy OUTPUT ARGUMENTS mobl double Mean obliquity of the ecliptic in degrees at jd_tdb tobl double True obliquity of the ecliptic in degrees at jd_tdb ee double Equation of th quinoxes
153. tion 2 or 3 2 length of newcat_id out of bounds Discussion Function transform_cat performs mean place to mean place transformations on star catalog data Only catalog reference data not observed quantities should be processed by this function C 57 For option 1 2 or 3 two dates date_incat and date_newcat must be specified the input data is associated with the first date and the output data is associated with the second date Two transformations are available option 1 The star s data is updated to account for the star s space motion between the first and second dates within a fixed reference system That is the epoch of the data is changed but not the equator and equinox or other system option 2 The reference frame within which the star s coordinates and proper motion are expressed is rotated corresponding to precession between the first and second dates The star s position in space is not changed That is the equator and equinox of the data are changed but not the epoch Setting option 3 requests both transformations and is the most common case The two date arguments date_incat and date_newcat may be specified either as a Julian date e g 2433282 5 ora Julian year and fraction e g 1950 0 Values less than 10000 0 are assumed to represent years The option 1 and option 3 transformations are appropriate only for objects with linear or no space motion do not use them for co
154. tly Requested Quantities C 38 Chapter 4 Data Structures and Functions C 41 4 1 Important Data Structures C 41 Structure cat_entry C 41 Structure object C 42 Structure on_surface C 42 Structure in_space C 43 C 3 Structure observer C 43 Structure sky_pos C 43 Structure ra_of_cio C 43 4 2 Function List table C 44 4 3 Important Functions in NOVAS C 47 place C 48 sidereal_time C 51 ter2cel C 53 cel2ter C 54 equ2hor C 55 transform_cat C 57 transform_hip C 59 app_star C 61 topo_star C 62 virtual_star C 64 local_star C 65 astro_star C 66 app_planet C 67 topo_planet C 68 virtual_planet C 70 local_planet C 71 astro_planet C 73 precession C 74 equ2ecl C 76 cio_ra C 78 era C 79 cel_pole C 80 e_tilt C 82 ephemeris C 83 solarsystem C 85 solarsystem Version 1 C 87 solarsystem Version 2 C 88 solarsystem Version 3 C 90 solarsystem_hp C 91 solarsystem_hp Version 1 C 92 solarsystem_hp Version 2 C 93 solarsystem_hp Version 3 C 95 Nutation Models C 96 Chapter 5 Equinox and CIO Based Paradigms Compared C 97 JA Computing Hour Angles C 97 5 2 Other Computational Considerations C 98 ee How NOVAS Implements the CIO Based Paradigm C 99 5 4 References C 100 C 4 Appendix A A l A 2 A 3 A 4 A 5 A 6 A 7 A 8 Appendix B B 1 B 2 B 3 B 4 B 5 Appendix C C 1 C2 Appendix D D 1 D 2 D 3 D 4 D 5 Overview of How NOVAS Has Changed Important Changes in Calls place A New General
155. ts ICRS RA and dec to galactic longitude and latitude Returns ERA 0 for given UT1 Julian date Transforms vector between dynamical reference system mean equator and equinox of J2000 0 and ICRS Converts GCRS RA and dec to coordinates with respect to the equator of date mean or true Computes geocentric position and velocity in GCRS of observer on or near the surface of Earth Computes total gravitational deflection of light for an object due to major solar system bodies Corrects position vector for deflection of light in gravitational field of given body light_time Computes position of a solar system body antedated for light time limb_angle Determines angle of object above or below Earth s limb horizon make_cat_entry Creates a structure of type cat_ent ry containing catalog data for a star or star like object make_object Creates structure of type ob ject specifying a celestial object based on the input parameters make_observer Creates structure of type observer specifying the location of the observer make_observer_at_geocenter Creates structure of type observer specifying that the observer is at the geocenter make_observer_in_space Creates structure of type observer specifying the position and velocity of an observer situated on a near Earth spacecraft make_observer_on_surface Creates structure of type observer spec
156. ty is the equation of the origins the direction of the true equinox measured in the equator eastward from the CIO Because the equinox and CIO are simply different right ascension origins on the instantaneous equator cio_ra provides the angular difference between the origins of these two systems The equation of the origins is also the difference expressed as an angle between the ERA and Greenwich apparent sidereal time 5 3 How NOVAS Implements the CIO Based Paradigm The equinox based paradigm is of course the historical basis for NOVAS One of its key pieces is the precession algorithm precession which uses the equinox as its azimuthal coordinate that is it transforms celestial coordinates from the mean equator and equinox of one date to the mean equator and equinox of another date Even though the recommended precession formulation has been replaced twice over the last half century this aspect of it has remained unchanged Another key piece is the algorithm for sidereal time sidereal_time which is based on a sidereal day that is defined by successive transits of the equinox The sidereal time formula must always be matched to the precession algorithm because mean sidereal time must account for the precession of the equinox in right ascension this has been consistently done in NOVAS To use the CIO based paradigm we must know where the CIO is in some well defined coordinate system Unlike the equinox the CIO is not defined by s
157. ulian date tes the Greenwich sidereal time at 4a _high 43d_low ither mean or apparent ERENCES Kaplan G 2005 US Naval Observatory Circular 179 UT UMENTS jd_high double High order part of UT1 Julian date jd_low double Low order part of UT1 Julian date delta_t double Difference TTI UT1 at of time gst_type short int 0 compute Greenwich mean sidereal tim 1 compute Greenwich apparent sidereal tim method short int Selection for method 0 CIO based method 1 quinox based method accuracy short int Selection for accuracy 0 full accuracy 1 reduced accuracy jJd_high jd_low in seconds OUTPUT ARGUMENTS gst double Greenwich mean or apparent sidereal time in hours RETURNE VALUE short int 0 1 2 10 D everything OK invalid value of accuracy invalid value of method 10 error from function v lt 30 NCLo vrar Discussion This function computes Greenwich sidereal time either mean if gst_type apparent if gst_type ass The input ensure the Julian date which must be in the UTI time scale may be split into two parts to highest precision in the computation For example set jd_high equal to the integral part of the Julian date and set jd_low equal to the fractional part Generally this C 51 split will be advantageous only if the low order part has been treated separately within the
158. upplied by JPL The JPL software must be tailored for your specific computer architecture This appendix describes how to set up the binary ephemeris file and the JPL software that reads it on your system After these tasks have been done successfully the JPL software must be linked into any NOVAS application that uses solarsystem version 2 Alternatively the binary ephemeris file can be used directly with solarsystem version I and the software in file eph_manager c The procedures outlined below worked on an Intel based Mac OS X system using the open source gfortran compiler and the JPL software available in February 2009 The resulting binary direct access ephemeris file was successfully transferred to and used on other Intel based computers running Microsoft Windows XP and Linux Thus our procedures are tailored for computers containing Intel processors including many systems running Microsoft Windows Mac OS X and Linux Providing specific procedures for all combinations of computer processors operating systems and compilers is beyond the scope of this user s guide Furthermore the procedures in this appendix are intended simply as a guide the USNO cannot provide technical support regarding JPL software C 2 Step by Step Guide Step 1 Connect to the JPL ftp site All the files needed to install the JPL ephemerides are available via anonymous ftp from ssd jpl1 nasa gov This can be accomplished through most modern Web browsers by typing
159. us Neptune Pluto Moon Implementation Notes In order to use NOVAS with solarsystem_hp version 2 you must first obtain the planetary ephemeris export package from JPL see Appendix C for details If the verification process is successful the ephemeris file is ready to use The ephemeris data is obtained from the binary file by calling the access subroutines provided in the JPL export package Version 2 of solarsystem_hp obtains ephemeris data from the binary file by calling Fortran subroutine jplihp_ which is in the NOVAS jplint f file Subroutine jplihp_ in turn calls JPL subroutine DPLEPH Fortran code and all other supporting Fortran subroutines The C function solarsystem_hp has a few features that make it possible for it to exchange data with the Fortran subroutine jplihp_ First all of the C arguments of the call to jplihp_ are C 93 addresses because Fortran uses call by address instead of call by value for arguments of subroutines Second all of the integer arguments in the call are designated as type Long INTEGER default The DOUBLE int in the C function to match the Fortran 1 PR ECI S arguments in the subroutine are designated as type double in the C function ON Probably the biggest hurdle in implementing version 2 of solarsystem_hp involves the proper compiling and linking of files containing different languages The procedures will be specific to your computing platform there
160. usage The ephemeris manager code eph_manager c and eph_manager h contains many minor internal updates including support for JPL s DE421 B 5 References Capitaine N Wallace P T amp Chapront J 2003 A amp A 412 567 P03 Capitaine N Wallace P T amp McCarthy D D 2003 A amp A 406 1135 Hilton J L amp Hohenkerk C Y 2004 A amp A 413 765 Kaplan G H 2003 in Proceeding of IAU Gen Assembly XXV Joint Discussion 16 The International Celestial Reference System Maintenance and Future Realizations ed R Guame D McCarthy amp J Souchay Sydney The Assembly 196 Kaplan G H 2005 The IAU Resolutions on Astronomical Reference Systems Time Scales and Earth Rotation Models USNO Circular 179 Washington DC USNO http www usno navy mil USNO astronomical applications publications circ 179 USNO Circular 179 Lieske J H Lederle T Fricke W amp Morando B 1977 A amp A 1 C 114 Lieske J H 1979 A amp A 282 Petit G amp Luzum B eds 2010 Chapter 5 in JERS Conventions IERS Tech Not 36 Frankfurt IERS Table 5 2e presented in the printed publication is a truncated series The full series which is used in NOVAS is available on the IERS Conventions Center website in file tab5 2e txt ftp tai bipm org iers conv2010 chapterS Simon J L Bretagnon P Chapront J Chapront Touze M Francou G amp Laskar J 1994 A amp A 282 663 Standish E M 1998
161. ut mp uses the full accuracy mode of NOVAS If you do not require accuracy better than about 1 milliarcsecond you may wish to consider the reduced accuracy mode discussed in Section 2 6 2 5 Using an External CIO File You have the option of using an external file of CIO right ascension values on the GCRS or of allowing NOVAS to calculate the true right ascension of the CIO the arc on the instantaneous equator from the equinox to the CIO using a series expansion Section 5 3 explains how NOVAS handles these two situations If you choose to use CIO_RA TXT which is provided with NOVAS you must first convert it to a binary direct access file as follows a Copy CIO_RA TXT and cio_file c to a directory on your local system b Compile cio_file c c Name the resulting application cio_file d Run the cio_file application If everything runs smoothly you should get the following message Results from program cio_file Input file identifier CIO RA P03 1 200d 182657 records read from the input file First Julian date 2341951 400000 Last Julian date 2561138 600000 Data interval 1 200000 days First data point 2341951 400000 1 948328 Last data point 2561138 600000 1 942125 Binary file cio_ra bin created e Verify the presence of cio_ra bin which should contain approximately 2 9 Mbytes The file CIO_RA TXT as supplied contains six centuries of data most of which is seldom used so trimming it down can reduce the file s
162. utation of the angular coordinates of solar system objects takes light time into account along with the other effects light bending aberration precession and nutation that also apply to stars To get the topocentric celestial coordinates of the Moon call topo_planet error topo_planet jd_tt amp moon delta_t amp geo_loc accuracy rat amp dect amp dist However a single function place can be used for all types of positions of both stars and solar system objects In fact app_star topo_star app_planet and topo_planet along with C 36 several other similar functions are actually just special purpose front ends to place place uses three structure arguments cel_ob ject of type object and location of type observer for input and output of type sky_pos for output as well as several scalar arguments The cel_ob ject of type object is same as was created earlier for use by app_planet and topo_planet in calculating the position of the Moon Function make_observer_on_surface creates a structure of type observer for an observer on or near the surface of the Earth make_observer_on_surface latitude longitude height temperature pressure amp o0bs_loc The call to place to obtain topocentric coordinates of the Moon with respect to the true equator and equinox of date then is error place jd_tt moon amp 0bs_loc delta_t 1 accuracy amp t_place where short integer coord_sys has been set to one 1 so that
163. which is available in Fortran Subroutine jplint contains a single call to JPL s Fortran subroutine PLEPH which in turn calls other Fortran subroutines in the JPL ephemeris software package Establishing a working copy of the JPL software and the DEnnn binary files on your system is not unfortunately a trivial process The files for doing that can be obtained directly from JPL as discussed in Appendix C Once you have generated a DEnnn binary file you can test the combined set up with solarsystem version on your system as follows a Compile and link files checkout stars full c novas c novascon c nutation c solsysl1 c eph_manager c and readeph0 c b Name the resulting application checkout stars full c Run the checkout stars full application http www usno navy mil USNO astronomical applications software products usnoae98 http ssd jpl nasa gov planet_eph_export C 28 d Compare the results that you get from checkout stars full with the data in file checkout stars full usno txt If the results agree the installation was probably successful but see the important note above The USNO output file checkout stars full usno txt was generated using DE405 The application check out stars full uses the full accuracy mode of NOVAS which includes the IAU 2000A nutation model a three body gravitational deflection model two part Julian dates in calls to function ephemeris and the full series when computing t
164. ww usno navy mil USNO astronomical applications publications circ 181 C 32 Chapter 3 Sample Calculations The sample C code discussed in this chapter can be found in the file example c which is distributed along with NOVAS it is a main function that can be linked to the NOVAS modules and executed It requires solarsystem version 1 a working copy of the JPL software and a DEnnn binary file To use it a compile and link files example c novas c novascon c nutation c solsys1 c eph_manager c and readeph0 c name the resulting application example c verify that the JPL ephemeris file JPLEPH or an alias to it is available in the same directory as example d execute the example application The results are given in the file example usno txt which was generated at the USNO with the JPL DE405 ephemeris example c may be modified for use with solarsystem version 2 see the comments within that file for details NOVAS has a number of high level functions that make obtaining frequently needed information on the positions of celestial objects easy some of these are described below In addition Chapter 4 describes many of the functions and all of the structures used in these examples The checkout programs used to validate a local installation checkout stars c checkout stars full c and checkout mp c also provide other examples of NOVAS function calls Note that all floating point arguments to NOVAS functions input or output are
165. y designation for the solar system body defined in novas h accuracy short int Code specifying the relative accuracy of the output position 0 full accuracy 1 reduced accuracy OUTPUT ARGUMENTS ra double Astrometric right ascension in hours referred to the ICRS without light deflection or aberration dec double Astrometric declination in degrees referred to the ICRS without light deflection or aberration dis double True distance from Earth to the body in AU RETURNED VALUE short int 0 Everything OK 1 Invalid value of type in structure ss_body gt 10 Error code from function place Discussion See the discussion for function app_planet Function astro_planet is identical to app_planet in input arguments and use Here however the output arguments provide the astrometric place of the planet in the ICRS Function astro_planet works by calling place with location gt where 0 and coord_sys 3 Return to Function List C 73 precession short int precession double jd_tdb1 double posl double jd_tdb2 double pos2 PURPOSE Precesses equatorial rectangular coordinates from one epoch to another One of the two epochs must be J2000 0 The coordinates are referred to the mean dynamical equator and equinox of the two respective epochs REFERENCES Explanatory Supplement To The Astronomi
166. ystem ITRS The arguments are as described above except that for cel2ter vec1 is the vector with respect to the GCRS in and vec2 is the vector with respect to the ITRS out Return to Function List http www usno navy mil USNO earth orientation eo products long term C 54 voide equ2hor qu2hor double jd_utl double delta_t short int accuracy double xp double yp on_surface location double ra double dec short int ref_option double zd double az double rar double decr PURPOSE This function transforms topocentric right ascension and declination to zenith distance and azimuth It uses a method that properly accounts for polar motion which is significant at the sub arcsecond level This function can also adjust coordinates for atmospheric refraction REF ERENCES INP Kaplan G 2008 USNO AA Technical Note of 28 Apr 2008 Refraction as a Vector UT ARGUMENTS jd_ut1 double UT1 Julian date delta_t double Difference TT UT1 at jd_utl in seconds accuracy short int Selection for method and accuracy 0 full accuracy 1 reduced accuracy xp double Conventionally defined x coordinate of celestial intermediate pole with respect to ITRS reference pole in arcseconds yp double Conventionally defined y coordinate of celestial intermediate pole with respect to ITRS reference pole in arcseconds location struct on_surface P

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