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SKYCALC USER`S MANUAL John Thorstensen, Dept. Physics and

1. and only its phase is printed If both the moon and the object are above the horizon and the sun is more than 9 degrees below the horizon an estimated value of the moon s contribution to the night sky brightness is given this is obviously only approximate and only holds under ideal conditions For comparison a dark site has about 21 5 V magnitude per square arcsec but this varies considerably If the sun is at a geometric altitude gt 18 but below the horizon twilight is reported The zenith distance at which the sun s upper limb reaches the horizon is taken to be 90 83 if the obser vatory elevation is zero the extra 0 83 account approximately for the refraction and average angular semidiameter of the sun A further correction appropriate to a sea level horizon is added if the site s elevation is nonzero In twilight an estimate of the brightness of twilight at the zenith is reported these numbers appear to match rather well the behavior of twilight in blue light In visual or red the enhancement due to twilight may be rather fainter If the sun s upper limb is above the horizon it is reported to be up In twilight or daylight the RA dec altitude and azimuth of the sun are given This altitude is not corrected for refraction If the sun s zenith distance is greater than 108 or more than roughly eighteen degrees below the horizon it is reported to be down Another feature checks to see if the position y
2. Note here that the default epoch for input coordinates is 2000 you can change the input epoch by typing say e 1950 As the example shows times right ascensions and declinations are generally entered as triplets of numbers separated by whitespace characters blanks tabs or newlines Colons will also work as 7 delimiters so you can cut and paste from other output When you use blanks to delimit the fields the leading parts hours degrees minutes of right ascensions and declinations can have a fractional part for instance RA 19 15 00 could be entered as r 19 25 and you don t have to enter the trailing zeros provided your next character is a valid command such as and there is at least one blank following your number The fractional hour and minute feature won t work with colon separated input but you can leave off the seconds 18 23 will be properly interpreted but 18 23 5 will be read incorrectly as 18 23 05 To enter a negative declination just make the first number negative as in d 0 18 30 0 is correctly handled to give a negative declination but there cannot be any space between the minus sign and the number following The h command which generates the hourly airmass table first prompts you for the Name of object The reason for this is that you may wish to redirect output from the program discussed later to make a hard copy the name then serves to label the output The name serves only as a la
3. The times of moonrise and moonset are given provided they occur at night or within a boundary on either side The algorithm used here can give some trouble if the site is very far from the center of its assumed time zone Ellipses are printed if the rise or set does not occur within the specified interval Note that rise and set times though they occur in successive columns do not always occur successively in time depending on the moon s phase Rise set times are again for 50 arcminutes below the horizon variations in the moon s semidiameter are ignored The lunar ephemerides are based on accurate formulae from Jean Meeus Astronomical Formulae for Calculators The next column gives the percentage of the moon s face which is illuminated New first quarter full and last quarter correspond approximately to values of 0 50 100 and 50 respectively for this quantity If 0 is the angle subtended by the sun and the moon at the observer the quantity tabulated is 1 Illuminated percentage 100 x zQ cos 6 Finally the last two columns give the RA and dec of the moon at local midnight whether or not the moon is up at that time The position is topocentric It is very useful to know the moon s position if you re trying to work around it Times in the Calendar program Note that the rise set and twilight times given in the calendar are for the local time zone The sidereal times are of course strictly local and have not
4. To include a new site on the menu modify the routine load_site Follow the examples there You can check that they are entered correctly by examining output from the 1 option in the program 33 Note carefully that the program expects longitude as west longitude in units of hours minutes and seconds where 1 h 15 degrees If your site uses an unsupported daylight savings time algorithm include your option in the routine find_dst_bounds using the current routine as a model Note that in the labels for dst conventions positive numbers refer to northern sites and negative to southern sites for which the logic has to be reversed The code at this point is packaged as a single enormous source file of over 7000 lines Function declarations are mostly in the old K amp R style I also have a rather older version in which the code is sliced into eight cross referenced pieces with ANSI style function declarations If you need this say to compile on a PC send me email The older version is without many of the newer bells and whistles Previous versions of the code included some widely published functions for sorting and JD conversion for which copyright was claimed by others In the 2000 version these have been replaced with non proprietary functions the JD conversions are an original coding of a Jean Meeus algorithm and the sort routine is coded from a heapsort algorithm published by Ken Iverson with the Association for Computing Ma
5. a warning is printed If you specify a time during the hour which is skipped when daylight savings time begins the computation is aborted and you are asked to specify a time 1 hour later If you specify a time during the double valued period when the time drops back the time defaults to standard and a rather sharper warning is printed This makes one hour of real time inaccessible unless you switch to greenwich time input with the g option and force the input time The routine to turn JD into calendar date is adapted from Astronomical Formulae for Calculators Third Edition 1985 Willman Bell Richmond The routine to generate the JD from the date and time was adapted from a routine originally based on a recipe in the old American Ephemeris Sun and Moon The lunar positions used are computed from Meeus Astronomical Formulae book op cit The routine corrects the time argument to approximate TDT because the moon moves quickly enough to make these small timing differences significant Spot checks against the Astronomical Almanac indicate that the routine generates geocentric lunar positions good to better than a few seconds of time in RA and a fraction of a minute of arc in declination A topocentric correction from geocentric to observatory centered which can be 1 is included based on an ellipsoidal earth and the true elevation of the observatory The topocentric correction appears to be somewhat more accurate than the lunar theo
6. Fitz of NOAO for chasing this down However the Open Software Foundation s gcc compiles this correctly I d urge users of Sun machines to acquire this compiler It s not always appropriate for the user to have write permission Accordingly it s possible to recompile the program with such permissions turned off to do this find the preprocesser definition define LOG_FILES_OK 1 and change the 1 toa 0 I encourage programmers to borrow from this code and to modify it for their purposes especially by adding sites However it s probably not a good idea to attempt major surgery on the code itself unless you study it for a while There are some subtle issues involved which took me a while to get right But most minor changes can be done safely and rather trivially If you find that the routines which depend on the system clock don t work you can turn them off by finding the line define SYS_CLOCK_OK 1 and changing the 1 to any other number I ve run into one curious issue regarding system clocks At Lick Observatory and perhaps others there is a tradition of maintaining standard time all year for scientific purposes even though civil timekeeping uses daylight savings time It s tempting to implement this simply by turning off daylight savings but then the T option doesn t work for half the year because the computer s clock is generally set to civil time I haven t yet coded in the complications needed to patch this
7. any real bugs but the program can behave in a peculiar fashion given some inputs If you depart from the specified input formats you can get peculiar behavior Some error checking is done and some prompting is given in some cases where really unexpected input is found but these routines are less than perfect It is difficult to crash the program but it can still be forced into an infinite loop in which it asks for a valid date at this point you ll just have to crash it and start over If you can document the inputs which cause this behavior I d appreciate if you would drop me a line so I can fix the problem The specification of times and dates is a little ungraceful see the discussions of the g and n options above the night date option patches one potentially confusing condition with another To some extent these confusions are inevitable astronomers are forced to work at night when dates change to suit the convenience of everyone else The definition of JD with its infernal half step difference against UT dates is a historical example of an ill considered attempt to get around these difficulties In former versions the day and date could in principle disagree within a very close tolerance of midnight I believe I have eliminated the possibility of this by using the same truncation of the Julian date to derive the day and the date The conversion from local to UT is tricky around the time when daylight savings time c
8. astronomers who may wish to match the airmass between program and standard star observation as closely as possible xS4 is especially for people chasing objects into the west it queries for a maximum acceptable airmass then sorts objects in order of how many minutes it will be before they reach this airmass Thus you can see exactly how urgent it is to get to each object xS5 sorts the list in order of the optional user defined number If this is a magnitude it will be in order of increasing magnitude it may be especially useful to put a priority in this field Naturally only limited information can be given about each item After you ve selected coor dinates it s advisable to type to get a complete listing of the observability information This is especially true if the moon is up the object you ve selected could be right next to the moon or even occulted Some more arcane commands Repeating phenomena xf and xv These two commands are useful for periodically repeating phenomena such as variable star eclipses xf prints a table of phases at regular intervals such as hourly xv prints the times at which a given phase such as an eclipse occurs Both use the celestial coordinates of the object to adjust the output to the geocentric time Both commands give the option of printing information only when the source is visible at night from the chosen observing site The error bars in the period and time zeropoint are p
9. not in itself cause any coordinates to be transformed but rather affects the interpretation of the input coordinates when computations are done The and h commands and others do these computations and show both the input coordinates and the coordinates in the epoch of date There s one little hook here specifying an input epoch lt 5000 prehistoric sets the input epoch exactly to the epoch of the currently specified date It s a little awkward to transform lots of coordinates this way because of all the baggage which comes along with it Therefore there is a command in the eXtra goodies menu xb which transforms a batch of coordinates from one epoch to another This prompts for input and output epochs and then for coordinates in the usual format It keeps going until you give it a negative RA such as 1 0 0 The batch precession is isolated from the rest of the program it does not affect any parameter values Conventional precession programs transform mean positions from one equinox to another but one sometimes wants the apparent place which includes aberration and nutation The extra goodies command xa computes this for the presently specified coordinates proper motion if any date and time It also includes an approximate refraction correction if the object is above the horizon Several stages of the calculation are printed out The accuracy of these routines is discussed later 18 Proper motions the p comm
10. the almanac for that date by typing y 2002 44a followed as always by a carriage return The command line syntaz of this program such as it is is nicely exemplified here The y command means set the date to the following date expressed y m d The reason for the somewhat non obvious choice of y to specify the date is that d is used for declination since the date format starts with the year this is at least a little bit mnemonic The a command means print the almanac information for This works on all systems I ve tested so far and the c library functions I use for this are supposedly standardized In case it doesn t work I ve built a switch into the source code to disable all the functions which rely on the system clock if the system clock is disabled it will tell you at this point and set the time and date to a default value of 2000 Jan 1 at midnight 6 the presently specified evening date Note that the commands are case sensitive so Y will not work in place of y The program seldom cares where carriage returns are placed but doesn t do anything until a carriage return is typed This command produces the following Almanac for the currently specified date Almanac for Kitt Peak MDM Obs long 7 26 28 h m s W lat 31 57 2 d m elev 1925 m Mountain Standard Time 7 hrs W in use all year For the night of Thu 2002 Apr 4 gt Fri 2002 Apr 5 Local midnight 2002 Apr 5 7 hr UT o
11. the line giving the UT date at local civil midnight also gives the moon s illuminated fraction and its angular distance from the object These are computed for local midnight whether or not the moon is actually up then A Word about Hard Copy and Log Files Users tell me that the h command is the most commonly used output feature and that it s especially nice to have hard copies of the tables for your main targets This therefore seems like a good place to mention two ways to direct the program s output to a file one can either redirect output using general operating system features like invoking the script command on UNIX before running the program or if your system supports it you can use a log file feature The log file should be the more convenient of the two possibilities However it has proven troublesome on some systems such as old Sun Sparcstations running Sun s vanilla flavor non ASCI C compiler Because of these troubles a version of the code is distributed which does not include the log file option There may be other circumstances in which your system manager decides to disable the log files Typing x will tell if the option is available if you see an xL option you have it The log file feature if enabled is invoked from the extra goodies menu by typing xL The big L in this case is meant as a mnemonic that something big is happening generally I reserve upper case for commands which change more than o
12. to lunar phase the dates selected are those of full and new moon they are selected to be those local evening dates on which new or full moon fall within 12 hours of the center of the night The tabulation starts with the lunation before your specified starting date At each date the object s hour angle and airmass actually sec z is given 1 at evening twilight 2 at the natural center of the night and 3 at morning twilight The natural center is time of the sun s lower culmination when its hour angle is 12 hours in general it differs from local clock midnight because of location in the time zone daylight savings time and the equation of time Finally the last three columns give the number of hours during the night that is past twilight for which the object is at airmasses less than 3 less than 2 and less than 1 5 These limits are arbitrary but representative of poor marginal and good observability Circumpolar objects can be observable both at the beginning and the end of a long winter night the code appears to tally the observable hours properly In high latitudes twilight does not occur in midsummer in this case twi all night appears in the columns for position at evening and morning twilight At extremely high latitudes the sun can remain below the specified twilight altitude all day and these columns then contain information for the times at which the sun is 12 hours from its lower culmination Looking at
13. type CALDRIVE OUT HOOPLE_1991 LIS to run the command file and direct the outfile to a file called HOOPLE_1991 LIS More on the TeX option The TeX output is based on a Dirty Trick given by Donald Knuth in The TeXbook on page 382 his macro verbatim simply prints a section of text using the tt font which has a fixed character width If you select the TeX option you ll have to edit your redirected output file to remove the fossil prompts which come at the beginning look for the line marked CUT HERE The edited file should then set up and print normally If you used option 1 you ll get one month on each page in portrait mode option 2 gives two months on one page in portrait mode and option 3 gives one month on each page in landscape mode You ll also get a cover page and the moon phase table Note that there are several parameters right at the start of the TeX file which may need to be tweaked to your local printer The portrait mode version sets magnification 835 hsize 7 6 truein and hoffset 0 7 truein These parameter values simply make the very wide output as large as possible on our local system Because TeX defaults to a magnification of 1000 the value 835 makes the print fairly small One can always play with these parameters to make the output come out nicely on your local printer You may wish to make the baselineskip a little larger if you find the lines to be too crowded vertically increasing it to 12 spre
14. when I ve finished with exposure I m working on The internal actions of this option are modified by the g and n options see below The program should do the right thing and set the date and time to reflect the present with whatever offset you specify as expressed in the prevailing time and date convention Note that this option does not cause the time to be continually updated the value of the time set by T remains in effect until you set the time to some other value But the next option xU enables this feature Enabling Disabling Automatic Clock Update xU Typing xU one time sets a flag which automatically reads the system clock whenever you ask for a time critical calculation just as with the T command you are prompted for a time offset when you invoke this option The time offset remains in effect so you can use a computer in a different time zone from where you are situated or keep track of what will be happening say 20 minutes into the future Typing xU again toggles this option off The output options which cause the clock to be read are m major planets xa apparent place and xf phase of a repeating phenomenon The options xZ set coordinates to the zenith and the list selection options will also read the clock The option is toggled off if you issue a command which indicates that you no longer want it including setting the time with t setting the date with y setting both with xJ or reading the system clock
15. Again this converts Julian to calendar dates but it also resets the date and time in the main program to the appropriate values An upper case J is used because two quantities are reset The routine takes into account the current input conventions toggled by g and n so the Julian date computed by immediately typing should reproduce the Julian date you have specified There is one almost unavoidable bug if daylight savings time is used then during the double valued hour when daylight savings time switches back to standard time the program interprets the input time by default as standard time a JD during the final hour of daylight time will create a time which will be interpreted later as standard A prominent warning is printed in this rare case Input outside the calendrical limits 1901 2099 is rejected Unlike the calculator like command xj this one doesn t loop xd Show the value of TDT UT It s occasionally useful to know the difference between UT based on the earth s rotation and TDT a uniform timescale see the Algorithms and Accuracy section for a more complete expla nation This computes an approximate value of this quantity for the currently specified date The approximations used should be good to better than a second from 1900 to 2000 and get increasingly more uncertain in the future because of the unpredictability of the earth s rotation The m command print a table of the major planets As on
16. Almanac convention Also you must specify the longitude in hours minutes and seconds and the latitude in degrees minutes and seconds Once you ve entered new site parameters you can check them with the 1 parameter list command then save them in a little ASCII file by typing s to reinvoke the site parameter command and then typing W to write a file The R option in the site menu prompts for a filename The program next attempts to read your computer s internal clock to establish the time and date Thus the program should wake up with the time and date set to right now This may not be what you want but among all possible times it s perhaps the most likely choice so it s the default The machine will tell you what it has set for time and date After this the program sets the RA and dec used for computations to the zenith for the specified site at the date and time which have just been set This again may not be what you want but if you re at the telescope it at least assures that the coordinates are above the horizon and you can easily change this later The default coordinate epoch is 2000 Finally the program suggests that new or rusty users take the fast tour sequence by typing f This should take about 10 minutes The rest of this discussion follows this tutorial introduction Specifying a date y and getting an almanac a The guided tour first suggests you specify an evening date as year month day and get
17. Kea the horizon correction can affect rise set times by some 10 minutes Spot checks Schaefer and Liller s table of observed times of sunset for Mauna Kea and Cerro Tololo gave for the most part agreement to within about a minute refraction variations preclude more accurate prediction The observatory elevation above its surroundings is used only in the rise set computations the barycentric corrections and the topocentric correction for the moon use the observatory s elevation above sea level which is a separate parameter Thus the elevation above the horizon may be adjusted to fit local circumstances The NOAO Newsletter tables for Kitt Peak for instance have sometimes included a correction of several hundred meters smaller than the 2 km elevation of the observatory the purpose being to correct approximately for the fact that Kitt Peak is higher than most of the mountains which define its horizon At very high latitudes where the moon and sun graze the horizon the program is less accurate since it iterates the rising setting and twilight times until the altitude of the object is within 0 1 of the desired altitude The rise and set algorithms are serviceable at circumpolar latitudes see the section on geographical limitations below but become increasingly unreliable within a couple of degrees of the poles where they are useless at the poles the diurnal rotation does not affect the altitudes of objects 27 The lunar sky br
18. SKYCALC USER S MANUAL John Thorstensen Dept Physics and Astronomy Dartmouth College Table of Contents 0 Introduction Why Bother eo 1 The Interactive Almanac 3 Overview i ne 1 1 Basic Use of the Program me Starting up ap Specifying a date y aad Colina an Aliane a 2 i tn ie Ms EE S Specifying RA and dec r and d and tabulating hourly airmass h TH A Word about Hard Copy and Log Files 9 Specifying time t and getting Instantaneous remesas wath 10 ff Observability through a Season 0 12 Looking at Current Parameters with 1 13 1 2 More Commands 15 Quitting the program Q 15 Printing a Menu 7 TA 15 A Word about eXtra Goodies x 16 Setting the Time to Now T 16 Enabling Disabling Automatic Clock Update xu 16 Changing the site s Ane 17 UT time input and night dates e and 5 coe Ge Ga 18 Coordinate epoch e batch precession xb apparent place xa 18 Proper Motions p 18 Coordinate conversions xc 19 Julian Date calculations and shite 23 ond xJ 19 20 TDT UT calculation xd 20 Major planets m 20 Setting to the Zenith xz 21 Object list handling xR xl xN a xS m 21 ff Predicting periodic phenomena xf and xv 23 xp Parallax factors and aberration 23 xy Day of year 24 1 3 Algorithms Accuracy and oi 25 Calendars and times 25 Sun and Moon 26 The Major Planets 28 Geographical Limitations 29 Precession and Apparent Place 29 Local Mean Sidereal Tim
19. ads things out a lot 38 Sample Output from the Calendar Program The following output came directly from a run of the program using the input quantities given in the example above It should be used to check the results in a new site Because the output is so wide I ve added carriage returns in the main body of the calendar they are consistently right after the columns relating to the sun This has a terrible effect on legibility but fits it on the page xo 1991 JANUARY k Calendar for Univ South North Dakota at Hoople west longitude h m s 6 16 56 latitude d m 44 44 7 Note that each line lists events of one night spanning two calendar dates Rise set times are given in Central time 6 hr W uncorrected for elevation DAYLIGHT time used shows night clocks are reset Moon coords and illum are for local midnight even if moon is down Program John Thorstensen Dartmouth College Date eve morn JDmid LMSTmidn Sun LST twilight Moons 3 1991 at start 2440000 set twi end twi beg rise eve morn rise set illum RA Dec Tue Jan 01 Wed Jan 02 8258 8 6 28 34 16 47 18 33 609 7 54 100 1238 1808 97 817 7 18 56 Wed Jan 02 Thu Jan 03 8259 8 6 32 31 16 48 18 33 609 7 54 105 1242 1929 91 915 5 13 45 Thu Jan 03 Fri Jan 04 8260 8 6 36 28 1649 18 34 609 7 54 110 1246 20 47 83 1008 9 7 58 3 CAUTIONS APPLYING TO BOTH PROGRAMS When these
20. am How you do this is dependent on your operating system On a UNIX system you just type the name of the program which is likely to be skycalc assuming it is in your current directory or has been installed in your path The program first asks you to select an observing site a little menu comes up which gives single character codes for a number of major and minor observatories Simply type the single character for the site you want followed by a carriage return Throughout the program nothing happens until you type a carriage return The examples computed below are for Kitt Peak k but you can give whatever you like Note that the input is case sensitive so K is not the same as k Here s what the menu should look like though you ll undoubtedly have more menu options Astronomical calculator program by John Thorstensen SELECT SITE Enter single character code 5 NEW SITE prompts for all parameters Write site parameters to a file Read new site parameters from a file exit without change current Kitt Peak Kitt Peak MDM Obs yx DSB and so on several different sites are available 1 Lick Observatory Your answer gt If the desired site is not on the menu type the letter for a new site n in this example and you ll be prompted for the characteristics of the site The prompts should be self explanatory Note that the longitude and time zone are positive westward unlike the
21. ame to label the output gives the following output Seasonal Observability of Flapdoodle s Variable Nebula RA amp dec 15 38 29 2 0 01 02 epoch 2000 0 Site long amp lat 7 26 28 0 h m s West 31 57 12 North Shown local eve date moon phase hr ang and sec z at 1 eve twilight 2 natural center of night and 3 morning twilight then comes number of nighttime hours during which object is at sec z less than 3 2 and 1 5 Night and twilight is defined by sun altitude lt 18 0 degrees Date eve moon eve cent morn night hrs sec z HA sec z HA sec z HA sec z lt 3 lt 2 lt 1 5 2002 Jan 23 F 11 49 down 6 52 down 1 33 1 3 2 9 2 0 1 0 2002 Feb 11 N 11 05 down 5 56 62 4 0 46 1 2 3 7 2 8 1 8 2002 Feb 26 F 9 55 down 4 58 4 4 0 02 1 2 4 4 3 6 2 5 2002 Mar 13 N 8 44 down 4 02 2 4 0 40 1 2 51 4 2 3 2 2002 Mar 27 F 7 39 down 3 11 1 8 1 16 1 2 57 4 8 3 8 2002 Apr 11 N 6 27 down 2 16 1 4 1 54 1 3 6 3 55 4 4 2002 Apr 26 F 5 14 5 9 1 20 1 3 2 32 1 5 7 0 6 1 5 1 2002 May 11 N 4 00 2 4 0 23 1 2 3 14 1 8 7 2 6 8 5 1 2002 May 25 F 2 51 1 6 0 33 1 2 3 57 2 3 6 8 6 4 5 1 2002 Jun 10 N 1 36 1 3 1 39 1 3 4 54 4 1 6 0 52 4 1 2002 Jun 24 F 0 36 1 2 2 37 1 5 5 50 28 3 5 0 4 2 3 1 2002 Jul 9 N 020 1 2 3 39 2 0 6 58 down 4 1 3 2 2 2 2002 Jul 23 F 106 1 2 4 35 3 3 8 05 down 3 3 2 5 1 4 2002 Aug 7 N 149 1 3 5 34 10 3 9 18 down 2 6 1 8 0 7 12 Because observing time requests are so intimately tied
22. an date which is the same in each case so they should always agree As noted earlier the program makes various transformations to account for zone time daylight savings time and such A subtler issue is the actual timebase which is used for the input time The distinctions between UT UTC TAI TDT are made authoritatively in the Astronomical Almanac but are widely ignored by astronomers so Pll explain them briefly here this isn t an authoritative discussion but I hope it s essentially correct Universal Time or UT is Greenwich time based on the true phase of rotation of the earth The earth s rotation gradually slows with time and it is sufficiently unpredictable that UT can t be determined accurately until after the fact There are a few minor variants of UT based on the state of the data reduction in this determination TAI is Inter national Atomic Time which is the best realizable uniform timescale UTC the famous coordinated universal time broadcast on WWV is a compromise between these it follows UT approximately but is maintained an integer number of seconds away from TAI by the insertion of an occasional leap second UTC and UT should be maintained so that they always agree to within 0 9 sec Finally TDT is Terrestrial Dynamical Time this is another uniform timescale offset for historical reasons I don t understand by a constant 32 184 seconds from TAI Strictly speaking before 1983 the apparent con ceptual equival
23. an this though none of the test stars had large proper motions If the object is above the horizon an estimate of the refracted position is printed This uses formulae adopted from the Astromomical Almanac for 1995 p B62 The air temperature is assumed to be 20 Celsius and the atmospheric pressure is computed using an exponential atmosphere and the elevation of the site above sea level The refracted position is printed even if the object is below the geometrical horizon provided refraction is estimated to bring it above the level surface horizon depression of the horizon due to elevation above surroundings is not included in this calculation The refraction correction in general cannot be expected to be very accurate but it s better than nothing Note that because of refinements in the reference frame proper motions should in principle be transformed at the same time as coordinates the current program ignores this Local Mean Sidereal Time Strictly speaking the local sidereal time equals the hour angle of the vernal equinox the mean sidereal time computed here is slightly different because the effect of nutation on the location of the equinox is not included This correction is called the equation of the equinoxes which is tabulated in the Astronomical Almanac it s generally less than 2 sec The algorithm used here is based on formulae and procedures explained in the 1992 Astronomical Almanac pp B7 and L2 Tests for the
24. and If you type p you will be prompted for annual proper motions of the object answer the prompts The specification of proper motion is complicated because there are at least two conventions in use for the units of the proper motion in RA One is the annual change of the RA itself generally given in seconds of time per year this is used in the SAO Catalog The other is the east west motion in seconds of arc on the sky which is the first times 15c0s0 The program will accept input of either type if you give seconds of time per annum you must follow your value with an s and if you give seconds of arc you must give an a The value is converted and passed internally in the first time convention Declination proper motions must always be entered as arcsec per annum If either of the proper motions are nonzero the output of will display the original coordinates in the standard epoch and equinox the coordinates updated for proper motion only current epoch but standard equinoz the coordinates updated for proper motion and precession current epoch and current equinox as well as the proper motions used The reason for doing this is that with most modern telescopes the coordinate readout can be set to a standard equinox but the actual sky is of course always in the present epoch regardless of what coordinates you apply to it So it s useful at times to display the updated position without changing the equinox Note that th
25. and date have their same numerical values but are now interpreted differently A message is printed to remind you of this Similarly the program awakens assuming that the date you specify is to be interpreted as the evening date for the entire night this is the night date condition For example if you print out an almanac for the night of October 20 and then specify a time after midnight 2 30 00 say and type the circumstances printed are those applying on the morning of October 21 The reason for doing this is to maintain some parallelism with the almanac which prints the phenomena for a given night Typing n once switches this option off so the current date is interpreted literally typing n again switches it back on unless you are in UT mode and so on This may seem confusing at first but it should be less confusing than the alternative It is at least always possible to interpret the output unambiguously the times and dates printed there are generated internally directly from the JD so they should always be reliable The g and n commands interact Going to UT input automatically turns off the night date option since UT dates should always be interpreted literally You are also prevented from turning on the night date condition when UT input is in effect Coordinate epoch e Batch precession xb and apparent place xa Typing e followed by an epoch sets the epoch for your input coordinates Setting the epoch does
26. aximum number of objects it warns you Now the fun begins The simplest thing you can do with the object list is type out some of the contents that s done with the x 1 list command You re prompted for the first and last items to print out by number in the list The program then prints out the presently defined date and time for which the hour angles and airmasses are computed and then simply types out the information for each object The last two columns give the hour angle and airmass sec z of each object The commands xN and xS are upper case letters because they cause more than one quantity to change at once xN searches for an object by name it must be an exact match including the upper or lower case of any letters and if a match is found the program sets both the RA and dec to that object If the epoch of the object s coordinates in the list is different from the currently defined input epoch the quantity controlled by the e command the object s coordinates are precessed to the input epoch and the coordinates are set to the precessed values that is the program handles this correctly It would have been possible instead to reset the input epoch as well as the RA and dec but this would have been even more confusing Like the xN command the xS command resets the RA and dec with precession if need be as above but now the user gets to select interactively which object to choose Ten objects at a time are presen
27. bel so you can give a random character if you want Here s what the output looks like xxx Hourly airmass for Flapdoodle s Variable Nebula Epoch 2000 00 RA 15 38 29 2 dec 0 01 02 Epoch 2002 26 RA 15 38 36 1 dec 0 01 28 At midnight UT date 2002 Apr 5 Moon 0 43 illum 64 degr from obj Local UT LMST HA secz par angl SunAlt MoonAlt 19 00 2 00 7 26 8 12 down 53 4 3 3 20 00 3 00 8 27 7 12 down 56 7 15 7 21 00 4 00 9 27 6 12 down 58 0 22 00 5 00 10 27 5 12 5 644 57 5 23 00 6 00 11 27 4 12 2 588 55 0 0 00 7 00 12 27 3 11 1 757 49 9 1 00 8 00 13 27 2 11 1 403 41 0 2 00 9 00 14 28 1 11 1 238 26 1 TE 3 00 10 00 15 28 0 11 1 180 4 4 5 7 4 00 11 00 16 28 O 49 1 207 18 9 15 4 5 00 12 00 17 28 1 49 1 327 36 4 15 3 23 6 6 00 13 00 18 28 2 50 1 597 47 2 2 9 29 8 Each line shows the local time the UT the local mean sidereal time and the object s hour angle the next quantity sec z the secant of the zenith angle is essentially the same thing as the airmass 8 The notation down in this column means the object is below the horizon v low will occasionally appear in this column meaning that the object is so near the horizon that sec z will overflow the space provided for it Note that the last two columns give the altitude of the sun and moon the sun is printed if it is higher than 18 and the moon if it is higher then 2 Otherwise ellipses are printed in those spaces Also notice that
28. chinery As far as legalities are concerned the code should now be pure as the driven snow 34 2 A NIGHTTIME ASTRONOMICAL CALENDAR PROGRAM This program prints an astronomical calendar for a given year from a single site The algorithms used are for the most part identical to those used for the circumstances calculator program described above but the input and output are different The program has been used for several years to print the nighttime astronomical calendar for Kitt Peak included in the NOAO Newsletter and is used to generate the calendars at a number of observatory websites Again this is a self contained C program which should run gracefully on various computers however the cautions listed above apply The output has a wide format 122 characters At the beginning some information is printed along with prompts for interactive use something which will probably seldom happen Then comes a page of information about the program and its accuracy which is largely redundant with this document Next follows a table of moon phases with the times given as local zone times accurate to a few minutes Next follow the results for each of the twelve months The output may optionally be formatted for input into the TeX typesetting program It can be set up to print two months to the page in portrait orientation or one month to the page in landscape orientation Further details on the TeX option are given later If TeX output is not s
29. ckets 30 Airmass The airmass as such is only reported by the instantaneous circumstances option elsewhere the secant of the zenith angle secant z is given For zenith distances less than 60 degrees these two quantities are equal to better than 1 per cent while at large distances they diverge somewhat The airmass used is based on a polynomial fit to a table given by C M Snell and A M Heiser in PASP vol 80 0 336 1968 Their table is calculated for a standard atmosphere and the elevation of Kitt Peak The fit is sec z airmass 2 87947 x 107 y 3 03310 x 107 y 1 35117 x 1073 y 4 717 x 107 yf where y secz 1 A small constant term in the fit is suppressed to force the airmass to unity at the zenith The fit is constrained for z lt 85 degrees and passes smoothly through the tabulated points to within 4 x 1074 which is probably smaller than natural variation due to atmospheric conditions I ve heard anecdotally that the airmass approaches about 35 toward the horizon while sec z tends toward infinity so this approach clearly is limited to z lt 85 degrees beyond that the value of sec z is printed Barycentric Heliocentric Corrections The algorithms used for the earth s orbit are derived from the solar ephemeris which in turn is from Jean Meeus Astronomical Formulae for Calculators pp 79ff It uses an elliptical earth orbit and a few of the most important perturbations The corre
30. codes are ported to a new system the results should be checked carefully for accuracy The sample output in this document should be reproduced correctly The user assumes responsibility for the correct operation of the programs and the sensible interpretation of their results The user s attention is drawn to the known limitations of the algorithms documented above While the programs have been tested carefully with the results given above the author makes no guarantee that this level of accuracy will obtain in all circumstances on all machines I explicitly disavow any responsibility express or implied for damages resulting from use of the program Output from this program should never be used as evidence in a court of law or to make decisions which might cause bodily harm if the results weren t right 39 Miscellany All the source code is usually kept in a single file which contains all the subroutines as well as the main program The size can cause difficulty with some compilers on personal computers though the gcc compiler which is standard in Linux takes it in stride It s possible to break the code up by using function prototypes in the usual manner but it s a lot of labor Maintainance If you find a real problem not due to your local machine and not documented above write John Thorstensen Dept of Physics and Astronomy Dartmouth College Hanover NH 03755 John Thorstensen dartmouth edu 40
31. ction to the solar system barycenter is also included using the same planetary calculations discussed earlier The earth s diurnal rotation assuming an ellipsoidal earth and including the observatory s elevation is included in the velocity calculation but the time of flight across the earth s radius 0 02 sec is not included Meeus solar theory does include a rough correction for the recoil of the earth due to the moon I tested these against the JPL DE200 ephemeris the basis for the Astronomical Almanac tables at 10 day intervals for 2000 days the maximum errors were 0 11 s and 3 02 m s and the RMS errors were 0 058 sec and 1 8 m s The most demanding applications such as analyses of pulsar timing and doppler based searches for extrasolar planets will require better accuracy but this should be adequate for almost everyone else Galactic and Ecliptic Coordinates The galactic coordinates conform strictly to the IAU definition and agree closely with those computed by IRAF they are based on a rotation matrix and do not suffer ambiguities due to the roots of inverse trig functions The input coordinates are precessed to 1950 before being transformed to galactic which introduces a slight uncertainty If 1950 input coordinates are supplied the only source of error should be double precision roundoff The ecliptic coordinates should be good to lt 0 001 degrees 31 BUGS and other ungraceful behavior I actually don t know of
32. current parameters with 1 At this point we ve set a fair number of parameters While many of the the current parameters are printed in the output from others are implicit and the display is crowded so they re hard to keep track of Thus the 1 look command simply prints out a nicely formatted list of input parameters Its output is Current INPUT parameter values DATE 2002 Apr 4 TIME 5 10 00 0 NIGHT_DATE ON date applies all evening next morning UT_INPUT OFF input times taken to be local USE_DST 0 Standard time in use all year AUTO UPDATE OFF system clock not automatically read on output RA 15 38 29 20 DEC 0 01 02 0 INPUT EPOCH 2000 00 PROPER MOTIONS OFF SITE Kitt Peak MDM Obs E longit 111 37 0 latit 31 57 2 degrees Standard zone 7 hrs West Elevation above horizon 700 m True elevation 1925 m 13 This is particularly useful for keeping track of the g and n commands which cause the interpreta tions of time and date to toggle between different cases Because the effect of each of these commands depends on the status when they are executed it s helpful to be able to look at their state without doing anything else Also the site latitude and longitude are converted here to a format which exactly matches the numbers in the Astronomical Almanac observatory list to make it easy to check them 14 1 2 MORE COMMANDS Quitting the program Q This stops the
33. e 30 Parallactic Angle 30 Airmass Sek AR why a A 30 Barycentric Heliocentric Corrections 31 Galactic and Ecliptic Coordinates 31 Bugs and other problems 32 ff Programmer s Notes 2 ee eee ee ee 8 2 A Nighttime Astronomical Calendar 2 2 2 2 2 2 2 42 35 General description Se es ee ae hee oe ah cos rag st ts Gem Ges Gay eens ay ae ees OD Times in the Calendar Program LA a ata ee e BS a EA a ak a a a 00 Running the Calendar o bo s misos sos e 8 8 A Gok SOT More on TeX Output 38 Sample Output nee chk eee A 3 Cautions Applying to Both A Misc poe bop CH o Bee end ke Sh Gk Ca RI 0 INTRODUCTION WHY BOTHER You ve just received your time assignment for Kitt Peak and you wonder whether the moon will interfere with your objects during those nights which weren t the one s you asked for Or you re sitting at the telescope at 1 AM wondering if you can squeeze in a 1 hour exposure before twilight at acceptable airmass on an object that s just rising now Maybe you want to set your spectrograph slit to neutralize atmospheric dispersion Maybe you want to precess a couple of objects coordinates or see what their galactic latitude is Perhaps you want to spot check the canned heliocentric corrections which IRAF has applied to all your data Perhaps you just want to know how high the sun will be above the horizon at 4 PM in October so you can see if it s safe to take a bik
34. e might expect this prints a table of the RA dec hour angle secant z altitude and azimuth of each of the major planets as well as the sun and moon The planetary positions are only modestly precise their pedigree and accuracy are explained later The sun and moon calculations are useful for finding their positions for times when they are below the horizon or past twilight for the sun and would therefore not be printed with the command The output is as follows W Long hms 7 26 28 0 lat dms 31 57 12 std time zone 7 hr W Local Date and time Thu 1995 Mar 23 time 4 50 00 0 MST UT Date and time Thu 1995 Mar 23 time 11 50 00 0 Julian date 2449799 993056 LMST 16 25 31 8 Planetary positions epoch of date accuracy about 0 1 deg RA dec HA sec Z alt az Sun 0 08 7 0 57 7 43 2 77 21 1 74 8 Moon 17 52 1 20 05 1 27 1 79 34 0 155 3 Mercury 23 01 3 8 46 6 36 4 74 12 2 92 8 Venus 21 47 2 13 45 5 22 86 46 0 7 106 7 Mars 9 06 7 20 11 7 19 11 61 4 9 297 5 20 Jupiter 16 56 1 21 49 0 31 1 71 35 7 171 2 Saturn 23 15 6 6 43 6 50 4 09 14 1 89 1 Uranus 20 08 6 20 41 3 43 3 85 15 0 126 8 Neptune 19 48 3 20 33 3 23 3 15 18 5 130 2 Pluto 16 04 1 6 41 o 21 1 29 51 0 188 5 lt least accurate It s entertaining to note that if your location is on the site menu you can get a table of planets for right now by starting the program giving your site s letter and typing m counting car
35. e proper motions are not computed with perfect rigor the current RA is just the old RA plus At and similarly for the dec This is inaccurate very close to the pole or over very long intervals of time The xc command coordinate conversions Typing xc causes the galactic and ecliptic coordinates to be printed As of this writing there is also a c at the main program level which still does this but it s not advertised on the main menu in order to keep the main menu to 24 lines The galactic coordinate algorithm complies strictly with the IAU definition which is specified in 1950 coordinates If the input coordinates are in a different epoch they are precessed internally to 1950 before being converted to galactic Both conversions work correctly over the entire sky The inverse conversions are not implemented The xj command calculate calendar dates from Julian dates The main program converts calendar to julian dates internally and prints out julian dates with among others the command It s sometimes useful to have the inverse which converts julian to calendar and the extra goodies command xj does this calculation The command loops until a negative julian date is given The routine expects all the leading digits of the julian date If the input is a true julian date the output is a UT date The date in the main program is unaffected by this command see xJ below 19 The xJ command Set to a Julian date
36. e ride Any competent astronomer armed with some reference materials and a calculator can answer these questions but it takes time Over the years I ve done this sort of thing many times I finally decided to encapsulate some utility routines of this sort into a couple of convenient easy to use portable packages This document describes two programs The more powerful and interesting one is an interactive astronomical circumstances calculator The other prints a 1 year nighttime calendar of phenomena for a single site this will generally be run in background to produce a table to be hung on the observatory wall or put in a notebook I wrote both these as standalone C language programs To maximize portability and ease of use I tried to make the user interface as simple as possible but no simpler to paraphrase Einstein There are no graphics no mouse driven menus or anything like that You type stuff and the computer types stuff back The commands are as terse as possible single letters so even hunt and peck typists should be able to use the programs efficiently Throughout this document I ve indicated things that you or the computer type with this typeface In some sense these programs and their documentation are a publication for me though not ina refereed journal Accordingly Pd like these to be disseminated as widely as possible in the community of professional astronomers Please feel free to pass them along If yo
37. elected a formfeed character is inserted at the top of each page At the head of each month is the year and month set off by asterisks Also given is the site name its longitude in hours minutes and seconds its latitude in degrees and decimal minutes and the standard time zone After some other information the user is reminded that the times listed except for sidereal are local zone times the name of the zone is given If daylight savings time is used the user is reminded of this as well The rest of the calendar is organized with one night per line Note that this choice is only sensible for nighttime astronomers a large but not all inclusive subset Though the calendar works at circumpolar latitudes this form of organization is not optimal during the midnight sun either A detailed description of the tabulated quantities follows The first column gives the day and date for both evening and morning This should minimize errors in reading dates A blank line appears between Saturday and Sunday nights The next column gives the JD at local midnight rounded off to the nearest 0 1 d to avoid any ambiguity The number given has the largest multiple of 10000 days figured for the first of the month subtracted away thus JD 2451020 5 will be printed as 1020 5 If daylight savings is in use the JD is the JD of daylight savings midnight The third column gives the Local Mean Sidereal Time see the earlier discussion for the distinctio
38. ent of TDT was called Ephemeris Time ET I m unclear as to the difference between ET and TDT On long timescales UT drifts parabolically away from TDT or its rough equivalent ET a perusal of pp K8 and K9 of the 1995 Almanac shows that they were equal in 1870 and 1902 and that the difference AT TDT UT has now reached about a minute Because calculations of solar system objects should be based on a uniform time scale the argu ment of these calculations is generally TDT But I ignore AT in the planetary calculations because the planets move rather slowly and the planetary theory used here is relatively primitive However TDT is used for the moon calculation where it is just significant because the moon moves so quickly and the sun where at present it changes the answer by about 3 arcsec From 1900 to the end of 1997 the values used are based on linear interpolations on 5 year intervals in the 2000 edition of the Almanac Accuracy appears to be less than a second when compared to the tabulated annual values After 1997 the correction used is a guess which is linearly extrapolated from present day values A parabolic extrapolation might be better but the behavior in the past has often been rather erratic so this seems adequate The calendrical routines break down before 1901 and after 2100 Input outside those dates causes the program to become uncooperative until you set a date inside the allowed range While it would be a sim
39. enwich USND Hoople Site name terminate with carriage return Central Time zone name terminated with carriage return 1 use daylight savings time USA post 1986 prescription 0 don t use TeX on output use 1 2 or 3 for TeX 1991 year 37 Naturally you should be especially careful about your site parameters anyone entering a new site in the source code should be downright compulsive since many people may depend on the accuracy The user should check the output to be sure the parameters repeated are correct the latitude longitude etc are printed at the top of every month s page Examples of how to run the program in background Pll show how to do this using UNIX or VMS systems note that these are trademark names This is not of complete generality but covers the bases for most users On a UNIX system if you ve named your executable task calendar you d edit a file called inputs containing the input data just as above Then type calendar lt inputs gt hoople_1991 amp which could be paraphrased as run the calendar program taking input from lt the file named inputs directing output to gt a file named hoople_1991 and do it in background amp On a VMS system you would edit a command file let s call it CALDRIVE COM which would have the first line run calendar with subsequent input on successive lines without dollar signs at the front again just as above Then you d
40. evious version of the h command prompted for the number of hours to print this is now computed automatically so old scripts designed for the previous version will have to be revised slightly Instantaneous Circumstances the Command The next action suggested in the guided tour is to specify a time of day and then display the instantaneous circumstances by typing for instance t 5 100 By default the time you enter is taken to be the local time but this can be changed to UT with the g option below Also by default a morning time such as this one is interpreted as referring to the morning of the date after the specified date this way morning and evening times refer to the same night This can also be changed using the n night date option These are all explained later The causes the instantaneous circumstances to be displayed for the present parameters the output is W Long hms 7 26 28 0 lat dms 31 57 12 std time zone 7 hr W Local Date and time Fri 2002 Apr 5 time 5 10 00 0 MST UT Date and time Fri 2002 Apr 5 time 12 10 00 0 Julian date 2452370 006944 LMST 17 38 02 4 Std epoch gt RA 15 38 29 2 dec 0 01 02 ep 2000 00 Current gt RA 15 38 36 1 dec 0 01 28 ep 2002 26 HA 1 59 26 airmass 1 358 altitude 47 36 azimuth 227 31 parallactic angle 38 6 141 4 In twilight sun alt 13 3 az 74 0 Sun at 0 57 15 1 6 06 56 Clear zenith twilight blue approx 2 2 mag over dar
41. explicitly with T As an example of the effect of this option when auto updating is toggled on successive invocations of will come back with slightly different times 16 Changing the Site with the command s You can change sites by typing the letter s and answering the prompts When you do this you will be given a menu of single character site codes from which to choose just as when you started the program Your local version of the program should be customized to offer the most common choices for your institution To choose a site just type the letter be sure to use the correct lower or upper case and hit carriage return You can also specify a site not on the menu by typing n or the appropriate character in your customized version If you select one of the canned sites all the parameters latitude longitude time zone info etc will be changed to their standard values for that site If you want a site which is not on the menu you ll have to give all its parameters Otherwise one would risk of changing the parameters piecemeal and having some parameters which are appropriate to the site and others which are not You ll need to know the latitude of your site in degrees minutes and seconds and the west longitude in hours minutes and seconds Like Jean Meeus I dislike the east longitude convention and I like to use hours for longitude because of the direct connection with time You ll also need the time zone in hou
42. fit to a graph on p 38 of A and M Meinel s lovely book Sunsets Twilights and Evening Skies Cambridge 1983 Comparison with measurements by E V Ashburn Journ Geophys Rsch v 57 p 85 1952 shows that the fit provides a fair match to the observed twilight in the blue 4400 A the V band is about a magnitude fainter and J should be a little fainter still The zero point of this number the dark night sky is quite problematic but the dependence on the sun s zenith distance should be reasonably accurate Ashburn s data were taken from a California mountain site at an elevation of 1653 meters 5415 feet The Planets The purpose of the planetary calculations is not to give definitive positions which are now derived from numerical integrations but to give rough positions for planning purposes e g is Jupiter visible Is it close to my object If you really need to point blindly exactly at a planet get another program or consult the Astronomical Almanac The positions are computed using formulae from the 1992 Astronomical Almanac p E4 The input data are heterogeneous For the planets through Mars the program uses mean elements from the old Explanatory Supplement to the Nautical Almanac These give very good results usually less than 1 arcmin for the inferior planets and satisfactory results a few arcmin for Mars For the outer planets Jupiter through Neptune the input data are from Jean Meeus Astronomical For
43. hanges to standard and vice versa The behavior has been rationalized in recent versions but it s still tricky The hour when daylight time changes back to standard time is ambiguous there is a default to standard time which may not be what the user wants The user is warned if there might be a problem Conversion from UT to local appears to be rigorously correct so specifying times as UT when there is a problem should get around any difficulties After you type g to toggle between greenwich and local time the time currently in effect changes to the value which is numerically the same in the new system not the time which is actually equivalent So if you are in the zone 7 hr west Mountain and you are using local time and the time is 1991 Jul 7 22 hr 0 mn 0 sec MST and you type g the time in force is now 1991 Jul 7 22 hr 0 mn O sec Universal time which is 7 hr earlier To get the same actual time you d have to enter y 1991 7 8 and t 500 The program reminds the user that this is happening Similar considerations apply to the n option Similarly typing e to change the epoch assumed for the input coordinates doesn t precess any actual coordinates it just changes how input will be interpreted On these points it may be helpful to emphasize that the program doesn t actually calculate or convert anything until it s asked for output the numbers you type in lie dormant until then Thus the g n and e options control only the
44. he date and time to right now plus a settable offset in case you re interested in say half an hour from now This is especially useful at the telescope There is also provision under extra goodies for reading in a list of objects in a simple format presenting this list sorted in various ways and selecting object coordinates from it Finally one can automatically set the coordinates to the zenith for the specified site date and time 4 There s flexibility as to how you specify dates and times which comes at a cost in consistency and simplicity By toggling software switches you can specify times either in UT or local zone time which can optionally include daylight savings also you can apply a date convention by which the evening date applies all night for nighttime continuity Later in this document I give a lengthy and detailed description of the level of accuracy expected for all the calculations My philosophy has been to compute everything as accurately as I could consistent with the requirement that the program be self contained and portable For example the precession and sidereal time calculations are very accurate because they are reasonably compact but the planetary positions are not definitive because that would require the inclusion of rather extensive data tables Which leads to A FEW CAUTIONS I ve made this code as accurate and as generally useful as I could but before using it for purposes where e
45. he quantity secant of the zenith distance is printed since the polynomial expression used to compute airmass from secant z becomes inaccurate If the object is at a particularly large airmass or below the horizon a comment is printed If the object s secant z is very large it is not printed to avoid overflowing the space provided The altitude is 90 degrees minus the zenith distance uncorrected for refraction the azimuth is as usual measured from the north through the east The parallactic angle is the position angle measured N through E at the object of the arc that connects the object to the zenith or loosely speaking the position angle of straight up This is useful for setting a spectrograph slit to catch the dispersed light See Filippenko 1982 PASP 94 715 for a discussion The parallactic angle may change sign at the meridian but actually varies smoothly Because some applications e g the placement of a spectrograph slit are indifferent to a 180 degree shift in the parallactic angle the angle 180 degrees is reported in square brackets If the moon could be interfering higher than 2 degrees below the uncorrected geometrical hori zon its phase fraction illuminated approximate RA dec altitude not corrected for refraction and azimuth are reported Also the angle subtended at the observer by the object and the moon is reported If the moon is more than two degrees below the horizon it is reported to be down
46. hich works correctly at the poles and gives mean positions good to much less than 1 arcsec in 50 years The set of test coordinates given by Smith et al 1989 A J 97 265 was reproduced to below 1 arcsec accuracy except for proper motions near the poles The present version of the program uses IAU 1976 precession parameters It computes epochs as Julian epochs i e epoch 2000 JD J2000 365 25 Hidden in the extra goodies menu is an apparent place routine which includes proper motion correction as above nutation and aberration Because the routine prints at several stages of the calculation you can pick the desired level of effects e g up to but not including refraction The nutation parameters are computed using approximate series given by J Meeus Astronomical Formulae for Calculators Willman Bell 1985 These include terms down to a few milli arcsec in amplitude The aberration uses a heliocentric not barycentric earth velocity derived from the accurate sun ephemeris and neglects relativistic terms and diurnal aberration More importantly parallax is not included in the apparent place calculations which for stars leads to errors which are always less than 0 7 arcsec and usually much less I spot checked the program by computing the apparent places of five FK5 stars at times to match the listings in the 1994 Apparent Places of Fundamental Stars 29 Agreement was generally 0 1 arcsec and never much worse th
47. hing to do with the time zone If you wish daylight savings times can be listed if you use this feature a site dependent prescription is used to select whether daylight or standard time is used The switchover occurs at 2 AM on Sunday morning This can in principle lead to an ambiguity around the time of time change in the fall it s 1 30 AM local time twice on the same night but you should be able to unravel this rare case from continuity with the preceding and following nights Several conventions are available for the dates of the time change The conventions coded are the USA convention 1st Sunday in April to last in October after 1986 last Sunday in April before 1986 more or less the Chilean convention the Spanish convention 36 and the Australian convention If you need another convention you ll have to add it to the source code in the routine find_dst_bounds The header that appears on each page makes a note if daylight savings time is used An asterisk is printed by the date on which the time is changed Running the calendar You will probably never want to actually run the calendar interactively it takes a while to run and it produces a very wide output It s more appropriate to run it in background with output redirected to a file you can print on some wide device e g a laserwriter in landscape mode I describe below the input that the program will call for when run non interactively However the program is a
48. ightness contribution is estimated if the sun is well down beneath 9 degrees altitude and both the moon and the object are in the sky This calculation follows K Krisciunas and B E Schaefer 1991 PASP 103 1033 The calculation will not be even roughly accurate unless the sky is quite clear haze cloud or even volcanic aerosols high in the atmosphere can greatly affect the scattered moonlight To the Krisciunas and Schaefer model I ve added a correction for variations in the apparent size of the moon and an extremely crude model of the opposition effect the surge of brightness just around full moon This is modeled as a 35 per cent brightening at full moon which tapers linearly in phase and goes to zero at 7 degrees from full moon The code does check for lunar eclipses but makes no attempt to account for their effect on the sky brightness It does print a disclaimer if the moon is in eclipse The brightness calculation assumes a zenith extinction of 0 172 mag in V typical of the 2800 m level on Mauna Kea Results are reported as equivalent V magnitude per square arcsecond for comparison the zenith night sky brightness in a dark site is quite variable but is very roughly 21 5 mag per square arcsec in V These estimates should be useful for planning purposes but unlike some of the other results in this program they are unlikely to be very precise Similar cautions apply to the zenith twilight brightness This is based on a polynomial
49. inates the state of the moon and its modeled sky brightness contribution twilight heliocentric barycentric actually corrections the parallactic angle the julian date and various other stuff There are little niceties such as a check as to whether a major planet might be near the line of sight and whether a solar or lunar eclipse is in progress o The seasonal observability command is designed to help you accurately assess the range of acceptable dates for observing a given object It tabulates how many hours an object will be observable at night at less than 3 2 and 1 5 airmasses from your site the tabulation is for each new and full moon between two specified dates Thus this serves as a lunar phase calendar as well m This major planets command types out rough positions of the major planets for your site date and time including their hour angles and airmasses In addition there are several special purpose calculators invoked by two character commands the two character commands are called extra goodies These list times of events for a variable star precess batches of coordinates convert julian to calendar dates give the offset from ephemeris time and such Specifying input parameters one by one can be tedious so there are a number of ways to make this more convenient Sites which include most of the world s major observatories are presented on a menu One can read the system clock with T and set t
50. interpretations of your input parameters The twilight and rise set times are slightly inaccurate at very high latitudes since the object comes into the appropriate altitude at a grazing angle Rise set can be erroneously reported as not 32 occurring at very high latitudes because the occurrance of rise set is judged using the position for local midnight and it s possible in principle for the program to try to find a rise set time which actually doesn t occur The correction used for the site elevation in the rise set calculations is approximate Note that it may or may not be appropriate to include altitude corrections for your site based on the details of your local horizon Notes for Programmers The skycalc calculator program allows the user to turn on a log file so that one can save results without having to go through the rigamarole of creating an input file and redirecting output I implemented this by replacing the appropriate calls to printf with a new routine oprntf which optionally writes to a file Because printf and hence oprntf have variable numbers of arguments I used the widely available ANSI standard framework which involves the inclusion of lt stdarg h gt see Kernighan and Ritchie 2nd edition p 155 ff It turns out that the cc compiler on older Sun workstations does not support this standard the supposedly ANSI standard version acc has a compiler bug affecting this particular feature I m indebted to Mike
51. k night sky Moon 19 53 48 24 38 4 alt 24 8 az 146 0 0 412 illum 0 9 days after last quarter Object is 66 3 degr from moon Lunar part of sky bright 21 7 V mag sq arcsec estimated Barycentric corrections add 380 7 sec 17 03 km sec to observed values Barycentric Julian date 2452370 011350 10 Notice all that has been computed After a brief summary of the site information the next block establishes the time in various systems The date and time are given in both local and UT If daylight savings time is selected a recipe which should be appropriate to the site is used to select whether the local time is reported in DST or Standard Note that the label on the local time will have either an S or a D as the second character to indicate standard or daylight LMST stands for the local mean sidereal time which is essentially the local sidereal time It disagrees with the true hour angle of the equinox by the equation of the equinoxes caused by nutation this amounts to a couple of seconds at most The next block refers to the object The object s coordinates are reported both for the standard epoch which is set with the e option and for the mean equinox of date The program wakes up assuming that input coordinates are for equinox 2000 Proper motions may be included see below If the coordinates given are more than about 5 degrees above the horizon the airmass is printed at lower elevations t
52. longitude of Greenwich in 1992 give agreement with the Astronomical Almanac tables of mean sidereal time to within a few msec Also the IRAF routine gives the same answers to within 0 1 sec However strictly speaking this accuracy will obtain only if your input time is based on the correct type of UT UTC broadcast by WWV is tied to atomic time and is corrected by whole seconds to agree with earth rotation If your input is based on UTC as all civil time is the computed local mean sidereal time will be incorrect by UT UTC which is less than 1 second Parallactic Angle This quantity the position angle of a great circle connecting the object to the zenith is often used for setting a spectrograph slit along the angle of atmospheric dispersion Its name arises because it is the position angle along which a very nearby object such as the moon will be displaced by topocentric parallax Its importance for spectrophotometry is emphasized and quantified by A Filippenko 1982 PASP 94 715 He gives formulae for the parallactic angle but leaves unclear a choice of root of an inverse trig function my own routine computes the altitude and azimuth which allows an unambiguous choice of root since more elements of the spherical triangle are known Because some applications such as spectrograph slit angles are indifferent to 180 degree rotations of this quantity the option also prints the antiparallel angle parallactic 180 in square bra
53. lso designed so that you can test run it interactively to reconnoiter the inputs it will require and the options available The program first asks you to select a site and prints a menu of canned possibilities You can select one of the canned sites or enter all the parameters for another site The last input prompted for is the year for which to print the calendar giving a negative year here exits the program Before producing the calendar then your first step should be to run the program interactively mostly to be sure which canned sites are available in your own version of the program After that you create a little procedure or job with the output redirected into a file to make the calendar itself The exact format of this job will depend on your system and on just what you want to do but the sequence of inputs you need is system independent Here are some annotated examples the text to the right is commentary not to be put in the job itself Example 1 for one of the canned sites k code for kitt peak assumed to be one of the canned sites 2 do format for TeX printing 2 months per page 2002 year for which calendar is to be run Example 2 for another site n new site not one of the canned ones 6 16 56 WEST longitude HOURS MINUTES SECONDS 44 44 42 latitude DEGREES MINUTES SECONDS 0 site elevation in meters above effective horizon 6 standard time zone hours WEST of Gre
54. mulae for Calculators Third Editions 1985 Willman Bell Richmond The outer planets have such large mutual attractions that satisfactory positions can only be had by including a fair number of perturbation terms I have included the largest ones from Meeus Chapter 24 The results are generally good to about 0 1 degree for Jupiter and to a few tenths of a degree for Saturn Uranus and Neptune For Pluto I have simply adopted the osculating elements for 1992 These give very good positions for 1992 which slowly deteriorate the farther one gets from this date The planetary positions are used in two ways They can be printed out in a table using the option m which stands for major planets p is already used for proper motion More subtly when one prints circumstances using the command the program computes the planetary positions and checks to see if your current RA and dec are within 3 degrees of any major planet If they are it 28 warns you The idea here is to avoid trying to observe some faint object with say Jupiter right next to it the 3 degree tolerance was chosen as being about the radius of a Schmidt plate For asteroids you re on your own Geographical limitations The daylight savings time conventions used are limited to those which are coded If you want to extend these to use at other sites you have to code the new convention into the program and assign it a numerical code negative numbers refer to sou
55. n between true and mean sidereal time at local midnight it is more accurate than the nearest second accuracy which is tabulated Again if daylight savings is in effect this is the JD at daylight savings midnight The next four columns give respectively the times of sunset evening 18 degree twilight morning twilight and sunrise Thus the columns are in the same sequence as the actual events which seems TeX is probably someone s trademark or something 35 natural The twilight given is 18 degree astronomical twilight and the rise set times given are when the center of the sun is 50 arcminutes below the horizon this is roughly the time when the sun s upper limb touches the horizon once refraction and the sun s angular diameter are taken into account If the observatory s elevation above its surroundings are specified the depression of the horizon is taken into account Only a single number is used for observatory elevation above the surroundings there s no attempt to account for the topography of the horizon Accuracy is as discussed above If an event doesn t happen during a night e g twilight at high latitudes in summer ellipses are printed in the appropriate column The next two columns give a very useful quantity namely the sidereal times at evening and morning twilight This defines the range of RA which is accessible on the meridian during the night The last five columns pertain to the moon
56. ne thing at a time You re prompted for the name of a log file When the log file is open almost all the program s output except for prompts is also written to the log file If you print out the online documentation such as the menu and fast tour this goes to the log file as well The log file has minor format differences from the terminal output to make it easier to read If you type xL again while the log file is open it closes the log file so xL acts as a toggle The log file is opened with append permission so if it exists already new output is written to the end More general output redirection using the operating system is not as simple unless your system supports something like the UNIX script command and details are system dependent You ll generally have to prepare a list of commands to feed into the program Here s a sample of what the command input file might look like in a typical case for the h command the comments are for human legibility and are not to be included input comment k selects Kitt Peak assuming it s in site menu y 1990 10 20 date e 2000 select epoch of input coordinates r 19 19 19 coordinates ra and dec d222 h hourly circumstances command Wholeflaffer 9 name of object the output will follow r 20 20 20 for next object d 12 12 12 h Flapdoodle s Variable Nebula and so on r d h followed by the name until Q exit program Note A pr
57. necessary to fit it onto PC class machines unless they re running Linux on which it runs perfectly There is a provision to write calculated results directly to a log file without having to use operating system output redirection The program is designed as follows You specify information about your site the coordinates of your object the date and time or whatever else is relevant using very simple commands and a flexible obvious format Then you give a command to do calculations and put out results some of these commands prompt for further needed information The output commands are explained in more detail later but the following summary gives some idea of the program s aims h The hourly airmass command prints a table of the airmass hour angle and other information for each hour during the night Users tell me that they use this more frequently than any of the other options It uses the date the site and the object coordinates a The nightly almanac command tabulates information about a single night including times of sunset sunrise twilight moonrise or set the moon phase moon coordinates the moon phase at midnight sidereal times at midnight and twilight and other such It uses only the site information and the date This prints the instantaneous circumstances for your observation it uses practically all the input information site RA and dec date and time and tells you the airmass precessed coord
58. ou ve specified is within 3 of the computed low 11 precision position of any major planet If it is the program warns you This way you won t try to set on your 98th magnitude object only to find that Jupiter is 5 arcminutes away Yet another feature reports if an eclipse of the sun or moon in progress The accuracy expected of these predictions is discussed later The o command Observability through a season The next suggestion in the guided tour is to explore the observability of your object through an observing season by typing o The output is designed for use before you apply for telescope time it supplies you with accurate information to allow you to decide the range of acceptable dates by printing a summary of the observability of your object as specified by the RA dec and epoch You are first prompted for the range of dates to cover in standard y m d form A 6 month span fits in a standard 24 line screen if that it important It next asks for the altitude of the sun to be used for twilight 18 is standard astronomical twilight but this is very dark so you may wish to relax this condition some if you can live with a little sky light Finally it prompts for an object name as in h above simply to use as a label for any hard copy you might make The object we ve been working with at about 15 hours would be in the first semester of the year Typing 2002 1 30 and 2002 7 30 and standard twilight plus a n
59. planets print 0 1 deg positions for specified instant x eXtra goodies galact eclipt var star predicts precess Q QUIT STOPS PROGRAM gt I ve found in testing that this menu is good for reminding advanced users of commands but poor for teaching new users how to run the program Hence it is not mentioned in the introductory banners but rather the user is referred to the fast guided tour invoked with f 15 A word about extra goodies x You ll notice an extra goodies option on the main menu This option hides some arcane and or complex commands Putting them here keeps the main menu short enough to fit into a 24 line display If you type x you ll get a summary which describes these selections The extra goodies command level does not loop but drops you immediately back into the main menu whatever you do so you will have to prefix any new extra goodies command with another x Setting the time to now T An UPPER CASE T sets both the time and the date using your machine s system clock As noted earlier this option may be switched off at compile time if there is some problem with this There s an interesting twist here you are prompted Set how many minutes into the future Answering O sets the time and date to right now any other number sets the time and date into the future or past for negative numbers This lets you quickly answer questions such as Can I get to this object half an hour from now
60. ple matter to extend the calendrical routines I worry about the wisdom of this because I have not tested the accuracy of the celestial calculations far outside of the present The routine which converts julian date back to calendar date which can be accessed directly with the xj command has 25 a wider range of validity it agrees with the Astronomical Almanac 1995 page K4 from 1600 to 2100 at least When printing the phenomena for a given night the program assumes implicitly that zone time at least grossly approximates local time Thus working from a California location zone 8 or Pacific time and attempting to get times printed as UT by giving a standard time zone as 0 will give peculiar behavior The g option allows input in UT As previously noted daylight savings time is implemented using hard coded algorithms to de termine the dates on which the clock time changes If your location uses some different algorithm you ll have to implement it in the source code Note that if you use daylight time and as is the default specify your input times in local time difficulties arise when daylight and standard times switch When daylight savings switches back to standard time fall back the numerical value of the time repeats for an hour going the other way spring forward there is an hour of non existent local times The program handles these conditions as follows If the specified time is within about 12 hours of the switch
61. program gracefully The Q must be upper case this should avoid accidents well enough Printing a menu This causes the following menu to print out Circumstance calculator type for output Commands are SINGLE lower case CHARACTERS as follows prints this menu other information options are i f i prints brief Instructions and examples f fast tour w prints info on internal Workings accuracy amp LEGALITIES TO SET PARAMETERS amp OPTIONS use these follow the formats r enter object RA in hr min sec example r 3 12 12 43 d enter object Dec in deg min sec example d 0 18 0 y enter date starting with Year example y 1994 10 12 t T t enter time e g t 22 18 02 see g and n T right now n toggles whether date is used as evening default or literal g toggles whether time is used as Greenwich or local e enter Epoch used to interpret input coords default 1950 p enter object Proper motions complicated follow prompts s change Site again follow prompts 1 Look at current parameter values no computation TO CALCULATE AND SEE RESULTS use these commands type out circumstances for specified instant of time ra dec a type out night s Almanac for specified evening date h type out Hourly airmass table for specified date ra dec o tabulate Observability at 2 week intervals at full amp new moon m Major
62. r JD 2452369 792 Local Mean Sidereal Time at midnight 12 27 11 4 Sunset 700 m horizon 18 52 MST Sunrise 6 06 MST Evening twilight 20 12 MST LMST at evening twilight 8 38 Morning twilight 4 47 MST LMST at morning twilight 17 15 12 degr twilight 19 42 MST gt 5 16 MST night center O 29 MST Moonrise 2 19 MST Moon at civil midnight illuminated fraction 0 430 0 6 days after last quarter RA and dec 19 43 32 24 32 7 The sun is down for 11 2 hr 8 6 hr from eve gt morn 18 deg twilight 6 1 dark hours after end of twilight and before moonrise Note that the date you have given is interpreted as the the local date for evening the rest is largely self explanatory but a few remarks are in order The times of moonrise or moonset are reported only if they occur when the sun is down or close to it The phase of the moon is printed together with its illuminated fraction and celestial coordinates these are computed for local midnight whether or not the moon is actually up at the time If the observatory elevation is non zero an approximate correction is applied to the times of moonrise and moonset this is discussed more fully in the section on algorithms and accuracy Specifying ra and dec r and d hourly airmass h Now let s get a little more specific and consider observing a particular object The guided tour now suggests you specify an RA and dec and make an hourly airmass table by typing r 15 38 29 2 d 0 01 02 h
63. rameters for your site The 1 command lays out the site parameters neatly for your inspection and many are echoed with other output Once you re happy with your site parameters you can save them in a file by re running the s command and writing them out with W after which you can read them with s and R The site files are ASCII the first line is the site name the next the zone name either of which can have blanks and the last line has the single character zone abbreviation the daylight savings switch parameter the longitude in decimal hours the latitude in decimal hours the zone offset the elevation above sea level and the elevation above the horizon terrain The parameters in the file are in the same form as the variables used in the program decimal hours etc so it can be used as a guide if you wish to hard code a frequently used site 17 UT time input and night dates g and n Typing the letters g or n switches between various options for the interpretation of input times and dates The g command switches between input in UT and in local time The program wakes up assuming that dates and times are input in local time typing g makes the program assume that the input date and time are in UT a little message is printed telling you this Typing g again switches back to local and so on Whichever time is assumed for the input is printed first on output Notice that when you type g the current time changes the input time
64. riage returns this is four keystrokes Setting to the zenith with xZ This command simply sets the right ascension and the declination to those of the zenith for the currently defined date time and site It is a capital letter because it changes two quantities at once The coordinates of the zenith are precessed from the present epoch for which they are just the sidereal time and the latitude to the standard input epoch defined by the e command Object lists the xR xl xN and xS commands Overview These commands were added in 1994 February they allow one to read coordinates from a file list them and select from them using various criteria These capabilities are quite powerful but add some complexity so the commands are hidden among the extra goodies At their simplest these commands allow you to set the RA and dec to those of a given object without having to type the RA and dec into the program But they are far more powerful than this for one thing the commands to display and select from the object lists also give the hour angle and airmass for each object at the currently defined site date and time This was designed for use at the telescope after setting the date and time to the present with T perhaps you can quickly scan a list of objects and select one which is well placed for observation The xS command goes further and presents the list sorted according to various criteria The file format for objects is s
65. ropagated xv prints error bars in minutes as to when the phenomenon is predicted to occur and xf prints the accumulated error in the phase at the tabulated time intervals Both tasks prompt extensively for input parameters but the celestial coordinates and site information are inherited from the main program xp Parallax factors This prints out the annual parallax components in RA and dec for the currently selected RA dec date and time in units of arcsec for a hypothetical star 1 pc distant real stars will have a parallax displacement equal to their parallax times these numbers The XYZ coordinates of the earth with respect to the solar system barycenter are also shown Finally the aberration of light due to the earth s motion is shown 23 xy Print day of year Computes and prints the day of the year UT The day number is given as a fraction the first moment of January 1 UT corresponds to day number 1 000 24 1 3 ALGORITHMS ACCURACY AND LIMITATIONS Calendar and times The time arguments for most of the routines are Julian dates implemented as double precision floating point numbers If your machine s double precision mantissa isn t reasonably long you can run into serious inaccuracy Digital s VAX machines express a JD to a few milliseconds accuracy but this should be checked when the code is ported to another architecture Calendar dates and days of the week are derived from a truncation of the Juli
66. rs west of Greenwich e g Pacific is 8 You can specify Eastern hemisphere sites by giving negative numbers for the longitude and time zone You ll also be prompted for the site s elevation above sea level which affects certain quantities slightly and its elevation above its horizon which is used only to adjust rising and setting times The last parameter prompted for is whether daylight savings time is to be used in converting between local and UT There are several options given here Typing 0 ignores daylight time Typing 1 invokes the conventions in use in the United States daylight savings starts on the first Sunday in April and ends on the last Sunday in October from 1986 on and from the last Sunday in April before that This ignores various wrinkles during wars energy crises etc Typing 2 gets you the Spanish Continental convention with daylight savings from the last Sunday in March to the last Sunday in September Negative numbers are used for southern sites typing 1 gets you the Chilean convention off daylight savings the second Sunday in March back on the 2nd Sunday in October and 2 gets the Australian convention Implementing other prescriptions would require fairly straightforward modifications to the source code The presently available prescriptions all assume that the time changes at 2 AM as reckoned in the time preceding the change as is standard in the US Naturally you should be sure that you have the correct pa
67. ry used As noted earlier under the command the program prints a notice if a solar or lunar eclipse is in progress The solar eclipse state is found very directly by computing the topocentric angular radii of the sun and moon and comparing with their topocentric angular separation The lunar eclipse calculation uses a simple geometrical model of the earth s shadow taking into account the distance of the sun at the moon s geocentric distance Although the program does not generate eclipse timings directly one can manually iterate to obtain times of eclipse contacts This provides an exacting test of the lunar ephemerides and the topocentric correction For solar eclipses the timings agree to within about 1 minute with the definitive ephemerides e g F Espenak and J Anderson NASA reference publications Nos 1301 and 1318 1993 a 1 minute timing error implies 30 uncertainty in the moon s longitude Lunar eclipse contacts are accurate to within about 5 minutes with residual 26 differences apparently due to the simple model used for the earth s shadow Thus these programs should not be used for the most critical eclipse calculations I do not know over what range of dates the lunar ephemeris can be expected to work well but it works nicely toward the end of the twentieth century The printed phases of the moon are based on Meeus algorithms which he claims are good to 2 minutes Explicitly printed positions of the
68. sun are also from algorithms derived from Jean Meeus As tronomical Formulae for Calculators These positions are referred to the mean equinox of date A topocentric correction is applied which amount to at most 8 8 arcsec Spot checks of the routine itself modified for this purpose to show geocentric apparent rather than mean coordinates gave agree ment to a few arcseconds Rise set times are derived using the Astronomical Almanac low precision formulae for the sun which are advertised as good to about 0 01 If the observatory elevation above its horizon is specified as zero the rising and setting times of the moon and sun are taken to be the times when the center of the object is 50 arcmin below the geometrical horizon This is about the time of contact of the upper limb with the horizon once refraction is taken into account Variations in the apparent diameter of the sun and moon are ignored If the observatory elevation above its horizon is non zero an approximate correction is added to the zenith angle at which rising and setting are reckoned this is horizon correction radians where e is the observatory s elevation above its surroundings and R is the radius of the earth In principle a more accurate correction would simultaneously consider the effect of elevation on the refraction although this doesn t make a great deal of difference see B E Schaefer and W Liller 1990 PASP 102 796 Table 4 In extreme cases Mauna
69. ted and the user selects an object and sets the RA and dec by giving its number on the list Typing m gives the next 10 objects while q quits the search without assigning coordinates The search continues until one selects and item or quits or until the list is exhausted The power and fun of this command comes in the order in which the objects are presented the objects are sorted according to various criteria hence the choice of letter There are at present five different options for the sort 22 xS1 sorts the objects in order of increasing distance from the presently defined RA and dec it s a find nearest There are many ways to use this If you set to the zenith with xZ the objects will be presented in order of zenith distance If you have an object on your list whose coordinates you remember roughly but you don t remember exactly what you called it you can find it quickly by setting to the rough coordinates and then finding the exact match You can also use it to find an object close to the one you re observing to avoid spending too much time slewing or to match airmasses but see option xS3 below xS2 sorts the objects in order of the absolute value of the hour angle Therefore it shows you which objects are closest to the meridian at the presently defined moment of time xS3 sorts the objects in order of proximity in airmass to the present coordinates This could be useful to photometrists and infrared
70. thern sites daylight savings in for instance December and positive to northern sites The routine to modify is called find_dst_bounds The algorithms used for rising and setting work at tropical and temperate latitudes and have been retrofitted to work at very high latitude As noted above rise set times are not as accurate at circumpolar latitudes as they are closer to the equator and they are meaningless at the geographical poles The code has not been tested exhaustively at very high latitudes nor has it been tested at length in the southern or eastern hemispheres but there are no reasons for expecting it won t work there Problems with computation of moonrise etc which should arise only at extreme latitudes are announced by a message reading Moonrise or set calculation not converging These problems can arise because at very high latitudes phenomena such as sunrise twilight and moonrise do not always occur Thus the almanac section of the program tests that each of these phenomena are likely to occur before attempting to compute when they do occur In this test it uses the declination of the relevant body computed for local midnight this can cause a mistake especially for the moon which can change declination quickly This should seldom be important Precession and apparent place The precession algorithm is coded from L Taff s very useful book Computational Spherical As tronomy Wiley It uses a rotation matrix w
71. traightforward Files are expected to contain one object per line the reading is done one line at a time so an error in one line does not propagate to the next Here s an example of a correctly formatted line vi727_cyg 21 29 36 2 47 04 08 1950 18 1 binary xrs 5hr The data fields in each line must be separated by blanks or tabs but otherwise are free format The first field is a name which must be less than 20 characters and cannot contain any blanks It is helpful if you keep the names simple so you can remember them exactly Next comes the RA in hours minutes and seconds three fields then the dec in degrees minutes and seconds three more fields All these numbers are read as floating point so for example 19 30 5 0 would be equivalent to 19 30 30 If the declination is negative the first character of the degree field must be a minus sign and there must be no space between the minus sign and the remainder of the number A 0 declination is handled correctly In the eighth mandatory field is the coordinate epoch Finally the ninth field may contain an optional user supplied floating point number which might be a magnitude 21 or perhaps a priority for observation Any further entries on the same line are ignored so you re free to put notes or other information there as above This information will not be read at all it s only there for your own benefit say for a printed copy of the list If the program does not successf
72. u would like your own copy it can be obtained via anonymous ftp from iraf noao edu it s in the contrib directory under the name skycalc You ll need access to a UNIX machine to unpack the tar file but the code itself should run on almost anything with a C compiler 1 THE INTERACTIVE ALMANAC Overview This is designed to provide quick and easy access to astronomical quantities of interest to an observer at the telescope and to ease the planning of especially nighttime observations This may seem unnecessary since so many powerful desktop planetarium programs are available While these are very impressive and often very useful they don t always provide the information needed by professional astronomers in the most useful form Furthermore they are generally wedded to a particular architecture and operating system generally PCs or Apple machines Elwood Downey s magnificent zephem program is an exception but it s aimed somewhat differently than the present program I wrote this program to serve my own needs which are broadly typical of the professional ob serving community Many amateurs may find it useful as well It is written in garden variety C and 3 has a dead simple user interface you type then it types no mouse no graphics nothing difficult to move from machine to machine Commands are terse The code is designed to run on workstation class machines ubiquitous in professional circles modifications are
73. ully read the eight mandatory fields from each line a complaint is printed and the line is ignored If the optional user supplied floating point number is not supplied it is automatically assigned a value of 99 9 This choice is arbitrary but it s a rational choice if you re using the field for magnitudes and it doesn t crowd any of the later displays You are allowed up to 2000 objects in your list If you want more you can change the defined constant MAX_OBJECTS in the source code and recompile The information stored for each object amounts to something like 48 bytes depending on your machine so the 2000 object limit is about 100 k most users could expand this without running into memory limitations The xR command reads objects from a file it briefly reviews the file format and then prompts for the name of the input file If you specify the filename QUIT all upper case it does not attempt to open a file If you specify a file which the program cannot open it complains and exits back to the main command level If the file does open but you already have objects in memory it asks you whether you would like to append the new objects or replace the old ones with the new ones The program then reads the file complaining if it finds any obvious anomalies blank lines non number in fields supposed to be numbers or whatever Finally it reports how many objects you have closes the input file and returns If you ve filled up to the m
74. xtreme accuracy is critical or for locations at extreme geographical positions it s a good idea to read up on the algorithms and their limitations And it s always the user s responsibility to be sure the answers are sensible Be especially careful when the code is recompiled on your local machine experience shows that compilers can generate different answers from the same code The examples below can be used to check that your local compiler gives good answers With these caveats we proceed to 1 1 BASIC USE OF THE PROGRAM Before starting note that you should ideally be able to get going with the program in about 10 minutes without referring to this document using the fast guided tour option and other help text I ve established this empirically with volunteer astronomer subjects Nonetheless going through the program with this document in hand should give a more thorough understanding of the program and draw attention to its capabilities This document should be a useful reference especially for questions of accuracy generality and such I assume that the user is familiar with the concepts of celestial coordinates sidereal time and so on the program has no provision for teaching complete novices Note that the program doesn t prompt you under most circumstances which can be disconcerting but typing a few empty carriage returns in a row will usually point you towards help Starting Up Begin by running the progr

SKYCALC USER`S MANUAL John Thorstensen, Dept. Physics and

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