This program is used to prepare for astronomical observations and for ephemeris calculation. It can calculate the position of the major planets, the satellites of Jupiter and Saturn, and minor planets and comets given either orbital elements or a tabulated ephemeris. Coordinates of stationary objects may also be input. It calculates (approximate) rise and set times, and transit times, of all objects to be observed. It calculates the (approximate) altitude and azimuth of objects at sun rise and set, and morning and evening twilight.
The information (coordinates, rise and set times) may be prepared for either a single time or a sequence of times.
Output includes a file containing the timetable of events for the
evening or evenings. The coordinates of the sun and moon, plus any
planets, minor planets, and comets are placed in a readable text file,
with other calculated values for solar system objects (e.g. distance to
earth). The coordinates of all objects to be observed are placed in
files in formats to be read by the other programs in this family, namely
dataconv and the starchart charting programs. A file listing the
(approximate) altitude and azimuth of objects at sun rise and set, and
morning and evening twilight is output. If satellite positions are to
be computed, these coordinates are placed in one file, and a separate
PostScript file graphically showing their positions relative to the
primary is also produced.
These output files are optional: any subset of the possible output files may be produced.
Parameters set the location of the observer, control what objects are to be observed, and designate the time or times of interest.
Currently, there are some limitations to the program:
Command line options set the location of the observer, control what objects are to be observed, control the output files produced, and designate the time or times of interest. The default location of the observer is set when the program is compiled.
Sample data files should be used for the file formats used for input files. The format names are `obj' for fixed objects, `ell_e' and `par_e' for parabolic orbital elements, and `emp', `empb', `aa', `st', and `iau' for tabulated ephemerides. The fixed object format contains the coordinates, name, magnitude, type and size of objects to be observed. The other file formats are used for comets and minor planets, and are discussed below.
The output files are named `outfile_root.XXX' where XXX is:
Planetary positions are calculated with moderate precision. Formulae are largely from Astronomical Formulae for Calculators by Jean Meesus. Minor corrections are currently ignored.
Comets and minor planet positions may be calculated either directly from the orbital elements or by interpolating a tabulated ephemeris. The calculation from orbital elements is most convenient, but the tabulated ephemeris is likely to be slightly more accurate.
The coordinates of objects in elliptical or parabolic orbits may be calculated from orbital elements given in files of format `ell_e' and `par_e' respectively.
Orbital elements are:
For elliptical orbits, `q' and `T' are not needed; for parabolic orbits, only `q', `i', `omega' and `Omega' are needed.
Orbital elements are referred to a certain equinox, and apply on a certain epoch date.
If `a' is not given, it may be calculated from `a' = `q'/(1-`e').
If `n' is not given, it may be calculated from `n' = 0.985609/(`a'*sqrt(`a')).
If `M' is not given, use `M' = (`Epoch_date' - `T') * `n'.
The magnitude of an asteroid may be calculated from two parameters: `G' and `H'. The magnitude of a comet may be calculated from the parameters `g' and `kappa'.
All ephemeris formats have date, RA, DEC, then other info. Date is the month in characters, then the day of the month. How the month is encoded is format dependent, examples include `IX' `Sept.' `Sep' `Sep.' `September'. Year is current year unless specified in command line, and is the year of the first date. Dates must be in increasing order: 3 followed by 4, December followed by January.
The formats are:
The `.obs' file contains a timeline of events for the night or nights of observation. Events include sun and moon rise and set, morning and evening twilight, and for each object to be observed: the rise and set times, the times when they are 20 degrees and 30 degrees above the horizon, and the time of their transit (when they cross the line from north to south passing directly overhead, and are at their maximum altitude above the horizon).
The times, especially sun and moon rise and set are approximate.
The timeline should help you plan your evening, so you know when to begin and end (twilight), and when the moon will interfere. It helps you plan to observe the objects when they are well placed, and ensure that you can observe an object before it has fallen too low in the sky.
The positions of the major satellites of Jupiter and Saturn are calculated for the time or times of interest. The Saturn satellite positions in particular are approximate, but are certainly good enough for identification purposes.
The positions are output in the `.sat' file, and also illustrated in PostScript in the `.sat_PS' file.
If more than one time is specified, the PostScript program draws many Jupiters and Saturns with their moons on a page (with separate pages for Jupiter and Saturn). If one time is specified, a single page is produced containing large drawings of Jupiter and Saturn with their moons. As a bonus, this single page also illustrates the phases and orientations of Mercury, Venus, Mars, Jupiter and Saturn (with rings), and the moon.