Observing

The observe Program

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:

• Minor corrections such as parallax and nutation are not made.
• Positions are accurate enough for most applications other than occultations.
• Rise and set times may be off by several minutes.
• Positions of Saturn's satellites are approximate, good enough for identification of satellites.

Controls and Input for the observe 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.

`-m'

Meridian of longitude, measured East of Greenwich. The USA is West of Greenwich, and the longitude is negative for all USA locations. One to three numbers can be used: degrees, degrees and minutes, or degrees minutes and seconds.

`-l'

Latitude. One to three numbers can be used: degrees, degrees and minutes, or degrees minutes and seconds.

`-a'

Altitude in meters.

`-z'

Time zone in hours East of Greenwich, again, this number is negative for USA locations. This does not include any effects of daylight savings.

`-d'

The date or dates of observation in UT are specified with this flag. The dates for the `-d' option are each specified as a string consisting of month, day, and optional year (use if different from the current year). The month may be encoded as above, e.g. `Aug' for august. Using the first three letters of the English name for the month always works, as does the Roman numeral form. Some other common abbreviations also work. The year may also be specified, the default is the current year. The day may be fractional, e.g. 1.25 is 6 am UT on the first. An optional third parameter is the increment of time to be used in stepping between the two dates.

`-o'

This sets names used for the output files and controls which output files are produced. The output file names all have the form `outfile_root.XXX', where outfile_root is set at compile time (usually to `planet'), or is set to the argument for this option. The `-o' may be followed with a letter or letters from the set "aeios" to select the altaz, eph, sif, obs, and star files, respectively. e.g. `-oae austin' would produce only the output files "austin.altaz" and "austin.eph".

`-p'

The positions of any or all of the major planets at the time(s) may be calculated. This is specified either as `-p' which causes the positions of all planets to be calculated, or individual planets may be specified by following the `-p' with a letter or letters from the sequence "MVmJsUN". The positions of the sun and moon are always calculated, since they always have some effect on observing conditions.

`-s'

The -s option causes the `.sat' and `.sat_PS' files to be produced for the satellites of Jupiter and Saturn. `-s' implies `-p'. With the `-si' option the drawings in the PS file are flipped north to south (if for one time) or east to west (if for multiple times) to produce an inverted view.

`-f'

Input object data is contained in files in several formats. The `-f filename format' option sets this input file.

`-n'

For some input file formats, the name of the object must be specified using the `-n' option.

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:

`altaz'

altitude and azimuth of objects at sun rise and set, and morning and evening twilight.

`eph'

ephemeris of sun, moon and objects specified.

`obs'

observability of objects: rise and set times of objects, twilight times, etc.

`star'

`lineread' format file containing coordinates (equinox 2000) of the object(s), sun, moon.

`sif'

`sif' format file containing the same information as the .star file. The separation character is `;'.

`sat'

Locations of the major satellites of Jupiter and Saturn with respect to the primary.

`sat_PS'

PostScript file drawing either: one page showing appearance of Jupiter and Saturn with satellites, and relative sizes and orientations of Mercury, Venus, Mars, Jupiter, and Saturn, and the Moon; or several pages showing Jupiter and Saturn with moons on a sequence of times if a range of dates was specified.

Planetary Positions

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 Planets

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.

Orbits

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:

`a'

semimajor axis, A.U.

`q'

perihelion distance, A.U.

`e'

eccentricity

`i'

inclination (degrees)

`omega'

argument of perihelion

`Omega'

longitude of ascending node

`n'

mean motion (degrees/day)

`M'

Mean anomaly at epoch

`T'

Time of perihelion.

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'.

Interpolation of Ephemerides

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:

`emp'

Format used in the Russian Ephemerides of minor planets.

`empb'

Format used in the Russian Ephemerides of minor planets for bright and unusual asteroids.

`aa'

Format used by the Astronomical Almanac.

`st'

Format commonly used by Sky and Telescope magazine.

`iau'

Format commonly used by IAU Circulars.

Time Line of Events

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.

Satellites of Jupiter and Saturn

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.