Notes for Skycalc (V4)

Skycalc is a program useful for performing many useful calculations, some of which I have provided a Web interface for. The code for skycalc, packaged with the skycalendar notes, may be obtained via anonymous ftp .

Skycalc is written by John Thorstensen (john.thorstensen@dartmouth.edu)
Web version by the User Support Departmnent (usd-help@eso.org)

You can jump to notes on accuracy , warnings and legalities , and the web usage of skycalc.

Accuracy Information

The distinctions between UTC, UT1, TDT (etc.) are ignored except that a rough correction to TDT is used for the moon. The solar ephemeris used is good to a few arcsec. Moon positions are topocentric and +- about 30 arcsec, hence solar eclipse paths are +- 50 km and +- 1 min. All rise/set times are computed to about +-1 min; non-level horizon, site elevation, and refraction uncertainties are often larger than this.

The lunar sky brightness model assumes ideal atmospheric conditions; true lunar contributions to sky will vary widely. To compare a dark site has about V=21.5 mag/sq.arcsec (variable)! Twilight brightness prediction is for blue, and only very approximate.

The planetary calculations are truncated, but the error should seldom exceed 0.1 degree; MV are best(1'), MJSU ok, Pluto worst.

Note that the local sidereal time given is Mean, not true, and that it assumes the input is true UT, not UTC (< 1 second)

Daylight savings time, if selected, is established using a site-specific convention (e.g., USA). Beware of ambiguities and nonexistent times when the clock is reset. If necessary, use the 'g' option and enter times and dates as Greenwich (UT), or disable DST in site params (see discussion under 'i').

The precession routine used is a 'rigorous' rotation matrix routine, taken from L. Taff's Computational Spherical Astronomy . It uses IAU1976 constants, is good to < 1 arcsec in 50 years, and has no troubles near the pole. Proper motion corrections are done crudely as x = x0 + mu * dt; this is inaccurate near the poles. Use another routine if sub-arcsec accuracy is critical. Apparent place (with nutation, aberration) is NOT computed.

The parallactic angle follows Filippenko (1982, PASP 94, 715).

The barycentric ('heliocentric') corrections are computed using an elliptical earth orbit with a few periodic perturbations including lunar recoil. The helio-to-barycentric transformation uses the same algorithms as the planetary positions. Output values are rounded to 10 m/s, which is slightly higher than the overall maximum error (< 5 m/s). Velocity correction includes Earth rotation and observer's elevation above sea level.

The galactic coordinate routine is rigorously accurate, and precesses to 1950 before transforming. The ecliptic coord. routine is for coordinates of date and is good to < 1 arcsec.

These routines are not necessarily correct at times very far from the present (1990s). The program rejects input outside 1900-2100.

When porting to a new machine, run tests to ensure correctness and accuracy. Experience shows that compiler peculiarities arise surprisingly often.

Cautions, Legalities

Many routines take a time argument which is a double-precision floating-point julian date; on most workstations, this gives time resolution of < 0.1 second. When porting to another machine or compiler, test that the accuracy is sufficient.

I (JT) cannot guarantee that this program is bug-free, and caution that not all routines are thoroughly precise and rigorous. The user of this program is responsible for interpreting results correctly. I disavow any legal liability for damages caused by use of this program.

Program copyright 1993, John Thorstensen, Dartmouth College Permission hereby granted for scientific/educational use. Commercial users must license. Please communicate problems or suggestions to the author, John Thorstensen, Dept. Physics and Astronomy, Dartmouth College, Hanover NH 03755 John.Thorstensen@Dartmouth.edu

Web Version

The web wrapper around skycalc has been written by the User Support Department for some specific calculations.