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

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 ignoredexcept that a rough correction to TDT is used for the moon.The solar ephemeris used is good to a few arcsec. Moon positionsare topocentric and +- about 30 arcsec, hence solareclipse paths are +- 50 km and +- 1 min. All rise/set times arecomputed 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 comparea dark site has about V=21.5 mag/sq.arcsec (variable)! Twilightbrightness prediction is for blue, and only very approximate.

The planetary calculations are truncated, but the error shouldseldom 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 asite-specific convention (e.g., USA). Beware of ambiguitiesand 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 correctionsare done crudely as x = x0 + mu * dt; this is inaccurate nearthe 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 anelliptical earth orbit with a few periodic perturbations includinglunar recoil. The helio-to-barycentric transformation uses the samealgorithms as the planetary positions. Output values are rounded to 10m/s, which is slightly higher than the overall maximum error (< 5m/s). Velocity correction includes Earth rotation and observer'selevation above sea level.

The galactic coordinate routine is rigorously accurate, andprecesses 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 fromthe present (1990s). The program rejects input outside 1900-2100.

When porting to a new machine, run tests to ensurecorrectness and accuracy. Experience shows that compilerpeculiarities arise surprisingly often.

Cautions, Legalities

Many routines take a time argument which is a double-precisionfloating-point julian date; on most workstations, this givestime resolution of < 0.1 second. When portingto another machine or compiler, test that the accuracy issufficient.

I (JT) cannot guarantee that this program is bug-free, and cautionthat 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 damagescaused by use of this program.

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

Web Version

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