Astronomical Almanac 2000

Various Definitions (some accurate some are approximations)


Universal time is defined in terms of greenwich mean sidereal time.

Greenwich mean sidereal time (GMST) is defined as the Greenwich hour angle of the mean equinox of date.


Page 1 This page defines sidereal time, then right ascension, declination etc etc and the equation of time

Page 2 This page finalizes with the calculation of rising and setting times.

As you can see (from above and below) sidereal time is related to the tropical year since both are referenced to the equinox however a sidereal *year* is independently referenced to the fixed stars. i.e. sidereal time and a sidereal year are defined differently. This is the point I tried to make on rec.astro? many years ago and only a man from NASA followed and agreed. I must admit it is weird and likely the result of some committee decision.


Also note the following year length definitions from page C1

The lengths of the principal years at 2000.0, as derived from the Sun's mean motion are:

Tropical Year (Equinox to Equinox) = 365.242190 = 365d 05hr 48min 45.2sec

Sidereal Year (Fixed Star to Fixed Star) = 365.256363 = 365d 06hr 09min 09.8sec

Anomalistic Year (Perigee to Perigee) = 365.259635 = 365d 06hr 13min 52.5sec

Eclipse Year (Node to Node) = 346.620075 = 346d 14hr 52min 54.5sec


Also note the day length definitions from page B6

In 2000:

1 Mean Solar Day = 1.00273790935 Mean Sidereal Days = 24hr 03min 56.55537sec

1 Mean Sidereal Day = 0.99726956633 Mean Solar Days = 23hr 56min 04.09053sec


Also note the lengths of mean lunar months from page D2

The lengths of the mean months at 2000.0, as derived from the mean orbital elements are:

Synodic Month (New Moon to New Moon) = 29.530589d = 29d 12hr 44min 02.9sec

Tropical Month (Equinox to Equinox) = 27.321582d = 27d 07hr 43min 04.7sec

Sidereal Month (Fixed Star to Fixed Star) = 27.321662d = 27d 07hr 43min 11.6sec

Anomalistic Month (Perigee to Perigee) = 27.554550d = 27d 13hr 18min 33.1sec

Draconic Month (Node to Node) = 27.212221d = 27d 05hr 05min 35.9sec


A Request

If anybody has high accuracy mean planetary periods (to many decimal places) please let me know. I'm particularly interested in the 5 visible planets.