A clock found in every astronomical observatory, which is set to register oh om os when 0° Aries is on the Zenith. Formerly a noon point, but since 1925 a midnight point, it moves forward in the zodiac by 1°, or 4 minutes, each day, hence the Sidereal Time at noon (or midnight if since 1925) on any day shows what sign and degree is on the M.C. at that particular moment. For example, ST at 0h, or midnight, on May 1, 1945 is 14h 34m 14s: approx. 874m / 4 = 218 degrees = approx. 8° Scorpio on M.C. The Sidereal Clock indicates 24h, while the solar chronometer registers 23h 56m 4.0906s of Mean Solar Time. It does not register A.M. or P.M., but divides the dial into 24 hourly periods. The so-called Army and Navy time of World War 11 indicates the eventual universal use of the same system applied to solar time, whereby for example, 2 P.M. will be known as 1400.
After the Sidereal clock has been set at 0h to coincide with the moment of the Earth's crossing the intersecting point of the Ecliptic and Equator, the next noon it will read something like 12:04 - the distance the Earth has travelled in orbit in one solar day, shown in units of time. Thus each successive day at noon it shows the cumulative amount of the Earth's orbital travel since noon on the day of the equinox. Thereby sidereal time becomes the hour angle of the Vernal Equinox, and the Earth's position at Greenwich Noon on any day can be expressed in terms of hours, minutes and seconds. Its position along the ecliptic is expressed in degrees and minutes of longitude, and along the equator in degrees and minutes of Right Ascension.
