Sidereal Clock

What is a sidereal clock? Astronomy buffs (radio and the other kind) often use a special clock which is based upon "Star time". If you used your normal clock and timed when a distant star passed due south of you each night, you would find that each night it passed about 4 minutes earlier. That happens because in addition to the 360 degrees the Earth rotates in 24 hours it also moves 1/365.25 of its orbit around the Sun. Thus, we swing into position to see a given point in the sky a Little earlier every night. Celestial objects are located on the sky by a coordinate system which includes a celestial longitude (Right Ascension) and latitude (Declination). The right ascension of a celestial object is same as the local mean Sidereal Time that the object will pass an imaginary line drawn from the celestial pole to due South ( 180 degrees azimuth). Thus, if you know the local mean sidereal time (LMST) you also know what objects are in MERIDIAN transit at your location. If this isn`t clear, most any astronomy text can can explain it in better detail.