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    Sidereal Time is based on the Earth's sidereal rotation period (rotation with respect to the stars). The length of the sidereal day is 24 sidereal hours, or 23h56m4s Solar time. The difference is due to the angular motion of the earth around the Sun. In a year there is one more sidereal day than Solar day. The sidereal time is a measured locally. The local sideral time (LST) is the right ascension of the zenith. The hour angle HA of an object with right ascension RA is given by HA=LST-RA GMST (Greenwich Mean Sidereal Time) at 0h UT = 24110.54841 + 8640184.812866 TU + 0.093104TU^2 - 6.2x10-6TUT^3 seconds GAST (Greenwich Apparent Sidereal Time) = GMST + the equation of the equinoxes. The Equation of the Equinoxes is the total nutation in longitude time the cosine of the true obliquity of the ecliptic. It ranges from +0.8 to +1.2 seconds. LAST (Local Apparent Sidereal Time) is the hour angle of the ascending node of the ecliptic. LMST (Local Mean Sidereal Time) = GMST - geode...

  Meridian, simply means line of longtitude. Long definition:  A (geographic) meridian (or line of longitude) is the half of an imaginary great circle on the Earth's surface, a coordinate line terminated by the North Pole and the South Pole, connecting points of equal longitude, as measured in angular degrees east or west of the Prime Meridian.  The term meridian comes from the Latin meridies, meaning "midday"; the subsolar point passes through a given meridian at solar noon, midway between the times of sunrise and sunset on that meridian.[ Methods of determining longitude and Earth rotation Traditional optical instruments for measuring Earth rotation included visual and photographic zenith tubes, circumzenithals, Danjon astrolabes, and transit telescopes, such as the Airy Transit Circle. These instruments measured local sidereal time by the passage of specially designated “clock stars” across a plane of reference established in part by the local direction of gravity. Th...