A method of time-reckoning based upon the period elapsing between two successive passages of some particular star, taken as a fixed celestial point, over a given point on the circumference of the Earth. During one such rotation the Sun's apparent orbital travel has amounted to approximately 1°, hence the return of a given point on the Earth to the same relationship with the Sun requires added travel to the extent of 1° of arc or 4 minutes of time. Thus each calendar anniversary shows an annual net gain of 1°, which is the basis of all systems of progressed influences. The S.T. at any moment is the angular distance along the Ecliptic from 0° Aries, the point of the Spring Equinox, to the meridian of a given place at noon on a given day, expressed in h. m. s. The Right Ascension of the Meridian (RAMC) is a similar angular distance along the Equator expressed in degrees and minutes of arc.
When the Spring equinoctial point is on the observer's meridian it is S.T. 0h. When that degree has moved 15° it is 1h S.T. Thus the time required for the equinoctial degree to move to a certain advanced position becomes the unit through which that position is expressed. To determine the sidereal time for a given moment at a certain place, take from the ephemeris the ST for that date and apply certain corrections, viz.: If the ephemeris is for any other meridian than Greenwich make sure to take that into account, adding or subtracting your distance from this meridian, not from Greenwich; also add or subtract 12 hours if you are calculating your time-interval from midnight.
Additions to this S.T. for stations west of the zone meridian are made in degrees expressed in solar mean time, four minutes for each degree, which must be further converted by adding 0s.657 for each degree to reduce the additions to sidereal time. The hours added for the elapsed time since oh must also be adjusted in the same proportion. v. Time.
