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.
Sidereal Day. The interval between two successive transits of the first point of Aries over the upper meridian
of any place. The Sidereal Day is equal to 23h 56m 4.09s of mean solar time, and it has sidereal hours, each of
60 sidereal minutes, each minute of 60 sidereal seconds.
Sidereal Time. 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.
Sidereal Year. v. Year.
Sign. One of the Twelve Signs of the Zodiac. The annual revolution of the Earth round the Sun is divided into
the 360° of a circle, a division that mathematically and astronomically is universally accepted. The
subdivisions of the circle into 12 equal arcs, distinguished by names, are known as the Signs of the Zodiac.
They no longer bear any relationship to the constellations of the same name.
These arcs are measured from the point where the Sun crosses the celestial equator at the beginning of Spring
on or about March 21st each year. As this is coincidental with the position of the Earth's axis at right angles to
the radius of its orbit, the days and nights are of equal duration all over the Earth. The point is termed the
Vernal Equinox. That the Zodiacal year seems at one period of history to have begun with Taurus indicates that
these records date from between 2,000 and 4,000 B.C., during which period the equinoctial point fell in Taurus.