Specifications

To calculate the sidereal time approximately, figure out the number of months and days that have
elapsed since the autumnal equinox. Allow 2 hours for each month and 4 minutes for each day in
excess of the months. Add this figure to the Greenwich Mean Time to get a rough estimate of the
Greenwich Sidereal Time. For example, on May 29 at 17h 30m GMT, 8 months and 8 days after
the autumnal equinox, figure as follows:
8 months at 2h per month
8 days at 4m per day
GMT
Total
=
16h
Oh 32m
17h 30m
33h 62m
=
=
Subtract 24h to obtain the approximate Greenwich Sidereal Time:
GST = 10h 02m.
Sidereal time is the elapsed sidereal interval since the vernal equinox was last on the meridian.
The equinox is the point on the celestial sphere where the sun crosses the equator in the spring.
Sometime called the point of Aries. To obtain a more accurate value of the sidereal time at a
given instant, corrections would have to be made for rotation and precession.
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