Solar and Sidereal Time
by Charlie Williams
The Earth’s rotation provides a means of timekeeping by the sun as well as by the stars. The time required by the sun to make two consecutive meridian transits is called an Apparent Solar Day. The time required by a star to make two consecutive meridian transits is a Sidereal Day. Solar time is kept with the sun. Sidereal time is measured with the stars.
Although these two methods of timekeeping are based upon rotation of the earth, a Solar Day and a Sidereal Day differ in length. Revolution causes the sun to appear about one degree of arc to the East each day. Therefore, the earth must turn an additional degree of arc or four minutes longer than earth’s rotational period.
Unlike the sun, the stars are not displaced in right ascension and declination by the earth’s revolution. Sidereal time or star time is reckoned by the transit of the March equinox each rotational period. Since the sun crosses the meridian four minutes later each day, the solar day and the Sidereal day are not compatible. On March 20/21, the sun and the equinox transit together, In six months, the sun at the September equinox transits 12 hours after the March equinox. A year will pass before the sun transits together with the March equinox again. Sidereal time measures the rotational period of the stars.
Written for my Astronomy 101 class in 1946. Still good though...Bill
[Return to Top]
[Go to Home Page]
Copyright © 2005 Muskegon Astronomical Society, Inc.
This web page was last updated 08/20/14