If we measure a day from noon to noon - from when the sun crosses the meridian (directly overhead) to when the sun crosses the meridian again - we will find the average solar day is about 24 hours. If we measure the day according to a particular star - from when that star crosses the meridian to when that star crosses the meridian again - we will find the average sidereal day is 23 hours and 56 minutes.
The reason for this is illustrated below. On the first day, point A points directly toward the sun at exactly twelve noon. After a full rotation of the Earth, point A is pointing in the same direction in space, but since the Earth has also proceeded along its orbit, point A is no longer facing the sun. It takes another 4 minutes of rotation for point A to again face the sun. (The effect is exaggerated in the picture.)
For example, the star Capella in the constellation Auriga
has the coordinates RA 5h 13m DEC 46deg. When Capella is directly overhead,
the sidereal time is 5:13.
This page was last update on 12 September 1997
and is © 1997 by Jim McDonald.