Sidereal Days and Solar Days

The sidereal day is defined to be the length of time for the vernal equinox to return to your celestial meridian.  The solar day is defined to be the length of time for the Sun to return to your celestial meridian.  The two are not the same, as illustrated in the following animation. 

The sidereal and solar day

Because the Earth is in motion on its orbit around the Sun in the course of a day, the Earth must turn about 4 minutes longer each day (3 minutes and 56 seconds, to be exact) to bring the Sun back to the celestial meridian than to bring the vernal equinox back to the celestial meridian.  Thus, the solar day is 3 minutes and 56 seconds longer than the sidereal day.  It is this almost 4 minute per day discrepancy that causes the difference in sidereal and solar time, and is responsible for the fact that different constellations are overhead at a given time of day during the Summer than in the Winter. 

10/01/2017