About time keeping, the definition of “sidereal” is “of or with respect to the distant stars (i.e. the constellations or the fixed stars, not the sun or planets)”, as defined after a google search. With that in mind, a quick and admittedly simplistic look at keeping earthly time is shown in Figure 1.
Figure 1 A comparison between solar days of 24 hours and sidereal days of 23 hours, 56 minutes, and 4.091 seconds.
As the earth moves around the sun in its yearly orbit, the absolute direction in which some fixed point on the earth’s surface that is aimed directly at the sun changes. We who sit on the earth at some particular place, maybe in our backyards, see the sun reach its peak in the sky once every twenty-four hours and we call that a solar day. Disregarding leap years, leap seconds, and the like; our wristwatches, tabletop alarm clocks, and other timepieces keep track of our personal time on a solar day basis.
Measured with respect to absolute space, however, the solar day requires an earth rotation per “day” of slightly more than 360°. Thus, when we take the universe at large as some fixed entity, with respect to that entity, the earth completes an exact 360° rotation in an average time of only 23 Hours, 56 Minutes, 4.091 Seconds which is a shorter time span.
That shorter time span is a sidereal day which differs from and is slightly less than a solar day.
Have you checked your watch lately?
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
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