From: utzoo!decvax!harpo!npoiv!npois!cbosgd!djb
Newsgroups: net.unix-wizards
Title: Re: length of day
Article-I.D.: cbosgd.2788
Posted: Sun Nov  7 16:12:14 1982
Received: Mon Nov  8 02:10:46 1982
Reply-To: djb@cbosgd.UUCP (David J. Bryant)
References: cca.3362

It's true nonetheless - a single rotation of Mother Earth takes
23 hours, 56 minutes and 4 seconds.  How come our clocks stay
close, and the Sun doesn't rise at midnight?  What we mean by
period of rotation is the time it takes to rotate through a full
360 degrees.  Go outside and look up.  After 23h 56m 4s, you've 
been rotated back and are facing outward into the universe in the 
same direction.

While the Earth was rotating, however, it was also orbiting the Sun,
processing around the Sun a tiny bit each day.  (Moving almost a degree
in its orbit while you were being rotated).  This movement leaves the
Sun slightly behind (in an angular sense).  It takes almost four more
minutes of rotating to compensate for this incremental daily precession
of the Sun.  This combination (23h 56m 4s rotation period plus ~4
minutes extra to return the Sun to its position of the day before)
produces a 24 hour solar day.  Well almost.  There's enough error there
to require an additional day be inserted into our calendar every four
years.

					David Bryant
					 cbosg!djb