Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/5/84; site mordor.UUCP
Path: utzoo!linus!philabs!cmcl2!seismo!lll-crg!mordor!@S1-A.ARPA,@MIT-MC.ARPA:mcgeer%ucbkim@Berkeley
From: @S1-A.ARPA,@MIT-MC.ARPA:mcgeer%ucbkim@Berkeley
Newsgroups: net.space
Subject: Re:  Tropics
Message-ID: <2849@mordor.UUCP>
Date: Mon, 29-Jul-85 16:09:38 EDT
Article-I.D.: mordor.2849
Posted: Mon Jul 29 16:09:38 1985
Date-Received: Wed, 31-Jul-85 22:43:16 EDT
Sender: daemon@mordor.UUCP
Organization: S-1 Project, LLNL
Lines: 37

From: mcgeer%ucbkim@Berkeley (Rick McGeer (on an aaa-60-s))

>
>          A book I was reading recently mentioned  that  over  time  the
>angle  of  the plane of the ecliptic changes.  The consequence is that
>the latitude of the tropics also move.  While this  makes  sense,  the
>book  also  stated that there is no formula which describes the motion
>over  time.  Is  this  really  true?  The  context  was  that  certain
>archeological  sites  are  solstice  oriented  and could be accurately
>dated if it was known in what year a Tropic was at  X  latitude.  Just
>curious.
>
>                                        jim@tycho
>


	Wrongo.  The phenomenon you're referring to is called the
"precession of the equinox" and the values have been calculated *very*
precisely.  Roger Bacon first pointed out the phenomenon is the 13th
Century, and showed that if the Julian calendar were not changed, then
sometime in the 30th Century Easter would occur in midsummer (the rate of
precession is about .75 days/century).  The solution he proposed was the one
adopted in the Gregorian calendar, in which Leap Years are not held in
century years and are held every 400th year: so there was no leap year in
1900, there will be one in 2000, but there won't be one in any of 2100,
2200, 2300.

	Incidentally, the world shifted to the Gregorian calendar at varying
times.  The Roman Catholic world did it first, in the 16th Century -- but
that was after the Schism, and so England didn't follow suit.  For 200 years
England's calendar trailed the European by first 9, then 10, then 11 days.
England finally converted in the mid-18th Century, to riots (11 days were
dropped from the calendar at the stroke of a pen).  Russia converted after
the Bolshevik revolution; this is why the "October revolution" was really
held, by Western calendars, in November.

						Rick.