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From: brent@phoenix.UUCP (Brent P. Callaghan)
Newsgroups: net.space
Subject: Re: Tropics
Message-ID: <1250@phoenix.UUCP>
Date: Mon, 29-Jul-85 09:37:43 EDT
Article-I.D.: phoenix.1250
Posted: Mon Jul 29 09:37:43 1985
Date-Received: Wed, 31-Jul-85 02:16:42 EDT
References: <2803@mordor.UUCP>
Organization: AT&T Information Systems, Lincroft NJ
Lines: 35

>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.

As far as I know, this process is known as "nutation".
The earth's axis of rotation precesses around a point in
space with a period of 20,000 years or so.  Superimposed on this
motion is another motion with a much shorter period.
I think nutation is caused by sun-moon gravitational effects on
the earth's equatorial bulge and tidal friction.

The effect over time is that the season's slowly shift around the
year, so that in 11,000 years time you'll be eating lettuce salads
for Christmas dinner like New Zealanders do!

As far as I know - nutation IS predictable.  There is even a clock
somewhere with extra dials for years, centuries etc.  The slowest
movement is the nutation dial.
Interestingly, the earth is not fixed to it's axis of rotation.
The north & south poles can move tens of yards every year.
I've seen a map showing the south pole moving erratically
(drunken walk) within a radius of 100 yards or so.
I don't think this movement is predictable.  I guess they
have the barber pole on wheels . :-)
-- 
				
Made in New Zealand -->		Brent Callaghan
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