From: dj@ssd.kodak.com (Dave Jones)
Newsgroups: sci.space
Subject: Re: Calendar and Zodiak
Keywords: Zodiak
Date: 28 Jul 92 19:47:36 GMT
Article-I.D.: pixel.1992Jul28.194736.11509
Organization: :noitazinagrO

In article <27JUL199222075838@judy.uh.edu> seds%cspara.decnet@Fedex.msfc.nasa.gov (This one works) writes:
>
>Here is what I know about the precession of the equinox.  the rate of precession
>is 1/72 of a degree a year. This gives a rate of precession of 25920 years for
>a full turn.  This rate is not constant due to the imbalances in the Earth's 
>mass distribution.  How this precession affects the seasons is thus: 
>
>Today in the Northern Hemisphere's summer, the earth is at the apogee of its 
>orbit around the sun.  As the precession continues on its merry way this will
>be the main parameter that will change. In 12,900 years the northern hemisphere
>will be in its summer when the Earth is at perigee. (I know these are the wrong
>terms, I just don't remember the right ones for sun centered orbits).
>
Aphelion and Perihelion.  We're going to need generic terms for nearest
& farthest points from the primary.  Perigee, periselenion, perihelion,
perijove, periarion (?)........betcha everyone just says apogee and
perigee in the end.

>This is why the calender will not change (I think) on the day of the year of the
>equinox. It is interesting to note that the change is about 1 degree per 
>72 years.  This effects lattitude lines so maybe my first guess is incorrect. 
>I remember reading that the great pyramid was at the tropic of cancer when it
>was built a few thousand years ago.
>
Whoops.  Precession does not affect the position of the tropics.  That's
nutation (I theenk) and it's much less then precession.  Moving the
Tropic of Cancer up to the Great Pyramid would mean increasing the tilt
of the axis by about 5 degrees: probably enough to make many parts of
the planet uninhabitable.  The lunatic fringe places the Pyramid on some
kind of Great Meridian, but meridians are where you say they are.

Just to try to keep this thread from wandering too much, I'll put up 
a quick precis:  We have a query about the calendar and the
equinoxes/solstices.  Do the equinoxes and solstices drift thru the
calendar because of precession?  If the vernal equinox occurs on a day
known as March 21st now, will the calendar say March 21st at the equinox
in, say, 12900 years time, assuming corrections for the odd fraction of
a day at the end of a year are done properly?

I say yes, because the unit of calendar measurement is the day, and the
length of the day already has precession built in to it.

I thought of the copout answer "they didn't have March 2000 years ago"
in reply to the original poster (who noted that precession has gone
1/12th of its cycle in 2000 years).  Fact is Julius Caesar ordered up
his calendar in 45BC.


>Maybe this info will help, no guarantees, no flames. :-)
>
>Dennis, University of Alabama in Huntsville
>
>Revive the Saturn Five!
>


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