@S1-A.ARPA,@MIT-MC.ARPA:koolish@bbncd2 (07/31/85)
From: Dick Koolish <koolish@bbncd2.arpa>
To get back to the original question that brought on all the
discussion about the Gregorian calendar:
The earths axis of rotation is subject to precession and change
of tilt. Both these effects would change the way that ancient
structures line up with astronomical objects. The difference is
that precession changes the positions of stars while tilt changes
the position of the sun.
Precession causes the pole to describe a circle in the sky
instead of remaining fixed at one place. This causes the
positions of stars and constellations to change. Polaris is the
pole star now because the axis is pointing near Polaris. 13,000
years ago, it was on the other side of the precessional circle.
In the 2,000 years or so since the constellations were named,
precession has moved around the zodiac by a full constellation so
that none of the astrological signs correspond to where the sun
really is in the sky.
The tilt or obliquity of the ecliptic also changes. This is a
long period change of 41,000 years that takes the tilt from 24.5
degrees to 22.1 degrees at about .47 seconds per year. This
change of tilt affects solar phenomena like sunrise, sunset, etc.
The formula for the tilt is:
23 degrees 26 minutes 21.448 seconds
- 46.8150*T seconds
- .00059*T*T seconds
+ .001813*T*T*T seconds
where T is the number of Julian centuries from 2000