These are not the same effects; The 21,000 years is the time it takes the orbit to precess if you track the motion of the perihelion point. The 28,000 years is the time it takes the earth's rotation axis to precess.
If you consider a spinning, wobbling top which is moving across a lazy susan (rotating turntable) you can see the difference; the 21,000 year figure is for the time it takes the lazy susan to rotate. The 28,000 year figure is the time for one wobble of the top. How's that? Jeff Adkins Andrew Pettit wrote: > There have been two recent subjects dealing, according to my understanding, > with the matter of precession: > > In the first "Two Questions" Jeff Atkins pointed to the site: > > http://riemann.usno.navy.mil/AA/faq/docs/seasons_orbit.html > > that refers to the period that the major axis of the Earth's orbit to rotate > through 360 degrees. This period is about 21,000 years. > > Under the heading "Length of the year" the period of precession is given as > 26,298 years and I have that figure from other sources. > > Am I correct in thinking that these are two seperate long term effects? > > Regards > > Andrew Pettit > (who cannot quote his latitude & longitude > shame on him!) > > ------------------------------Andrew Pettit-------------------------------- > > e-mail: [EMAIL PROTECTED] > > Postman Pat: 3, Lucastes Road, HAYWARDS HEATH, West Sussex, RH16 1JJ, > ENGLAND > > Tel. UK: (+44) (0)1444 453111 Content-Type: text/x-vcard; charset=us-ascii; name="Astronomer.vcf" Content-Transfer-Encoding: 7bit Content-Description: Card for Jeff Adkins Content-Disposition: attachment; filename="Astronomer.vcf" Attachment converted: Macintosh HD:Astronomer.vcf (TEXT/ttxt) (0001D95E)
