Re: [guide-user] Precession in Guide

Chris Marriott Nov 2, 2002

----- Original Message -----
From: "Richard Mallett" <100114.573@...>
To: <Blind.Copy.Receiver@...>
Sent: Friday, November 01, 2002 8:39 PM
Subject: [guide-user] Precession in Guide


>
> Given the above, and on a different, but related, matter, how certain can
> we be that, as has been stated, the next opposition of Mars will be the
> closest for 100,000 years ?

Very sure. Don't confuse the approximations to such things as precession,
planetary motion, etc, used by star charting programs such as Guide, SkyMap,
etc, with the numerical integrations they are derived from.

The IAU precession formula is a polynomial approximation which is highly
accurate for a few millennia either side of its "origin" of J2000.0, but
which very quickly degenerates into random numbers once you go outside its
range of validity. The numerical integration models of the solar system
which these approximations are derived from are a completely different
matter; they can be run forwards and backwards in time almost arbitrarily
and are certainly good for millions of years.

Regards,

Chris
---------------------------------------------------------------
Chris Marriott, SkyMap Software, UK
e-mail: chris@... Web site: http://www.skymap.com
Astronomy software written by astronomers, for astronomers