----- Original Message -----
From: Laurent Zimmermann <zmn@...>
To: <guide-user@yahoogroups.com>
Sent: Friday, April 06, 2001 2:55 PM
Subject: Re: [guide-user] "Star" of Bethlehem


> A quick look in "Help -> Quick Info" to see that Guide uses
> delta T = 9908.6 s.
> A difference of about 1/4 h with respect to SkyMap Pro.

Interesting - that's exactly the value that old versions of SkyMap also use
for that date.

It appears as though Bill is using a delta T formula devised by F R
Stephenson and M A Houlden in a book called "Atlas of Historical Eclipse
Maps", published by Cambridge University Press in 1986. This formula applies
to dates prior to 948 AD and is:

B = (Year - 948.0)/100.0;
deltaT = (46.5 * B - 405.0) * B + 1830.0;

If you work that out for 17th April 6BC you'll get the value +9908.6
seconds, as both Guide and old versions of SkyMap Pro use.

However, this formula is based on a "secular acceleration" for the Moon
which differs from that used by the modern "ELP2000" lunar theory which both
Guide and SkyMap Pro use. Based on more modern research, and to be
consistent with the ELP2000 theory, a correction needs to be added to the
value derived from the above formula of:

0.6 - 0.6*T - 2*T*T

seconds, where "T" is the number of Julian centuries from J2000.0. This
correction is trivial for modern times, but quite significant for the
distant past. For the year 6BC where T=20.05 centuries the correction
amounts to -816 seconds, or 13.6 minutes.

In case Bill doesn't read this I'll sent this correction to him directly -
he really should be using the corrected formula for "delta T" in Guide,
since it has significant effects on the paths of ancient eclipses.

Regards,

Chris
-----------------------------------------------------------------------
Chris Marriott, SkyMap Software, UK (chris@...)
Visit our web site at http://www.skymap.com
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