Re: [guide-user] Occult3 vs. Guide8b Times

Bill J Gray May 23, 2004

Hi John,

Kevin Fetter suggested Delta-T and lunar limb conditions as
possible causes. While these do matter, I don't think we can
use either as an explanation here. Guide is projecting a value
of Delta-T=66 seconds for this event, and I doubt Occult would
disagree by much more than a second or so. (Could be worth a
check, but it's unlikely that Occult would be very far off.)

The lunar limb changes might contribute a few seconds. (The
variations in the limb are a few kilometers, and the moon is
moving about a kilometer a second. The limb contributes a _lot_
of odd variations for grazing events, where you can sometimes
see a star disappear and reappear from behind mountains. But
this is more of a "head-on" occultation.) As Kevin mentioned,
Occult includes the variations in the lunar limb. Guide doesn't.
But again, this wouldn't amount to a yawning gulf of 34 seconds.

I can confirm the time you got from Guide. Dunno why Occult
would differ by more than a few seconds... perhaps some odd setting
buried in it? (I'm not really familiar with the workings of the
program.)

Possibly of help in diagnosing this: what are the predicted
second, third, and fourth contact times from Occult? With
Guide, I get

first contact: 08:49:26
second contact: 08:50:28
third contact: 09:53:18
fourth contact: 09:54:30

The time you cite from Occult looks suspiciously close to the
time the center of Jupiter is occulted. If your first and second
contact times are essentially the same, and the third and fourth
are also similar, then I'd start to suspect that Occult hasn't
been fed the right angular diameter of Jupiter and is treating it
as a point source.

-- Bill