Hi Chris,

Errrmmm... we _do_ have something odd going on here.

Instead of getting SkyMap's value of Delta-T=+9117.5 seconds, I
got Delta-T=+9921.5 seconds! (This information is given in "Help...
Quick Info".) This is a difference of 804 seconds. Not enough to
push this event into visibility, by any means, but it did surprise me.

I ran that test thinking that we probably were both using the Delta-T
formula from Meeus' _Astronomical Algorithms_ for "dates before A.D. 948":

Delta-T = 2715.6 + 573.36 * T + 46.5 * T^2

...where T=centuries from J2000 = (JD-2451545.0) / 36525.

Therefore, I assumed that we'd have virtually identical values of
Delta-T, which would have allowed me to repeat comments about not taking
agreement between any two programs too seriously; they may just be
copying each other's mistakes.

Checking by hand (T=-20.0566) again got me Delta-T=+9921.5. Perhaps
you've dug up a more recent expression? The one given in Meeus dates
back to 1986, and it wouldn't surprise me at all if something better
has been determined since then.

The code I use for Delta-T is available as part of the "basic
astronomical source code" at

http://www.projectpluto.com/source.htm#astrocalc

Robert, thanks for the list of "possible Christmas stars". I should
probably use it as a starter for an "event" file for Guide's Miscellaneous
Tables function. The list could easily grow to some dozens of events; I
know I've read of several over the years. As Chris mentions, it seems
entirely possible that the "star" was added as an afterthought; short of
time travel, though, I don't see how you could prove the matter one way
or the other.

-- Bill