Paul Schlyter Apr 6, 2001
> Hi Chris,There are indeed a number of different Delta-T formulae around.
>
> 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