wrote:
Hi - I'd be grateful for some help with this!

I've found it stated in a few places that

GMST (in seconds at UT1=0) = 24110.54841 + 8640184.812866 * T +
0.093104 * T^2 - 0.0000062 * T^3

where T is in Julian centuries from 2000 Jan. 1 12h UT1

Could someone please explain what that 24110.54841 figure is doing
there. In seconds, this comes to about 7 hours.

When was UT1=0 ? Is GMST about 7 hours in advance of UT1? What am I
getting wrong?

Thanks!

Michael

Sure. At 2000 Jan 01 12:00 UT, the sun was more or less on the meridian
at Greenwich. Remember that the winter solstice was about 9 days
earlier -- at that time the sun was at 18h RA, so add another 9 degrees
so the sun would have been at about 18h36m.

But the equation is for 0h UTC. Midnight at Greenwich, so the local
sidereal time would be 12h opposite to the above -- that is, 6h 36m or
thereabouts.

That 24110 seconds is 6h41m50s. So my BOTE calculation is off by only 5
minutes or so, and that's the equation of time.

-- Bill