Quote:
Originally Posted by grant hutchison
Ah, you're right, it's my mistake.
Meeus is using Dynamical Time, which is rather more than a minute ahead of UT. By 2012 the difference will be something like 78 seconds, which accounts for the USNO's figure, and also for their coy avoidance of exact seconds: they can't know exactly by how much DT and UT will differ until after the event.
Grant Hutchison
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Ah, so.
Hee, hee. I remember you've mentioned that GR calculations sort of waft over your head. Well, this kind of stuff does the same for me. When you get to talking about the various astronomical timekeeping standards, then throw that into calculations of the various positions of objects as seen in various coordinate systems, I'll get a blank, deer-in-the-headlights look on my face.
However, I did have to look up dynamical time. I got that look, but I did notice something that caught my GR interest: Geocentric Coordinate Time and Barycentric Coordinate Time.
The former (always referenced to the clock rate of an observer "stationary at infinity", and assuming asymptotic Schwarzschild-like flatness) is the proper time of a clock following the motion of the center of mass of the earth, but without the earth's gravity well.
IOW, this is the proper time of a clock following the earth's world line through space and in all the other gravitational fields the earth is immersed in.
Barycentric coordinate time is that of a clock following the solar barycenter's world line through space, but absent the surrounding solar system field.
Doing that allows you to separate all the different effects on clock rates in a structured sort of way.
-Richard