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Marcel, you know, this whole business is actually very closely related to the 
issue of leap seconds. Fact is, we can predict (and retrodict) the positions 
of the planets and Sun and Moon relative to one another with high accuracy 
for many thousands of years in either direction, future or past. But the 
orientation of the Earth is tougher, in part because we're asking for a fine 
order of difference and also because dissipative forces are responsible for 
the variations of the Earth's rotation. 

While we worry about leap seconds once every year or two, if you want to work 
the same problem 2,000 years backward in time, you need something like three 
hours worth of leap seconds, give or take 500. In other words, we can specify 
atomic time or ephemeris time without much ambiguity and calculate most 
phenomena in the heavens, like occultations and eclipses, with great 
accuracy. Their relative timings will be accurate. But we don't know the 
exact local time when those events occurred as seen from the Earth since we 
don't know exactly how much mean time differed from "true" time back then. 
Was it 4:30 or 4:35 local apparent time? Right now, there's no way to know.

-FER



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