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    Re: The Star of Bethlehem and Navigation
    From: Frank Reed
    Date: 2009 Jan 2, 22:06 -0800

    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.
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