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    Re: DeltaT fits
    From: Frank Reed
    Date: 2010 Apr 3, 13:20 -0700

    Peter H, you wrote:
    "It goes way back in time and I assume (!) that this is as accurate as possible with today's knowledge, coming from a NASA site."

    That's Fred Espenak. He's a big authority on eclipses, and through his position at NASA he has great leeway to publish his material on the Internet, but this is not "NASA info" in the sense of having been curated for definitive accuracy or validated for space flight or anything like that. It's "Espenak info" designed for a particular purpose and audience --and highly trustworthy based on his reputation rather than NASA's institutional reputation. Maybe we could consider it comparable to a physics professor's private web site hosted under a university's overall web-hosting structure.

    Want more? Visit the Wikipedia page on "Delta-T" and then dig around in the references (that's my wiki-philosophy: the refs are frequently useful, the article almost always questionable). If your algorithms match the year-by-year tables you'll find through those references, then you're ok. On the other hand, why use an algorithm to generate tabulated data?? Just grab the table and interpolate. Again, the accuracy is around 1 or 2 seconds of time in the late 18th century and around 0.1 to 0.2 seconds of time in the late 19th century, and it's very unlikley that there will EVER be any improvement in these acccuracies. Given the rate of the Moon's motion among the stars, lunar distance calculations typically would be shifted by one second of arc for two seconds of uncertainty in delta-T. Naturally actual topocentric positions like GHA will be shifted at the usual rate of one minute of arc of longitude for 4 seconds of time.

    PS: For the curious, here's a slice of the table used in my web site calculators:
    DTtable(0) = 13 '1750
    DTtable(1) = 15
    DTtable(2) = 16
    DTtable(3) = 17
    DTtable(4) = 17
    DTtable(5) = 13.5 '1800
    DTtable(6) = 12.5
    DTtable(7) = 11.9
    DTtable(8) = 7.5
    DTtable(9) = 5.6
    DTtable(10) = 7.0 '1850
    DTtable(11) = 7.9
    DTtable(12) = 1.5
    DTtable(13) = -5.5
    DTtable(14) = -5.9
    DTtable(15) = -2.7 '1900

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