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    Re: Nautical almanac 1773
    From: Michael Dorl
    Date: 2007 May 01, 17:03 -0500

    Thanks to Frank Reed and Dan Walden who helped me understand what was going on here.  First I ever heard of GAT time.

    For whatever it's worth, my windows almanac program based on routines from Mosier at JPL gives the following....

    On 8/1/1773 at 12:05:50 (I can't set time to any finer precision)

    12:05:50 UT
    dT = 15.314
    GHA of Sun is 359-59-54
    apparent lunar distance of antares 48-18-16.05

    note that this is 'apparent' in the true sense of the Astronomical Almanac.  'Thus the position at which the object would actually be seen from the center of the Earth, displaced by planetary aberration (except the diurnal part).'

    At 02:55 AM 5/1/2007, you wrote:
    DW, you wrote of the lunar distances:
    "Using xephem, a fast, easy to use linux program, with open source:"
    "1200 GMT geocentric 48-14-59"
    "1200 GAT geocentric 48-18-20"
    Those values differ 3 to 4 arcseconds from the ones I quoted previously. Which are right? Is the difference real?? I do believe that we can quote lunar distances to 1 arcsecond accuracy, based on modern ephemerides, even for dates back in 1773. Of course, such small differences are below the level of significance for navigational observations.
    42.0N 87.7W, or 41.4N 72.1W.

    See what's free at AOL.com.

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