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    Re: GMT from Moon and Body Observed Altitudes...??
    From: Fred Hebard
    Date: 2019 Feb 20, 19:54 -0500
    The moons of Jupiter also provide a clock, but it was impractical for use at sea (not possible to keep telescope trained on Jupiter on board a rocking vessel).

    Fred Hebard

    On Feb 20, 2019, at 19:31, Wendel Brunner <NoReply_Brunner@fer3.com> wrote:

    To determine longitude on a steadily spinning planet requires a clock. In the absence of a timepiece on the vessel, the heavens provide a number of clocks which can be read from astronomic observation. The most practical celestial clock - really, the only practical one - is the moon. The moon passes through its cycle of phases moving 360 degrees with respect to the sun in 28 days, or about 30 minutes of arc per hour. If you can measure the distance between the sun and moon to within 0.5 minutes of arc, you can determine time to within one minute, or longitude to within plus or minus 15 minutes.  
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