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    From: Roger W. Sinnott
    Date: 2018 Jan 29, 07:15 -0500

    Wednesday morning's total lunar eclipse, visible from the western half of North America as well as Pacific Rim countries, offers a simple way to determine your longitude to roughly the same accuracy you'd get from a traditional lunar-distance measurement (between the Moon and a star, planet, or Sun) and a full-blown analysis.

    On the Sky & Telescope website I've put detailed predictions of the Universal Time (= GMT) when individual craters and bright spots will enter or leave the well-defined central "umbra" (Earth's shadow):
    Experience shows that observers with small telescopes can time these events with an accuracy of about 30 seconds.  So here's the method:

    (1) First, cheat a little and use your known longitude to calculate the Local Mean Time.  Set a watch to this LMT, and then forget that you already know your longitude.

    (2) Using a telescope, write down the exact LMT when you see a crater cross the edge of the umbra.

    (3) Subtract your LMT from the predicted UT and convert this difference to degrees.  That's your longitude!

    For improved accuracy, observe 5 or 10 craters this way, at both entrances and exits, and average the longitude values obtained.  (If anyone wants, I can send the same list of craters showing predicted times to the nearest second.  But these can't be PERFECTLY accurate because the shadow's size varies slightly from one eclipse to another, for unknown reasons.)

    For purists:  In step (1) you could determine LMT from altitude sights with a sextant without knowing your longitude in advance.


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