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    Re: 1491 The year China discovered longitude
    From: Frank Reed CT
    Date: 2004 May 9, 13:51 EDT
    George H wrote:
    "Menzies tells us that the key event that had to be timed (with a star), was what he calls U3, at the first signs of emergence from full umbra. This seems surprising, as U2, the entry into full umbra, would have been equally useful, and the combination of the two more useful still."

    The combination of U2 and U3 would require a basic clock and more detailed recording. That's not a showstopper, but obviously if we can live without a clock, it's an easier observation. I can think of a couple of reasons to prefer U3 over U2. You get to see U2 first. It's a practice run for U3. You're prepared for the change in lighting that will occur when the events play out "in reverse" as the Moon emerges from the umbra, and you get to see the unique shading phenomena for this particular lunar eclipse. In addition, the observer's eyes would be fully dark-adapted by the time of U3, and he would have had plenty of time during the full phase of the eclipse to locate faint stars near the meridian.

    Frank R
    [X] Mystic, Connecticut
    [ ] Chicago, Illinois
       
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