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    Re: Moon Occultation of Jupiter
    From: Frank Reed CT
    Date: 2004 Dec 1, 01:32 EST
    George H wrote:
    "1. Jupiter doesn't extinguish all that suddenly. Jupiter has a semidiameter
    of nearly an arc-minute, a diameter of nearly 2'. The Moon moves with
    respect to the background of stars at about 30 minutes in an hour, so it
    would take about 4 minutes in all to shut down Jupiter's light altogether,
    and more if Jupiter wasn't crossing the midplane of the Moon. So, you might
    suggest, just time the last-gasp of the light, which should be pretty
    sudden. But then, that moment would depend on the light-gathering power of
    your telescope, and different observers are likely to disagree about the
    moment. For this reason, an occultation of a star would be better: that
    happens instantaneously."
    I'm guessing that you've never watched an occultation of a planet by the Moon. The last-gasp is actually rather easy to time and different observers (at the same site) are not likely to disagree by more than a couple of seconds. You could also time the initial contact.

    And wrote:
    "2. The Moon, because it is so close to Earth, isn't in the same direction
    when seen by different observers, at different places on Earth. Because of
    parallax, the apparent Moon can be shifted by a whole degree from the point
    in the stars where an observer exactly below the Moon would see it. The
    Moon is only about 30 minutes across, so many observers wouldn't see the
    occultation at all, and others would see Jupiter cross the Moon at very
    different times and at different "levels", taking different times to cross.
    An observer could allow for this parallax if he knew just where he was on
    the Earth's surface. Unfortunately, that's just what he needs to find out.
    So the "very elementary lunar calculation" that David describes is
    Unattainable? That's just not so. It's a fairly tedious calculation, that's for sure.
    "3. The extinguishing of Jupiter's light as it's overtaken by the Moon is
    easy to see, if that part of the Moon's disc is dark, as it is around first
    quarter. But when it happens at last quarter, that part of the Moon is
    brightly lit, and then it's difficult (impossible?) to make out the planet
    from the Moon as it's appproached and swallowed-up."
    Jupiter is very bright. It can be seen right up to the last moment of the occulation using binoculars (or a small telescope).
    "It's more difficult to time re-emergence than extinction, because it takes the observer rather by surprise."
    That's true.
    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
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