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    Re: Moon Occultation of Jupiter
    From: George Huxtable
    Date: 2004 Nov 30, 21:01 +0000

    David Edwards asked quite a sensible question-
    >            Apparently there will be an occultation of Jupiter by the Moon
    >on Tuesday 12-07-2004 at approximately 0355 EDT.  It will end at
    >approximately 0505.  I would assume that for a celestial navigator an
    >accurate determination of local time, or even better an LOP, could be
    >computed from a knowledge of when the beginning and/or ending of this event
    >will occur in universal time.  Would that not be a very elementary lunar
    >calculation?  Look at the moon with binoculars and set a chronometer to the
    >correct universal time at the exact moment that Jupiter disappears behind
    But there are several reasons against such a procedure-
    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.
    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
    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. It's more difficult to
    time re-emergence than extinction, because it takes the observer rather by
    When Peter refers to "an accurate determination of local time", I presume
    he really intended to imply determining Greenwich time, in line with his
    wish to "set a chronometer to the correct universal time at the exact
    moment..." Local time is better determined from a morning or afternoon Sun
    altitude, or a star in the Western or Eastern sky near dusk, and is rather
    easy to measure.
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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