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    Re: Moon Occultation of Jupiter
    From: George Huxtable
    Date: 2004 Dec 3, 14:50 +0000

    Referring to my of discussion the brightness of the Moon's limb at various
    >> But the recent Jupiter occultation of November 9th, which Frank was
    >> presumably taking as his example, occurred 5.5 days after Moon's last
    >> quarter, which was on Nov 5th. The Moon must have appeared as a faint
    >> thin
    >> crescent. By that time the Moon's phase had changed through 70
    >> degrees, and
    >> the direction of sunlight falling on the limb had changed by the same
    >> amount; so the resulting brightness at the Moon's surface would then
    >> be 4.4
    >> cos 70, or about 1.5 times as bright as the surface brightness of
    >> Jupiter.
    Fred Hebart commented-
    >I wonder whether the non-level surface of the moon --there are craters,
    >etc-- would alter the brightness, perhaps making it less variable with
    >changes in sun angles.  Alternatively, I suppose, integrating over the
    >various surface angles might give the same brightness as if the moon's
    >surface were smooth.  I would guess the surface brightness might have
    >been measured fairly thoroughly by astronomers?
    Response from George.
    Yes, the apparent brightness of the Moon's surface in different areas would
    be profoundly affected by the surface slope, in places where the Sun, as
    seen by a Moonman, is near his horizon. In the situation I was discussing,
    at the limb the Sun was so slow that a slope of more than 20 degrees, in
    the wrong direction, would put that bit of the Moon into shadow. That's why
    the Moon'mountains and craters show up so well near the terminator of the
    It's true, as well, that different regions do have different surface
    reflectivity (albedo), which is why we can still see some pattern on the
    surface, even when the Sun is overhead to a Moonman.
    So my argument was an over-simplification, of course, in treating the Moon
    as having a constant albedo of 7% all over, and also in assuming that it
    behaved like a perfectly diffusing surface, similar to a matt-white
    plastered wall. Moondust probably makes a good diffuser, if a poor
    reflector. But if someone had scattered some of the particles you get in
    retro-reflective paint, that would upset matters!
    I've not seen any actual mapping of measured albedo over the Moon's
    surface, but I expect someone has made those measurements.
    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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