Re: ISS transit across sun

From: Thomas Fly (thomasfly@charter.net)
Date: Sat Mar 01 2003 - 17:16:53 EST

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    From: "Tom Wagner" <sciteach@mchsi.com>
    >
    > In my thinking---and this is not a mathematical analysis for sure and may
    be
    > totally wrong :~) but, if the observer was looking toward the low earth
    > orbit (LEO) satellite that was about to enter the moon's shadow the
    observer
    > would have to be relatively close to the line of sight of the sun/moon.
    That
    > would make it so the sunlight would be on the far side of the satellite
    > which would make it a silhouette (thereby invisible).
    
    Well actually, at an altitude of 250 miles, the ISS sees the horizon about
    1500 miles away (and so it's about 1500 miles away, when you observe it
    rising above your horizon).
    
    The moon has a radius of about 1000 miles,  so there's 500 miles where the
    space station would neither be especially in the direction of the sun (and
    hence would have plenty of light reflecting off it), nor in any part of the
    moon's shadow.
    
    Owing to the fact that the sun is not a point source of light- but like the
    moon is about 1/2 degree in angular size- the moon's shadow on the earth
    (like an airplane passing overhead at low altitude) is fuzzy, and the area
    of total darkness (i.e., the umbra) is relatively small.  So in reality,
    during an eclipse transit, you'd see the brightness of the space station
    begin to diminish as it entered the shadow, drop to nothing when it crossed
    the face of the moon (that is, while in the umbra), and then increase in
    brightness as it emerged from the umbra.
    
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