RE: Station not a silhouette

From: Matson, Robert (ROBERT.D.MATSON@saic.com)
Date: Mon Dec 30 2002 - 16:31:09 EST

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    Hi Tom,
    
    > I observed the ISS crossing in front of the 23% illuminated moon a couple
    > days ago. I could easily see the +1.5 magnitude station superimposed as a
    > white dot over the illuminated limb of the moon. I assume the albedo of
    the
    > station is brighter than the moon.
    
    You're absolutely right.  It is significantly higher than that of the Moon.
    I believe the moon has an Albedo of less than 10% -- 7% sticks in my head
    for some reason.
    
    > I am however confused as to why the moon's brightness is commonly stated
    > as being in the negative magnitudes yet it was not as bright as the ISS
    > at the moment they converged.
    
    Ahh -- you're talking about the difference between radiance and irradiance.
    While the Moon is certainly dimmer per unit solid angle (the angular
    equivalent
    of area), it has a much much larger solid angle than that of ISS.  The
    integrated
    energy over all that area is what gives the moon its large negative
    magnitude.
    If ISS had the same solid angle as the moon, you wouldn't be able to see
    the stars:  it would look like daylight.  Or, you can look at it this way:
    I'm sure you've seen your shadow cast by the full moon, but certainly not
    by ISS.
    
    Cheers,
    Rob
    
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