Re: ISS saturn transit

From: Ed Davies (edavies@nildram.co.uk)
Date: Mon Jan 19 2004 - 06:33:26 EST

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    Thomas Fly wrote:
    > ....
    > It's hard to say (without writing a computer program to calculate it) precisely
    > what "after shadow" means, because of the earth's atmosphere. It's fairly well
    > known (among amateur astronomer types) that at sea-level, etc., a "horizontal"
    > light ray has been refracted by about 35 minutes of arc, so that the setting
    > sun/moon (having a diameter of about 30 minutes of arc) is in fact entirely
    > below the physical (i.e., unrefracted) horizon when it first begins to "set."
    > ...
    > So the atmosphere acts somewhat as a lens (or prism), bending the light from the
    > sun into what would otherwise be "shadow."
    
    ...and remember that for a satellite entering shadow this effect is doubled
    as a light ray which just skims the Earth's surface would be refracted twice, 
    once on the way into the atmosphere and once on the way out.
    
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