Re: ISS saturn transit

From: Thomas Fly (
Date: Sun Jan 18 2004 - 23:35:06 EST

  • Next message: Robin R. Wier: "Re: ISS saturn transit"

    >> Has anyone tried to view the ISS in shadow through a
    guided telescope?
    > Up to ~40 seconds after shadow entry on passes directly over
    > Phoenix, AZ. Assumed reflected city lighting.
    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."
    Another important consideration is the fact that the sun is "pretty big" (i.e.,
    it isn't like a laser beam, or a "point source of light"), so that- even without
    atmospheric lensing- the earth's "shadow" would not be sharp.
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