Re: Morning Pass Of ISS/Venus

From: Tom Wagner (
Date: Fri Apr 27 2001 - 22:25:46 PDT

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    Jonathan T. Wojack asked:
    Does anyone else have the same problem that I have when attempting to
    observe daytime objects (i.e., daytime Iridium flares, etc.) - lots and lots
    of semi-clear "things" in my field of vision (they originate in my eyes,
    that all I know)?  I am light-sensitive ( a roomful of lights can often
    decrease my thinking capabilities), so maybe I'm unique in this respect....?
    I think many of you may be interested in learning the explanation to this
    mysterious phenomenon Jonathan has described.
    Those "clear things" that you see are red blood cells (RBCs) passing through
    the capillaries that are situated on the surface of your retina. Any RBC you
    see is moving across the top of your retina casting a shadow for only about
    a second---then it follows a vessel deeper into the tissue. The shadow they
    cast is normally only visible when you look at an evenly lit bright source
    of light, like the blue sky. [I suppose that the blue sky works better than
    a white sky because the red absorbs a lot of blue, creating a contrastier
    The reason that you don't also see the treelike pattern of capillaries that
    cover your retina is because your visual system ignores their presence; that
    is as long as the shadow they create doesn't move. You can trick your eye
    into seeing them however. Move a penlight around while shining it into the
    very corner of your eye, through the white part--the sclera. This creates a
    shifting shadow of the vessels on top of the retina that is detectable only
    while the shadow is changing. When it stops moving, fortunately for us, our
    nervous system adapts and we don't see it again.
    If you look closely, you can see the RBCs move in step with your heart beat.
    Some capillaries bifurcate (spilt) and you can see some RBCs go one way and
    others going the other way.
    As far as I know, sensitivity to light should have nothing to do with the
    seeing of RBCs.
    (human anatomy is one of four subjects I teach)
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