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> Hi, everybody!
>
> A friend of mine sent me this email about an optical phenomenon. He'd
like
> to know what it is he is seeing. Does anyone here know? His explanation
is
> kind of long.
>
> Bev
> cloudbow_at_primenet.com
> =========================================================
>
>>Bev,
>>
>>that meteorological / optical problem I mentioned
>>a while ago - it burbled to the surface again while I watched
>>the sun lower to the Blue Pacific.
>>
>>Observation:
>>
>>While flying my little Cessna over the greensward of Wisconsin,
>>I frequently noted this phenomenon.
>>
>>Below on the ground is a bright spot of sunlight directly
>>opposite from the sun's position in the sky.
>>
>>If the sun is behind me, the spot is in front of me on the ground.
>>If the sun is to the right, the spot is on the left. That is what I mean
>>by "opposite" the sun's position. Shoot a line from me to the
>>sun, then extend it to the ground, and the spot is where the
>>line hits the ground.
>>
>>This is a constant thing, regardless of season, weather,
>>ground surface, etc.
>>
>>When I get low, as in landing, the spot enlarges to contain the
>>shadow of my plane.
>>
>>It is as if the ground was preferentially reflecting back directly
>>along the line from the spot to the illumination source.
>>
>>This phenomenon is perhaps akin to the one seen in a
>>small boat on cloudy water: A shaft of bright sunlight goes
>>down into the water, extending the line drawn between Sun
>>and the observer.
>>
>>I understand that the road signs that fall into my airplane's
>>little spot of light should glow brighter, because they are designed
>>to reflect directly back in the direction of illumination.
>>
>>But I do not understand why totally random objects (cornfield,
>>snow, forest, water) should seem to reflect directly back in
>>the direction of illumination.
>>
>>Is this akin to seeing the sun or moon over the ocean light a
>>path toward the viewer?
>>
>>None of the engineers and amateur physicists I have
>>mentioned this to have been able to explain this phenomenon.
>>Gotta be simple, must have been explored.
>>
>>Any thoughts?
This is the so-called "opposition effect". On any rough surface, all
shadows will disapppear at a point directly opposite the Sun. Most
of the shadows are too small to be seen, but they do affect the total
brightness of the area. When all those little shadows disappear, the
area will look brighter.
This is also the reason why the full moon is some 10 times brighter
than the half moon, instead of just twice as bright (as one could
naively expect), or PI (3.14159....) times brighter (as would be the
case if the moon's surface was perfectly smooth and also a perfect
diffuse reflector of light). When the moon is full, all those tiny
shadows on the lunar surface completely disappear, and the moon will
look brighter.
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pausch_at_saaf.se psr_at_net.ausys.se paul_at_inorbit.com
WWW: http://www.raditex.se/~pausch/ http://spitfire.ausys.se/psr/
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