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In the visible spectrum, how many photons reach the Earth per square millimeter, per second? (Assuming a nice, clear day with the sun nearly directly overhead, etc.)

This is not an easy question to answer because it depends upon many variables. Even "assuming a nice, clear day with the sun nearly directly overhead, etc." First, you have to integrate over the visible region of the electromagnetic spectrum. Second, both water and oxygen have absorptions in the visible range -- weak but present in the long path length of the atmosphere. And these are just the start of the complications. The NASA web site:

gives the basis for determining the answer, but you will need a long sharp pencil!!

Vince Calder

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