In article <408498ee$1@news.povray.org>,
 Dan P <dan### [at] yahoocom> wrote:

> I must be misunderstanding radiosity -- I figured because it was so far 
> away, it wasn't close enough to the moon to reflect any light to it 
> because of fall-off.

Light doesn't magically become less intense with distance, it spreads 
out. The inverse square law comes from the area of the surface of a 
sphere around the origin point. The moon reflects light from many points 
on its sunlit surface. At the distance from it of Earth, the inverse 
square law approximates the falloff pretty well, but nearer the surface, 
most of the surface area is hidden, and you can only be really near a 
small area, unlike a true point source where all light comes from a 
single point. Yes, the earth only intercepts a small portion of the 
reflected light, but the moon reflects a lot of light. Haven't you ever 
been outside on a night with a full moon?

In the case of an object like the moon, an object twice as far away 
would have to be twice the radius to cover the same area of sky. At 
twice the radius, it would have four times the visible area, exactly 
compensating for the fact that one fourth the reflected light per unit 
of surface area will reach an observer on Earth. From the lunar surface, 
the Earth is much bigger and brighter, and it lights the surface 
brightly enough to be seen on the dark side from earth. (That is, enough 
light reflects off Earth onto the moon, and back to earth again, to be 
visible with the naked eye.)

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: <chr### [at] tagpovrayorg>
http://tag.povray.org/

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