Christopher James Huff wrote:

> 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. 

Yes, in this case,
you speak of space,
but in the air,
you must take care,
to remember the particle,
(he's quite the article!),
his friends and foes,
and others he knows,
absorbs the light,
with all its might,
its effort made,
light seems to fade!

> The inverse square law comes from the area of the surface of a 
> sphere around the origin point.

Bah, I consider that more a guideline than a law :-)

> 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?

What you are describing is called the "albedo" of an object. This 
website[1] defines albedo as, "the fraction of light that is reflected 
by a body or surface." This site contains some albedos that illustrate 
your point:

(higher = brighter)
Venus: 0.76
Earth: 0.33
  Moon: 0.12

The Earth is 21% brighter than the moon (as you've said), probably 
because of all the water on the surface and in the clouds. Venus is 43% 
brighter because it has a lot more clouds to reflect the light. Venus 
has been so bright to us that people have mistaken it for the light of 
an on-coming train!

> 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.)

Lemme get out my slide-rule (hmmm... carry the three)... sounds good to 
me! :-)

[1] http://zebu.uoregon.edu/~js/glossary/albedo.html
-- 
Respectfully,
Dan P
http://<broken link>

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