Christopher James Huff wrote:
> [...]
> 
> 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.)
> 

Note that the same effect causes the moon to be visible during a lunar 
eclipse as well, just that in this case the earth faces its dark side to 
the moon so the light reaching the moon and making it visible to us is 
mostly scattered light.  Therefore the appearance of the moon during a 
lunar eclipse strongly varies depending on the amount of dust in the 
atmosphere.

Christoph

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