Why can I sometimes see the unlit part of the moon?

On Thu, 24 Aug 2000, fay wrote:

> a friend recently mentioned that he could see the features, craters etc.
> of the dark face on a crescent moon . what causes this effect to be seen
> at some times and not others. does it have to do with the seasons?
> thanks.

The phenomenon you are describing is called "Earth shine." When the moon
is in a crescent phase as seen from Earth, the Earth looks nearly full to
an observer on the moon. You know that the full moon is bright enough to
light up the ground on Earth, but the full Earth is much brighter -- the
Earth appears larger than our moon, and the Earth reflects more sunlight
than the moon.

The full Earth is so bright on the moon that we can actually see the light
from Earth bouncing off of the moon and back to Earth again! This is how
we can see the features on the crescent moon, even if the sun isn't
shining on that part of the moon yet.

Thanks for writing!

Sincerely,
Kurtis Williams

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