On Tue, 2 Nov 1999 you wrote:

> If we lived on the Moon, would we see the Earth go through the same phases as
> the moon does? How long would it take for the Earth to go through each phase
> if it does go through phases?

The reason that we see the moon and other planets go through phases is due
to geometry. Think about the New Moon - the moon is between the Earth and
the sun. When we look at the moon at this time, we are seeing the
nighttime side of the moon. If we lived on the earth-facing side of
the moon at this point, the
sun would be below the horizon. Now suppose that we look back at the
Earth from the moon. Since we are between the Earth and the sun, we are
only going to see the sunlit side of the Earth.

Likewise, when we would see a full moon from the Earth, if we were on the
moon looking back toward the Earth, we would see the dark face of the
Earth.

So, to go from seeing the sunlit side of the Earth to seeing the nighttime
side of the Earth, we would have to see the Earth go through partially-lit
phases.

So, yes, the Earth goes through phases as seen from the moon in the same
amount of time that it takes the moon to go throught a cycle of phases.
The only difference is that the Earth's phase is opposite the moon's
phase. If the moon is full, the Earth is new; if the moon is new, the
Earth is full, etc.

Thanks for writing!

Sincerely,
Kurtis Williams

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