the moon

[debomecha]

Why the moon sometimes appear completely like circle and sometimes not?

[Ethel]

Do you mean the waxing and waning of the moon through each monthly cycle? If so there is a good explanation and graphics here:

http://en.wikipedia.org/wiki/Lunar_phase

Otherwise it is just clouds that make the moon look as if there are bits missing. Or the passing of a star or planet obscuring the moon.

[delboy3]

Ethel, you were doing so well until your last sentence. Think about it...

[mibn2cweus]

Like the Earth, the Moon also experiences day and night.

By considering how the Moon is illuminated by the Sun the position of the Sun relative to an observer on Earth can be determined at night by the part of the Moon that is being illuminated.

A general idea of what time it is can also be deduced by considering the position and Phase of the Moon

[Loosehead]

I think it'd be very noticeable if a star or planet was between us and the moon ethel!