The moon doesn't really change shape, it is always a full, whole, spherical moon. We just can't always see all of it, we can only see the light side. The sun shines on a specific side of the moon, and because earth (and the moon) are always moving, the sun is shining on different areas of the moon every day. During a new moon, the sun is shining on the part of the moon we can't see.
the earth blocks it from the sun
The revolving of the earth and the rotating of the moon.
wax and wane
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rise set wax wane
During the time at sea, the sailors saw the moon wax and wane three times.
Two weeks, and two weeks to wane.
Wax and wane
That completely depends on when you start watching it. The question is like asking: "Does it get light or dark outside first ?"
The opposite of to wax is to wane. Wax means to grow and wane means to diminish
It's the earth shadow
The moon orbits the Earth every month and as it does so, we see different parts of the moon lit up by the sun, which causes the moon to go through 'phases'.