Why does the Moon change shape?

Watch the Moon over several weeks and it seems to transform. One night it is a thin sliver, later a glowing half, then a brilliant full circle, before shrinking away again. These shapes are called the phases of the Moon, and they repeat in a steady cycle. It looks as though the Moon is changing shape, but here is the surprising truth: the Moon's shape never actually changes at all. It is always a round ball. What changes is how much of it we can see.

The first key fact is that the Moon does not produce its own light. It shines only because it reflects sunlight, the light of the Sun bouncing off its surface. Because the Moon is a sphere, the Sun can only ever light up one half of it at a time, just as the Sun only lights up one half of the Earth at a time. The other half of the Moon is always in darkness. So at every moment, the Moon has a bright sunlit side and a dark unlit side.

The Moon travels in an orbit around the Earth, taking about a month to complete one full trip. As it moves around us, our viewpoint on its sunlit half keeps changing. Sometimes the lit half faces almost directly toward us, and we see a full circle of light. Sometimes the lit half faces mostly away from us, and we see only a thin curved sliver, or nothing at all. The different phases are simply the different amounts of the Moon's bright side that happen to be turned toward Earth.

A very common misconception is that the Moon's phases are caused by the Earth's shadow falling on the Moon. This is not correct. The Earth's shadow does land on the Moon sometimes, but that is a separate and much rarer event called a lunar eclipse. The ordinary monthly phases have nothing to do with shadow. They are entirely about geometry: the changing positions of the Sun, the Earth, and the Moon, and how much of the Moon's permanently sunlit half is visible from where we stand.

This article was written using information from Wikipedia.