Why the moon changes in appearance over the course of the month?

Answer 1

The most important thing to know about moon phases is that the moon does not produce it's own light - it reflects the sun's light to the Earth.

The moon orbits the Earth, so it's position with respect to the Earth and Sun change throughout a month. When the moon is in the same direction as the sun (when viewed from Earth), it cannot be seen as this is called the New Moon. As it orbits the Earth, in a few days we start to see a thin crescent and after a week has passed, we can see one half of the moon's lighted side. After another week, the moon is on the opposite side of Earth as the sun and we can see all of the moon's sunlit side (this is called the Full Moon). After another week we see less of the moon's lighted side and we see a half moon again (except the opposite side of the moon is lit up). We see even less of the moon (it becomes a crescent again) and it eventually becomes a New Moon again.

This cycle repeats itself every 29 days or so.