Teach Engineering: Lunar Learning

Why does the Moon not always look the same to us? Sometimes it is a big, bright, circle, but, other times, it is only a tiny sliver, if we can see it at all. The different shapes and sizes of the slivers of the Moon are referred to as its phases, and they change periodically over the course of a lunar month, which is twenty-eight days long. The phases are caused by the relative positions of the Earth, Sun, and Moon at different times during the month.