The Motion and Phases of the Moon
The orbit of the moon about the Earth is very nearly circular. It takes part in the nightly rotation of the night sky, but moves quickly relative to the fixed stars, making a complete orbit in about a month, and also changing it's appearance – the moon has phases, depending on it's position relative to the Earth and Sun. We see the moon because of the light it reflects from the Sun. The amount of reflected light varies with the Sun – moon – Eerth alignment as shown in the diagram below.