Moon

The Moon revolves with Earth around the Sun as the Moon is revolving around Earth. 

The Moon revolves around Earth in 29½ Earth days. 

Similarly to Earth, as the Moon revolves, it is rotating or spinning on its axis. The rotation time for the Moon is a little over 27 Earth days. 

Because the Moon rotates and revolves in nearly the same amount of time, the same side of the Moon always faces Earth. 

As the position of the Moon changes as it revolves around Earth, the

 observations of the Moon from Earth are seen as phases.

The cause of the phases of the Moon depends on how much of the

 sunlit side of the Moon faces Earth.