Centre of Mass

Earth and Moon taken by
the Galileo Spacecraft
Credit: NASA
Mass is the amount of "stuff", or matter, and object has and this amount remains the same wherever the object is, even though it's weight will change depending upon gravity.

Phases of the Moon

It takes the Moon around 27 days to orbit around the Earth, during this time we see the Moon go through a series of different shapes, known as the "phases" of the Moon. It takes around 29.5 days for the Moon to get back to exactly the same phase, this is a little longer than it takes it orbit the Earth, due to the fact that the Earth is also moving around the Sun at the same time.