From the Earth, we can see multiple phases of the Moon in a 28 days cycle. These phases are not changes int he Moon or how much of it is lightened, but instead it is a result of the positions that the Earth, Moon, and Sun have in relation to each other. The Sun is always lighting one half of the Moon, which is the half that is turned toward it. From the Earth it doesn't seem that way, as we can see Moon that seems fully light, partially light on the left side, partially light on the right side, or totally dark, but all of that depends on how much of the lighted side of the Moon is turned toward the Earth.
