Phases of the Moon are based on what observable portion of the Moon is illuminated by the sun. At all times, half the Moon is illuminated by sunlight. We on Earth however, don’t always see all of the illuminated side. As a result, a new Moon, full Moon, and everything in-between is a result of the position of the Moon in relation to the Earth and the sun. As the Moon orbits around Earth, these relative positions change, causing the Moon to appear differently each night.