A third quarter moon, sometimes called a last quarter moon, occurs about three weeks after a new moon. The moon reflects light from the sun with varying degrees of illumination during different phases.
The moon goes through many phases each lunar month due to its position in relation to the Earth and sun. The new moon occurs when there is no reflected sunlight visible on its surface, leaving most of the moon's disk in darkness. During a third quarter moon, which occurs about a week before the moon is full, the majority of its surface area is visible with only a quarter remaining dark.