Moon phases reveal the passage of time in the night sky. Some nights when we look up at the moon, it is full and bright; sometimes it is just a sliver of silvery light. These changes in appearance are the phases of the moon. As the moon orbits Earth, it cycles through eight distinct phases. The four primary phases of the moon (new moon, first quarter, full moon, last quarter) occur about a week apart, with the full moon its most dazzling stage. | Encyclopedia of Science