Moon Phases
The Moon's shape and brightness for each of the next four weeks, and the exact dates of the next new moon, first quarter, full moon and last quarter.
The lunar cycle
A full cycle of phases — new moon to new moon — takes 29.53 days on average, called the synodic month. Because that is shorter than most calendar months, a month occasionally contains two full moons; the second is popularly called a blue moon. The Moon's illuminated fraction follows the angle between Sun, Moon and Earth, which is why it waxes and wanes smoothly rather than in steps.