The Moon does not emit its own light, shining instead by reflecting sunlight. Depending on the relative positions of the Earth, Sun and Moon, varying amounts of the lunar surface appear to be illuminated.

A few days after the Full Moon, the illumination of the Moon’s disk as seen from the Earth begins to decrease (position F in the diagram below). The waning (shrinking) gibbous Moon rises after sunset, transits the meridian after midnight and sets after sunrise. The waning gibbous phase repeats every 29.531 days – one synodic month.

The Moon’s motion around the Earth, with the Sun illuminating only one side of the Earth and Moon.