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 week after New Moon, the Moon reaches its First Quarter. In this phase, the Moon is in quadrature (elongation = 90^o, position C in the diagram below), and one half of the Moon’s disk is illuminated as seen from Earth.

The First Quarter Moon rises at noon, transits the meridian at sunset and sets at midnight. The First Quarter phase repeats every 29.531 days – one synodic month.