Phases of the Moon

The appearance of the illuminated part of the Moon as seen from the Earth varies as the relative positions of the Earth, Sun and Moon change; the fastest of these movements is the translation of the Moon around the Earth. The change in appearance gives rise to the so-called lunar phases or phases of the Moon.

The period of synodic rotation of the Moon, i.e. the time between two consecutive new moons, is called lunation. By convention, the lunation begins at the instant of the new moon, i.e. when the geocentric longitudes of the Moon and the Sun coincide. The instant when these longitudes differ by 180º is called full moon, and when they differ by 90º or 270º, respectively, it is known as first or last quarter. The phases of the Moon for the current calendar year are listed in the following table.