The bright lunar side appears to change continuously, because of the sunlight reflection, according to the lunar position in relation to Earth and Sun. The changes of its apparent shape are called moon phases.
The time between identical moon phases is called synodic month and its duration is 29,530589 days. The noun month has as etymological origin the indoeuropean root: mēns- (–>moon),(1) the moon-sickle of the first and last days of the lunar circle.
The Sun and the Moon are in synod (conjuction) during the New Moon, while they have opposites positions during Full Moon.
The moon phases are illustrated in the figure according to the lunar position. The shapes during a complete revolution are symmetrical during Moon waxing and Moon waning, i.e. from New Moon to Full Moon and, again, from Full Moon to New Moon. Therefore, the bright side has a D shape, while the Moon is waxing, and a C shape, while it is waning.
Another interesting phenomenon is the light of the dark side of the Moon close to the New Moon, i.e. when the bright side is a very thin crescent. This light is called ashen glow; it is the earthshine – the reflected light from Earth to Moon which returns to our eyes.
- Etymological dictionary of Greek, by Robert Beekes, with the assistance of Lucien van Beek, Brill, Leiden-Boston, 2010, vol. ΙΙ, p. 945.