The lunar phases: why the Moon changes its face every night
From new moon to full moon: how the eight lunar phases arise, what the terminator and the quarters are, and which symbols tradition attached to them.
No other celestial body changes its appearance so visibly and so regularly. Every evening the Moon shows a different portion of its lit disc, and this transformation, repeated before the eyes of every people, was humanity's first clock. Understanding the phases means understanding a simple geometry: the relative positions of Sun, Earth and Moon, and the point from which we observe them.
A sphere lit by half
The Moon produces no light of its own; it reflects the light of the Sun. At every moment half of its surface is lit and half lies in shadow, just as day and night divide the Earth. What changes during the month is how much of the lit hemisphere faces us. As early as the fifth century BC the philosopher Anaxagoras of Clazomenae argued that the Moon receives its light from the Sun, an idea that was far from obvious at the time.
The line separating the bright part from the dark part is called the terminator. Along this line, for an imaginary observer standing on the Moon, the Sun is rising or setting. The shadows of craters and mountains grow long, which is why through a telescope the lunar relief looks far sharper near the terminator than in the flat light of the full moon.
The eight stages of the cycle
By convention eight phases are distinguished. At new moon the satellite lies roughly between the Earth and the Sun and turns its dark face towards us, invisible in the glare of day. Then come the waxing crescent, the first quarter, with half the disc lit, and the waxing gibbous phase. Full moon arrives when the Moon is opposite the Sun: it rises at sunset and sets at dawn. The cycle then runs in reverse through waning gibbous, last quarter and waning crescent.
The first and last quarters are called quadratures, because the Moon and the Sun, seen from the Earth, form an angle of ninety degrees. New moon and full moon are instead called syzygies, from a Greek term meaning being yoked together. Since the Moon moves eastwards by about thirteen degrees a day against the stars, it rises on average some fifty minutes later each day.
Proverbs for recognising them
To tell the waxing moon from the waning one, Italian popular tradition coined a saying that is still well known: «hump to the west, waxing moon; hump to the east, waning moon». The hump is the convex side of the crescent. An old Latin mnemonic instead called the Moon «deceitful», because when it draws a C it is decreasing and when it draws a D it is increasing. Both rules apply in the northern hemisphere; south of the equator the figure appears reversed.
The symbol of becoming
Since antiquity the phases have been read as an image of birth, maturity and decline. In the late Greek world, above all in the Orphic hymns and in magical texts, Selene, Artemis and Hecate were at times associated with the different aspects of the Moon. The Moon that vanishes and returns each month became in tradition a symbol of regeneration, and its constant return a promise that darkness too comes to an end.
Watching the phases remains the simplest and most instructive astronomical exercise: it is enough to look at the sky at the same hour for a few consecutive evenings, noting the shape and position of the crescent. In that slow metamorphosis one reads both the geometry of the solar system and the root of countless calendars, myths and rites that humanity has built around the lamp of the night.