Earthshine: Leonardo and the glow of the Earth on the Moon

On the evenings after new moon, when the crescent is still thin, one notices that the rest of the disc is not entirely black: it appears faintly lit, an ashen grey. This is earthshine, called in Italian luce cinerea, the ashen light, a phenomenon that intrigued observers and poets for centuries and whose correct explanation is owed to one of the greatest minds of the Italian Renaissance.

The Earth as a mirror

The part of the Moon not lit by the Sun receives light reflected by the Earth. When it is new moon or nearly so for us, for an observer on the Moon it would be full Earth: our planet, much larger and more reflective than the Moon, appears in the lunar sky as a bright disc that brightens the satellite's night. Part of that light returns towards us, and this is what we see as earthshine.

The phenomenon is most evident with the waxing crescent after sunset and the waning crescent before dawn, when the sky is dark and the Moon is still close to the Sun. As the days go by, the Earth seen from the Moon enters its own waning phase, and earthshine fades until it is lost in the glare of the part lit by the Sun.

Leonardo's explanation

The correct explanation appears in Leonardo da Vinci's Codex Leicester, written roughly between 1506 and 1510. Leonardo understood that the glow of the dark part came from sunlight reflected by the Earth. He attributed the main role to the surface of the seas, whereas we now know that clouds and ice are the main reflectors of light, but the principle was correct.

Leonardo's notes long remained unknown. The idea was put forward again independently at the end of the sixteenth century by the German astronomer Michael Mästlin, Kepler's teacher, and Galileo discussed it at length in the Sidereus Nuncius of 1610. For Galileo earthshine showed that the Earth shines like the other heavenly bodies and is not a dark, motionless place cut off from the rest of the cosmos.

Between poetry and folklore

Earthshine also entered popular tradition. In the British Isles the phenomenon was described as the old moon in the arms of the new, and an old Scottish ballad, Sir Patrick Spens, presents it as an omen of storm for sailors. In Italian the very name, tied to the colour of ash, underlines the pale and almost ghostly character of that glow, which in many cultures accompanied the idea of the reborn Moon.

A measure of climate

In the twentieth century the French astronomer André Danjon systematically measured the intensity of earthshine in order to estimate the amount of light reflected by the Earth, its albedo. Similar programmes have also been carried out in recent times, because variations in earthshine mirror changes in our planet's cloud cover and provide useful data for climate studies.

Whenever we observe the thin crescent with its grey disc, we are in fact seeing sunlight that has touched the Earth, reached the Moon and returned to our eyes. In that faint reflection, which Leonardo was the first to read, the Earth shows itself for an instant as a star among the stars.