Lunar eclipses: why our satellite turns red
Umbra, penumbra and light bent by the atmosphere: how a lunar eclipse happens, what the Danjon scale measures and how the ancients interpreted it.
During a total lunar eclipse the full disc does not vanish: it fades slowly and takes on hues ranging from copper to dark red, sometimes almost to black. The spectacle, visible to the naked eye from the whole night side of the Earth, is among the celestial phenomena that most struck ancient civilisations, and its explanation is both simple and elegant.
The shadow of the Earth
A lunar eclipse occurs when the Earth comes between the Sun and the full Moon, and the Moon passes through the cone of shadow cast by our planet. Since the lunar orbit is tilted, this does not happen at every full moon, but only when the full moon falls near one of the nodes. Around the true shadow, called the umbra, lies the penumbra, a region where the Sun is only partly hidden.
Eclipses are therefore classed as penumbral, often almost imperceptible, partial and total. Totality can last just over an hour and forty minutes in the most favourable cases, when the Moon crosses the centre of the shadow. Aristotle, in his treatise On the Heavens, already observed that the always curved outline of the shadow cast on the Moon was proof of the spherical shape of the Earth.
The colour of copper
The red comes from the Earth's atmosphere. Sunlight grazing the edge of the planet is bent by refraction into the cone of shadow, while the air scatters mainly blue light, the same reason why the sky is blue and sunsets are red. The eclipsed Moon is thus lit by all the sunrises and sunsets taking place on Earth at that moment.
The shade changes from one eclipse to another according to the state of the atmosphere. After major volcanic eruptions, such as that of Pinatubo in 1991, dust high in the air can make the eclipsed Moon very dark. To describe these differences the French astronomer André Danjon proposed in the twentieth century a scale running from 0, for almost black eclipses, to 4, for bright orange eclipses with a bluish rim.
Omens and stories
Ancient chronicles often record lunar eclipses as omens. In Mesopotamia in the first millennium BC they were considered threatening signs for the king, and cuneiform texts describe the rite of the «substitute king», a man who temporarily occupied the throne to draw the announced misfortune upon himself. In many cultures it was believed that an animal or a monster was devouring the Moon, and people made noise to drive it away.
The expression «blood moon», now widely used for total eclipses, takes up a biblical image: the prophet Joel announces that before the day of the Lord the moon will be turned into blood. Its use as a popular name for the astronomical phenomenon, however, has spread only in recent times, above all through the media and popular astronomy.
A mirror of the atmosphere
For modern science the lunar eclipse is also a measuring instrument. The brightness of the eclipsed Moon allows the transparency of the upper atmosphere to be estimated, and observing the shadow already helped in antiquity to calculate the relative sizes of the Earth and the Moon, as Aristarchus of Samos did in the third century BC with a geometrical method that became famous.
Today lunar eclipses are predicted to the minute centuries in advance and can be watched without any eye protection. Yet they remain one of the few moments when the motion of the heavenly bodies becomes visible to everyone, and when the colour of the Moon tells of the state of the air we breathe.