Galileo and the Sidereus Nuncius: the Moon through the telescope

In March 1610 a short Latin book of a few dozen pages appeared in Venice and changed for ever the way people looked at the sky. It was called Sidereus Nuncius, the announcement or messenger of the stars, and it was signed by Galileo Galilei, then professor of mathematics at the University of Padua. Among the novelties it announced, the first and most immediate concerned the Moon: not a perfect, polished sphere, as the Aristotelian tradition held, but a world of mountains and valleys.

A telescope aimed at the Moon

In the summer of 1609 Galileo had heard of the Dutch invention of a tube with lenses able to bring distant objects closer. He built ever more powerful versions, up to instruments magnifying about twenty times, and in the autumn of that year he began to observe the Moon regularly. What struck him above all was the terminator, the line dividing the lit part from the part in shadow: instead of being sharp, it looked jagged.

Galileo noticed bright points appearing in the dark zone and spreading as the Sun rose higher, just as the peaks of earthly mountains catch the light at dawn before the valleys. He deduced that the lunar surface was covered with heights and hollows, and from the length of the shadows he calculated that some mountains on the Moon were taller than many on Earth.

Drawings and engravings

The lunar observations are documented by a series of watercolours made by Galileo himself, now kept at the National Central Library in Florence. Galileo had studied drawing and perspective, and his trained hand could render with a few washes of ink the effect of raking light on the relief. In the printed book the images were translated into engravings showing the Moon in different phases.

He was not the first to draw the Moon through a telescope: the Englishman Thomas Harriot had already made a sketch of it in the summer of 1609. But Harriot published nothing, while Galileo was able to interpret what he saw and communicate it to the whole of learned Europe. The book, dedicated to Cosimo II de' Medici, also announced the discovery of the four satellites of Jupiter, named the Medicean stars.

Earthshine and the echo in art

The Sidereus Nuncius also dealt with earthshine, the faint glow that makes the dark part of the Moon visible in the days around the new moon. Galileo explained it as sunlight reflected from the Earth towards the Moon: an idea Leonardo da Vinci had already noted in his manuscripts, which remained unpublished. The Earth thus became, in its turn, a moon for the Moon.

The discovery had an immediate echo in painting as well. Between 1610 and 1612 Ludovico Cardi, known as Cigoli, a friend of Galileo, frescoed in the Pauline Chapel of Santa Maria Maggiore in Rome an Immaculate Virgin resting her feet on a Moon marked with craters, just like the one described in the Nuncius. It was perhaps the first time the new astronomy had entered a church.

With the Sidereus Nuncius the Moon ceased to be an incorruptible star and became a body similar to the Earth, observable and measurable. That apparently simple discovery undermined the distinction between the celestial and the earthly world on which ancient physics rested, and opened the way to the defence of the Copernican system that would cost Galileo his trial in 1633.