Galileo turns his telescope to the sky

The story

A spyglass from Holland

In the autumn of 1608, in Holland, a lens-maker, Hans Lippershey, asked the authorities for permission to produce in exclusive an instrument made with two lenses, which made faraway things look close. It was not granted, because other people claimed to have had the same idea. The news, however, spread quickly across Europe. At first that instrument was useful mainly to soldiers and sailors, to see ships and armies from far away.

In the summer of 1609 Galileo Galilei, professor of mathematics at the University of Padua, heard about it during a trip to Venice. He was forty-five, he had been born in Pisa in 1564 and had already done important studies on the movement of bodies. He had not yet seen one, but he understood how it must be made, went back to Padua and built by himself, with a convex and a concave lens in a lead tube, a spyglass that magnified about three times. Then he kept improving it: he reached eight times and finally about thirty.

The gift to the Republic

At the end of August 1609 (around 21–25 August) Galileo took his instrument up the bell tower of St Mark's in Venice, and let the senators of the Republic try it: from the top they saw ships arriving hours before they could be seen with the naked eye. For this gift Galileo obtained a raise in pay and the confirmation of his post for life. Galileo understood, though, that the spyglass could do something much more important than looking at ships. He pointed it at the sky.

A sky never seen before

In the autumn and winter of 1609 Galileo observed the Moon. He expected a smooth, perfect sphere, as the ancient philosophers had taught. Instead he saw mountains, craters and plains, a world made like the Earth. Then he looked at the Milky Way, the great milky “river” of the sky, and discovered that it was made of countless stars too small and distant to be told apart with the naked eye.

The most surprising discovery came on 7 January 1610, when Galileo pointed his instrument at Jupiter. Next to the planet he saw three tiny bright dots, in a line. In the following days he observed them again, and noticed that they moved and that there were four of them: they were not fixed stars, but bodies circling Jupiter, like the Moon around the Earth. Today they are called Io, Europa, Ganymede and Callisto. Galileo named them the “Medicean stars”, in honour of the Medici family, the grand dukes of Tuscany.

The account of these observations was published in March 1610 in a little book of a few pages, written in Latin, the “Sidereus Nuncius”, that is, “The Starry Messenger”, which was an immediate success. Within a few months Galileo left Padua and returned to Florence, as “mathematician and philosopher” to Grand Duke Cosimo II de' Medici.

Evidence for Copernicus

During the rest of 1610 Galileo also observed the phases of Venus, that is, the fact that, like the Moon, Venus sometimes appears as a full circle and sometimes as a thin crescent. This could not happen if Venus went around the Earth, as Ptolemy's system said, but it was explained very well if it went around the Sun. He then saw dark spots on the Sun and Saturn with a strange appearance: in his instrument it seemed to have two “ears”, because Galileo could not make out the rings.

These observations convinced him that Copernicus was right. Galileo began to defend this view strongly, and in the following years it brought him into conflict with many scholars and with the Church. In 1633 he was put on trial in Rome and forced to declare that he no longer believed in the Copernican system. He spent his last years under house arrest in his home at Arcetri, near Florence, where he died in 1642.