Galileo had spent the autumn of 1609 building better telescopes. On the night of January 7, 1610, he pointed one at Jupiter and noticed three faint 'stars' in a straight line through the planet. The next night they had moved the wrong way for fixed stars. Over the following weeks he worked out that four bodies were circling Jupiter. It was the first proof that not everything in the heavens orbited the Earth, and it gave strong support to the Copernican system.
At first Galileo thought he was looking at three ordinary background stars, lined up by chance through Jupiter. What caught his attention came a night later, when they seemed to have shifted in the wrong direction. He kept watching and making sketches, and on January 13 he spotted a fourth. These were not stars. They were moons, carried along with Jupiter the way our Moon travels with the Earth.
In March 1610 he rushed his findings into print in a short book, Sidereus Nuncius, or 'Starry Messenger'. With an eye on patronage, he named the moons the Medicean Stars after Cosimo II de' Medici, Grand Duke of Tuscany. It worked: Galileo soon became court mathematician in Florence.
The name did not last. Today Io, Europa, Ganymede and Callisto are called the Galilean moons. The discovery mattered far beyond astronomy. A planet with its own moons showed that Earth was not the single centre around which everything turned, and that argument would eventually bring Galileo before the Inquisition.
Key people: Galileo Galilei