Jupiter's moons

The absolute novelty described in Sidereus nuncius for the first time and already shown on the work's title page was the discovery of Jupiter’s moons with the help of a telescope. Galileo had built his first telescope in 1609 after learning that a Dutchman had been making such instruments. The results of his precise calculations of the periods of circulation of Jupiter's moons are amazingly close to modern calculations. Galileo seemed to have sensed that his extraordinary calculations represented a further confirmation of the Copernican world view.