When word of a simple lens device patented in the Netherlands in 1608 reached Padua in the summer of 1609, Galileo Galilei built his own telescope from the description alone; within months he had improved it to roughly twentyfold magnification. Until then the telescope had been treated mainly as a maritime and military instrument; Galileo's innovative step was to turn it on the night sky.

In the few months between late 1609 and early 1610 his observations conflicted one after another with received cosmology. The Moon's surface was not smooth and perfect but covered with craters and mountains — from shadow lengths Galileo could even roughly compute the heights of lunar peaks. Around Jupiter, four small points of light shifted positions from night to night; these were moons of the planet, a concrete instance of an orbital system not centred on Earth. The Milky Way, examined through the lens, dissolved into countless individual stars invisible to the naked eye.

Galileo published his findings in March 1610 in a slim Latin pamphlet printed in Venice — Sidereus Nuncius, 'Starry Messenger'. The booklet spread across Europe with unusual speed; Kepler wrote a supportive reply almost at once. By naming the new moons the 'Medicean Stars' Galileo secured the post of court mathematician in Tuscany and returned to Florence. Later observations would add the phases of Venus and sunspots.

The observations were the first direct, public, repeatable evidence in favour of the Copernican system; but a clash with authority was unavoidable. In 1616 Rome warned Galileo not to defend heliocentrism beyond the level of 'hypothesis', and after the Dialogue of 1632 the 1633 Inquisition trial forced him to recant and placed him under house arrest for the rest of his life. The episode became the lasting symbol of the tension between observational evidence and established authority — while the telescope itself became the central instrument of astronomy.