Galileo Galilei demonstrates his first telescope to Venetian lawmakers.

Galileo Galilei was an Italian astronomer, physicist, and mathematician who played a major role in the Scientific Revolution. In 1609, Galileo learned about a new Dutch invention called the “spyglass,” which used lenses to make distant objects appear closer. Although he did not invent the telescope, Galileo quickly understood its potential and began constructing improved versions of the instrument. By carefully grinding lenses and experimenting with their arrangement, he produced telescopes that were considerably more powerful than the early Dutch designs. Galileo then turned his telescope toward the night sky, becoming one of the first people to systematically use the instrument for astronomical research.

Beginning in late 1609 and early 1610, Galileo made a series of observations that transformed humanity’s understanding of the heavens. He discovered that the Moon had mountains, valleys, and an uneven surface, contradicting the traditional belief that heavenly bodies were perfectly smooth. He observed countless previously invisible stars in the Milky Way and discovered four moons orbiting Jupiter—Io, Europa, Ganymede, and Callisto—which are now known as the Galilean moons. Galileo also observed the phases of Venus and studied sunspots on the Sun. These discoveries provided powerful evidence that the heavens were far more complex and changeable than many ancient philosophers had believed.

Galileo’s telescopic observations also strengthened his support for the heliocentric model associated with Nicolaus Copernicus, in which Earth and the other planets orbit the Sun. The moons of Jupiter demonstrated that not everything in the universe revolved around Earth, while the full range of Venus’s phases was incompatible with the traditional Ptolemaic model. Galileo published many of his early discoveries in *Sidereus Nuncius* (*The Starry Messenger*) in 1610, bringing him international fame. His defence of heliocentrism later brought him into conflict with the Roman Catholic Church, culminating in his trial by the Roman Inquisition in 1633. Despite this controversy, Galileo’s use of the telescope helped establish observation and experimentation as essential tools of modern science and permanently changed the way people viewed the universe.