The first celestial body Galileo observed with his telescope was the Moon. In late 1609, he turned one of his improved telescopes toward the lunar surface and saw that it was not a perfectly smooth, polished sphere, as many Aristotelian philosophers believed.
Galileo noticed bright and dark regions, jagged boundaries, and shifting shadows near the line between lunar day and night. He correctly interpreted these patterns as evidence of mountains, valleys, and craters. The Moon therefore became an early and powerful example of the telescope changing ideas about the heavens.
A common mix-up is Jupiter. Galileo soon made his most famous discovery there: four moons now called Io, Europa, Ganymede, and Callisto, observed in January 1610. Those observations were published in Sidereus Nuncius, but they came after his initial telescopic study of the Moon. English astronomer Thomas Harriot had also viewed the Moon earlier, but Galileo was the first to explain its surface features systematically.