What specific observation in the Moon revealed celestial imperfection to Galileo?

Mountains, valleys, and craters

One of the primary components of the ancient cosmological model, upheld by Aristotelian and Ptolemaic thought, was the belief that celestial bodies, especially the Moon, were composed of perfect, ethereal material, rendering them flawlessly smooth spheres. Galileo's telescopic observations directly refuted this centuries-old dogma. When he aimed his improved instrument skyward, he discovered that the Moon's surface was visibly rugged, marred by topographical features such as mountains, valleys, and impact craters. This observation demonstrated that the Moon was a physical world, much like Earth, subject to physical processes and imperfections, thereby shattering the traditional metaphysical separation between the corruptible terrestrial realm and the eternal, pure celestial realm.