What features did Galileo confirm existed on the Moon's surface starting in 1609?

Mountains and valleys

Galileo's crucial observations, initiated around November 1609, definitively proved that the Moon was not the perfect, smooth, and unchanging celestial body described by ancient Greek philosophy, particularly the doctrines derived from Aristotle. Instead of a flawless orb, his rudimentary telescope revealed features analogous to Earth's surface. He observed high relief structures, specifically identifying mountains whose peaks caught the light while their bases remained in shadow near the terminator. He concluded that the Moon possessed both 'great prominences and depressions,' indicating the presence of elevated terrain and deep valleys, thereby establishing the Moon as a world with physical, terrestrial-like geology rather than a pristine celestial object.