What did Galileo Galilei observe on the Moon through his telescope?

In late 1609, Galileo Galilei began using his improved telescope to observe the heavens, leading to discoveries that fundamentally challenged the prevailing Aristotelian view of the universe.12

Through his observations, Galileo discovered that the Moon is not a smooth, polished, or perfect sphere as previously believed. Instead, he observed that its surface is rough, uneven, and characterized by mountains, hills, and valleys. By tracking the phases of the Moon and observing the movement of the terminator across its disk, he was able to reveal the relief of these lunar features.1234

These findings were significant because they disproved the traditional cosmological assumption that there was an absolute difference between the "corrupt" earthly region and the "perfect" and unchanging heavens. Galileo documented these observations in his 1610 work, The Sidereal Messenger.1