How did Galileo use a telescope to study the Moon in 1609?

In 1609, Galileo Galilei began using a telescope of his own construction to study the heavens, shifting the focus of astronomy from the movement of celestial bodies to their physical character.12

After learning of the invention of the spyglass in the Netherlands, Galileo improved the design by teaching himself the art of lens grinding, eventually producing instruments that magnified up to 20 times. When he turned these telescopes toward the Moon, he observed that its surface was not smooth, as had been previously thought, but was instead rough and uneven, featuring hills and valleys.23

In December 1609, he documented these findings by drawing the lunar phases as they appeared through his telescope. These observations were significant because they challenged the prevailing Aristotelian belief that the heavens were perfect and unchanging, providing empirical evidence that the Moon was a world with a complex, mountainous surface.23