These were the first realistic Moon images. Galileo’s art training and knowledge of chiaroscuro (a technique for shading light and dark) allowed him to recognize the shadows as mountains and craters. Galileo created this famous set of six watercolors of the Moon in its various phases “from life” in the autumn of 1609. Galileo’s drawings are outstanding in their detail and accuracy. Galileo had a weak vision in one eye and finally lost both.
You can also read Galileo’s beautiful book Dialogue Concerning the Two Chief World Systems and the biography from Springer, Galileo Galilei: At the Threshold of the Scientific Age.