The first thing he observed is that the Moon has valleys and mountains, the height of which he calculated from its shadow on the lunar surface. It was so similar to Earth that from these observations many thought that the Moon could be a place with life. He also discovers that Jupiter has satellites that obey Kepler’s two laws that were already known at the time.
The figures on the left show the drawings of the lunar phases and the reliefs of the Moon made by Galileo from his observations with the telescope.
In the summer of 1612, Galileo observed, with the help of his telescope in projection mode, the presence and evolution of sunspots, which contradicted the immutability of the Sun. In figure 5 we can see an original drawing by Galileo and below ( figure 6) a modern photograph of the same phenomenon.
To make these observations, Galileo projected the real image of his telescope on a screen, since if the Sun is observed directly (even with the naked eye), retinal burns occur.