Galileo refers in this to a book he had published earlier, the "Starry Messenger". It is well worth reading.

Galileo had just build the first ever telescope and he describes what he has discovered: there are incredibly more stars than can be seen by the naked eye; Jupiter has satellites; there are mountains on the moon and he has figured out their height from the length of their shadows... The book is full of the quiet excitement of a man who has discovered a new world, and seen what no man had seen before.

https://archive.org/details/siderealmessenge80gali/page/6/mo...