Galileo's First Glimpse

First off, I'll dispel the idea that Galileo invented the telescope; he merely improved upon its design. Unlike any human at the time, however, he chose to point the telescope up to the night sky and document what he saw. He saw that the moon was pockmarked by craters and had a surface that resembled Earth's crust. It was no longer a perfect* celestial body but a whole other world with imperfections. To give you a taste of the resolution of his lenses, here's a shot from a modern telescope (I highly recommend clicking on the image for a better view):



*Although details of the moon's surface could not be seen by the naked eye, the dark splotches of its surface could easily be discerned by any earthling. These splotches were explained by two different ideas: a) the moon was so close to Earth that it was "corrupted" by our sins or b) the moon reflected Earth's continents. (Now we think that the dark regions are three billion year old lava flows instigated by early, Everest-size asteroid impacts.)