The Telescope
Dutch spectacle makers invented the telescope, but Galileo was the first to use the telescope to study the heavens systematically. His little telescope was poorer than any cheap modern amateur telescope, but what he observed rocked the very foundations of Aristotle's universe and the theological-philosophical worldview it supported. It is said that what Galileo saw was so disturbing for some officials of the Church that they refused to even look through his telescope; they reasoned that the Devil was capable of making anything appear in the telescope, so it was best not to look through it.

The Sidereal Messenger
Galileo published this small book describing his discoveries using the telescope. It included his first 3 results:

1) Craters on the Moon
He reported that the moon was not perfect, but instead had deep shadows from valleys and mountains on its surface. He also showed it was a world not unlike Earth.

2) The Milky Way was made of Stars
He showed that the great "cloud" called the Milky Way (which we now know to be the disk of our spiral galaxy) was composed of enormous numbers of stars that had not been seen before.

3) Moons of Jupiter
Galileo observed 4 points of light that changed their positions with time around the planet Jupiter. He concluded that these were objects in orbit around Jupiter. Io, Europa, Ganymede, and Callisto are shown below with approximately the correct scale of their orbit and motion to the size of Jupiter's disk.

These observations showed there were new things in the heavens that Aristotle and Ptolemy had known nothing about. Jupiter's moons demonstrated that a planet could have moons circling it (other centers of motion) that would not be left behind as the planet moved around its orbit!