Museo Galileo - Museum of the History of Science in Florence
Galileo invented many mechanical devices besides the telescope, such as the hydrostatic balance, a pendulum clock and a high power water pump powered by one horse. Of course, his most famous invention was the telescope. Galileo made his first telescope in 1609, modeled after telescopes produced in other parts of Europe that could magnify objects three times. He created a telescope later that same year that could magnify objects twenty times. You might argue that although he didn't invent the first telescope, he obviously improved upon it.
With this telescope, he was able to look at the moon, discover the four satellites of Jupiter, observe a supernova, verify the phases of Venus, and discover sunspots. His discoveries proved the Copernican system which states that the earth and other planets revolve around the sun. Prior to the Copernican system, it was held that the universe was geocentric, meaning the sun revolved around the earth