Galileo

Galilei Galileo
On Sunspots
The Assayer

Within two years after the telescope's invention by Dutch optician Hans Lippershey (1587-1619), Galilei Galileo (1564-1642) used it to make his revolutionary observations. Using the telescope, he discovered the moons of Jupiter (known as the Galilean satellites), spots on the Sun, and mountains and craters on the Moon. Also he discovered that the Milky Way is a dense collection of stars.

He used his discovery of Jupiter's moons to show that the Earth was not unique in having a moon. Also, he used his discovery of Moon's irregular surface to argue that the Earth was not unique in being imperfect.

Galileo defended the Copernican system against Aristotelian theory. He also studied motion and wrote one of the first books on mechanics.

His argument that the book of nature is not verbal but mathematical is considered to be a foundation of modern science, in which use of the experimental method to discover the facts of nature became a recognized method.