Galileo discovered several key laws of motion, such as the Principle of Inertia, and used a telescope to view the Moon and later Jupiter, where he discovered its moons for the first time. He wrote a book supporting the Copernican heliocentric model, in which the Sun is at the center of the universe, but had to recant this view during his trial by the Inquisition in 1633.
Galileo Galilei (1564-1652) trained in mathematics and used it to help refine the scientific method by taking precise measurements during experiments and tabulating the results. His experiments showed a pendulum’s swing always takes the same amount of time, no matter the size. Other experiments using balls rolling along an inclined plane led to the discovery that all objects accelerate at the same rate. He made other contributions to engineering, surveying, mathematics and the scientific method that helped later scientists develop their work.
Galileo’s fame comes from his telescope observations. He did not invent the telescope, but he was one of the first to use it, observing the planets and stars and then published the results. He observed that the moon was not smooth but had craters and mountains. He observed Saturn’s rings but mistook them for two bodies. He showed the sun had spots moving across its surface and found many more stars than could be seen with the naked eye.