Galileo Galilei was an Italian astronomer, physicist, and engineer who made fundamental contributions to science in the late 16th and early 17th centuries. He improved the telescope and used it to make groundbreaking astronomical observations, including the moons of Jupiter and the phases of Venus, which supported the heliocentric model of the solar system. Galileo also conducted influential studies on motion and mechanics, developing laws of falling bodies and pendulum motion, which laid the groundwork for classical physics.