Galileo Galilei (1564 - 1642)
Galileo was born in Pisa, and was the son of a musician. He started to study medicine at the University of Pisa, but he later changed to philosophy and mathematics. He became a professor of mathematics at the University in 1589. Galileo was interested in many aspects of the world around him; he studied velocity, gravity, relativity, projectile motion, pendulums, and compasses.
In 1609, Galileo heard of the invention of the telescope in The Netherlands. Based only on uncertain descriptions of the Dutch telescope, Galileo designed a telescope with 3x magnification, and he later improved his design to achieve up to 30x magnification. Using a Galilean telescope, the observer could view magnified, upright images on the Earth - this design of telescope is commonly called a spyglass today.
Galileo also used his telescope to observe the sky. On 7th January 1610, he described "three fixed stars, totally invisible by their smallness" close to Jupiter. On subsequent nights, observations of the "fixed stars" showed that they moved relative to Jupiter, and sometimes disappeared behind Jupiter. He soon concluded that the objects were orbiting around the planet: he had discovered three of the Jupiter's four largest moons. He discovered the fourth on 13th January, and they became known as the Galilean Moons of Jupiter.
During his lifetime, Galileo was a firm believer in the Copernican model of the solar system, in which the planets orbit around the Sun (a heliocentric model). His other notable discoveries in astronomy include the phases of Venus, Saturn's rings, and sunspots.