He made important astronomical discoveries.
Galileo was able to observe and comprehend several astronomical phenomena as a result of his telescope observations. He noted the moons around Jupiter and gave descriptions of the Venus and Moon phases. He also recorded observations of the Milky Way, Neptune, and the Sun's sunspots.
Using a telescope, Galileo saw what he initially characterized as "three fixed stars, utterly invisible because of their smallness" on January 7, 1610; these stars were all near Jupiter and formed a straight line that passed through the planet. The positions of these "stars" about Jupiter were observed to change on the nights that followed, in ways that were incomprehensible if they were fixed stars. He quickly concluded that they were orbiting Jupiter after finding three of the planet's four largest moons.
Galileo first noticed in September 1610 that Venus displays a broad spectrum of phases that are comparable to those of the Moon. All phases of Venus should be visible, according to the heliocentric model of the Solar System proposed by Nicolaus Copernicus, because Venus's orbit around the Sun causes its lit hemisphere to face Earth when Venus is on the other side Sun and facing away. Earth when it is positioned on the Earth-Sun side. The orbits of any planet cannot cross those of the Sun's spherical shell in Ptolemy's geocentric model.
About the heliocentric theory, which held that the planets circled the Sun rather than the Earth, all these discoveries demonstrated that Copernicus was correct. Galileo's observation of craters on the moon demonstrated that Aristotle was mistaken when he claimed that all celestial bodies were perfect beginning with the moon.
