Galileo (1564-1642) was an Italian astronomer who, with the German astronomer Johannes Kepler, began the scientific revolution that led to the successful works of the English scientist Sir Isaac Newton.

Galileo’s main contributions to astronomy were the use of the telescope in observation and the discovery of sunspots, mountains and valleys on the moon, the four largest moons of Jupiter, and the phases of Venus.

By December 1609, Galileo had built a telescope that magnified objects 20 times their real size. He used it to discover mountains and craters on the moon. He also saw that the Milky Way was composed of stars, and he discovered the four largest moons of Jupiter. By December 1610 he had observed the phases of Venus, which disagreed with Ptolemy’s theory of the earth at the center of the universe and helped support Copernicus’ theory that the sun was at the center.

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