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Galileo's First Sightings

For thousands of years, people believed the Sun and all the heavens orbited around the Earth.

In the sixteenth century, Nicolaus Copernicus concluded that the planets traveled around the Sun. This was a very controversial idea, because it seemed to contradict the Bible and all the accepted traditions of the time.

Galileo Galilei
Galileo Galilei

In 1610, Galileo Galilei used a new optical instrument -- known today as a telescope -- to look at the night sky. He discovered several points of light close to the planet Jupiter. He assumed they were stars, fixed in the heavens, but their strange alignment intrigued him. The next night he looked again, and saw that they had changed position. The bright points were dancing around Jupiter --- they were not acting like stars at all. Galileo realized he was seeing bodies in orbit around Jupiter. Copernicus was right - the heavens do not revolve around Earth.

Originally, Galileo named Jupiter's four bright moons after his sponsor, the Medici family. Later, the moons were called Io, Europa, Ganymede, and Callisto, after mythical persons who were companions of the god Jupiter. In honor of Galileo, they are now called the Galilean moons.

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