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Four hundred years ago, in 1609, Galileo Galilei made his first telescope and turned it to the night sky. In January, 1610, he discovered four moons of Jupiter: Io, Europa, Callisto, and Ganymede.
The International Year of Astronomy is being celebrated in 2009 to honor both this event and the publication of _Astronomia nova_ by Johannes Kepler the same year.
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