In January 1610, Galileo Galilei turned his improved telescope toward Jupiter and discovered four celestial bodies orbiting the planet — now known as the Galilean moons: Io, Europa, Ganymede, and Callisto. He published his observations in Sidereus Nuncius (Starry Messenger) in March 1610. The discovery was revolutionary: it proved that not everything orbited the Earth, directly contradicting the geocentric model. Combined with his observations of Venus's phases, lunar craters, and sunspots, Galileo's telescopic discoveries provided powerful empirical support for the Copernican heliocentric model. His advocacy eventually led to his trial before the Inquisition in 1633.
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