Galileo discovers Jupiter's largest moons
On this day · 7 January 1610
Four faint dots beside Jupiter refused to behave like stars, and quietly demolished the idea that everything circles Earth.
On January 7, 1610, Galileo Galilei pointed his improved 20-power telescope at Jupiter and noticed three little points of light strung beside the planet. He first took them for background stars, but night after night they shifted position, clinging to Jupiter rather than drifting past it.
Within days he spotted a fourth and reached an unavoidable conclusion: these were moons, circling a planet other than Earth. He published the findings that March in Sidereus Nuncius, the first scientific work built on telescopic observation.
The moons removed a key objection to Copernicus: that a Sun-centered cosmos required more than one center of motion.
Here was visible proof that not everything orbited our world. The discovery helped pry open the heliocentric debate that would later put Galileo at odds with the Church. Today the four bodies, Io, Europa, Ganymede, and Callisto, are still called the Galilean moons in his honor.
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