Astronomy

On January 7, 1610, Galileo Galilei observed with his telescope what he described at the time as "three fixed stars, totally invisible by their smallness," all within a short distance of Jupiter, and lying on a straight line through it. Observations on subsequent nights showed that the positions of these "stars" relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars. On January 10, Galileo noted that one of them had disappeared, an observation which he attributed to its being hidden behind Jupiter. Within a few days he concluded that they were orbiting Jupiter: he had discovered three of Jupiter's four largest satellites (moons): Io, Europa, and Callisto. He discovered the fourth, Ganymede, on January 13. Galileo named the four satellites he had discovered Medicean stars, in honour of his future patron, Cosimo II de' Medici, Grand Duke of Tuscany, and Cosimo's three brothers. Later astronomers, however, renamed them Galilean satellites in honour of Galileo himself.

Galileo later published an account of his telescopic observations of the moons of Jupiter, using this observation to argue in favor of the sun-centered, Copernican theory of the universe against the dominant earth-centered Ptolemaic and Aristotelian theories. The geocentric view had been dominant since the time of Aristotle, and the controversy engendered by Galileo's opposition to this view resulted in the Catholic Church's prohibiting the advocacy of heliocentrism as potentially factual, because that theory was contrary to the literal meaning of Scripture. Galileo was eventually forced to recant his heliocentrism and spent the last years of his life under house arrest on orders of the Inquisition.

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