Galileo Spots Jupiter's Moons: Universe Shakes

Three faint specks of light near Jupiter kept shifting positions, and Galileo Galilei could not explain why. Through a homemade telescope with roughly 20x magnification, the 45-year-old professor of mathematics at the University of Padua observed what he initially described as "three fixed stars, totally invisible by their smallness," all aligned in a straight line through Jupiter. Over the following nights, their positions changed in ways that were impossible if they were actually stars. On January 7, 1610, Galileo began the systematic observations that would overturn two thousand years of cosmological certainty. By January 13, he had identified four distinct objects orbiting Jupiter. He named them the Medicean Stars, honoring Cosimo II de'' Medici, Grand Duke of Tuscany, whose patronage Galileo was actively courting. Later astronomers renamed them the Galilean moons: Io, Europa, Ganymede, and Callisto. Ganymede proved to be larger than the planet Mercury. The discovery was devastating to Aristotelian cosmology, which held that all celestial bodies revolved around the Earth. Here was direct observational evidence of a second center of motion in the universe. Objects were clearly orbiting something other than our planet. If Jupiter could have satellites, then Earth''s special status as the center of all celestial motion was no longer defensible. The philosophical and theological implications were enormous. Galileo published his findings in March 1610 in Sidereus Nuncius, the Starry Messenger, which sold out immediately. The observatory of Christopher Clavius, the Vatican''s top astronomer, confirmed the observations. Galileo received a hero''s welcome in Rome in 1611. But the implications of his discovery created enemies among both philosophers and churchmen. Within two decades, the same institution that had celebrated him would put him on trial for heresy. The four moons he spotted through a crude telescope became the cornerstone evidence for heliocentrism and launched the scientific revolution that remade humanity''s understanding of its place in the cosmos.