In early January 1610, Galileo Galilei trained his telescope on Jupiter and recorded the positions of what he initially took for fixed stars near the planet, observations that within days he recognized as moons orbiting Jupiter itself. The discovery undercut the Aristotelian idea that all celestial bodies revolved around Earth.
Using a telescope he had built himself, with roughly 20x magnification, Galileo began systematically observing Jupiter from Padua in the first days of January 1610. On the night of January 7, he noticed three small 'stars' in a straight line near the planet; returning to the same patch of sky the following nights, he realized their positions shifted in a way inconsistent with fixed stars.
By mid-January he had identified a fourth body and confirmed that all four were orbiting Jupiter — moons now known as Io, Europa, Ganymede, and Callisto, the Galilean moons. It was the first time anyone had observed bodies clearly orbiting something other than Earth, a direct blow to the geocentric model that held all heavenly bodies circled our planet.
Galileo rushed his findings into print in March 1610 as Sidereus Nuncius (The Starry Messenger), dedicating the discovery to his patron Cosimo II de' Medici by naming the moons the 'Medicean Stars' — a name that didn't stick, but the political gambit secured him a court position. The discovery became one of the pivotal pieces of evidence in the Copernican Revolution, the very revolution for which Galileo would later be condemned by the Inquisition in the twilight of his life.