Galilean moons

On January 7, 1610, Galileo Galilei pointed a telescope at Jupiter and saw something that contradicted two millennia of cosmology: three small stars near the planet that moved from night to night. Within a week he had observed a fourth. These were not stars but moons, and their orbits around Jupiter proved that not everything revolved around Earth.

The discovery required no new physics, only better optics. Galileo had improved Dutch telescope designs, grinding lenses that could magnify roughly 20 times. He was not the first to look at the sky through a telescope—Thomas Harriot had observed the Moon months earlier—but he was the first to observe systematically and publish.

The cosmological implications were profound. Aristotelian physics held that all celestial motion centered on Earth. The Ptolemaic system placed Earth at the universe's center with everything orbiting around it. Jupiter's moons demonstrated a counter-example: here was a celestial body with its own satellites, a miniature solar system within the solar system.

Galileo named the moons the Medicean Stars, hoping for patronage from the Medici family. The name did not stick; we call them the Galilean moons instead—Io, Europa, Ganymede, and Callisto, named by Simon Marius who claimed independent discovery.

The adjacent possible required the telescope, which had existed for barely two years. Dutch spectacle makers had created the instrument; Galileo improved it; the combination of optical technology and systematic observation produced the discovery. Similar instruments existed across Europe; multiple observers were looking at the same sky. The moons would have been discovered regardless of whether Galileo had turned his attention to Jupiter.

Today, the Galilean moons are targets for astrobiology research. Europa's subsurface ocean may harbor life; Ganymede has its own magnetic field. What Galileo observed to challenge geocentrism became objects of scientific investigation in their own right.