Who Discovered Jupiter's Four Largest Moons? Galileo, 1610

Q: Who discovered Jupiter's four largest moons?

Italian astronomer Galileo Galilei discovered Jupiter’s four largest moons, Io, Europa, Ganymede and Callisto, in January 1610, using a telescope he built himself. They’re known as the Galilean moons. The discovery showed that not everything orbits Earth, giving strong support to the idea that Earth orbits the Sun.

Four “stars” that moved

On January 7, 1610, Galileo pointed his telescope at Jupiter and saw three small points of light in a line beside it. Over the following nights, they moved, and a fourth appeared. By January 13 he had concluded that they were moons orbiting Jupiter.

He published his findings in March 1610 in a short book, Sidereus Nuncius (“The Starry Messenger”). Hoping for support, he named the moons the “Medicean Stars” after his patrons, the powerful Medici family of Florence.

The Galilean moons

Moon	Notable for
Io	Hundreds of active volcanoes
Europa	An icy crust over a hidden ocean
Ganymede	The largest moon in the solar system; see Jupiter’s largest moon
Callisto	One of the most heavily cratered surfaces known

Their modern names were proposed by German astronomer Simon Marius, who claimed to have seen them around the same time. They’re named after lovers of the god Zeus, Jupiter’s Greek equivalent.

Why it mattered

In 1610, most scholars believed in a geocentric universe, with everything circling Earth. Jupiter’s moons proved that some objects orbit another body. This supported Copernicus’s theory that the planets, including Earth, orbit the Sun.

Galileo’s other discoveries

Mountains and craters on the Moon
Phases of Venus, further evidence for a Sun-centered system
Countless stars in the Milky Way
Sunspots

Conflict with the Church

In 1633, the Roman Inquisition found Galileo “vehemently suspect of heresy” for supporting a Sun-centered universe. He was forced to recant and spent his last years under house arrest. In 1992, Pope John Paul II formally acknowledged the Church’s error.

Galileo’s legacy

NASA’s Galileo spacecraft orbited Jupiter from 1995 to 2003. For the planet itself, see what the largest planet is. For a later famous space telescope, see the first space telescope.