The Galilean moons, or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They are the most readily visible Solar System objects after the unaided visible Saturn, the dimmest of the classical planets, allowing observation with common binoculars, even under night sky conditions of high light pollution. The invention of the telescope enabled the discovery of the moons in 1610. Through this, they became the first Solar System objects discovered since humans have started tracking the classical planets, and the first objects to be found to orbit any planet beyond Earth.
Montage of Jupiter's four Galilean moons, in a composite image depicting part of Jupiter and their relative sizes (positions are illustrative, not actual). From top to bottom: Io, Europa, Ganymede, Callisto.
Galileo Galilei, the discoverer of the four moons
The Medician stars in the Sidereus Nuncius (the 'starry messenger'), 1610. The moons are drawn in changing positions.
A Jovilabe: an apparatus from the mid-18th century for demonstrating the orbits of Jupiter's satellites
There are 95 moons of Jupiter with confirmed orbits as of 5 February 2024. This number does not include a number of meter-sized moonlets thought to be shed from the inner moons, nor hundreds of possible kilometer-sized outer irregular moons that were only briefly captured by telescopes. All together, Jupiter's moons form a satellite system called the Jovian system. The most massive of the moons are the four Galilean moons: Io, Europa, Ganymede, and Callisto, which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body that was neither Earth nor the Sun. Much more recently, beginning in 1892, dozens of far smaller Jovian moons have been detected and have received the names of lovers or daughters of the Roman god Jupiter or his Greek equivalent Zeus. The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 91 known moons and the rings together composing just 0.003% of the total orbiting mass.
Jupiter and the Galilean moons as seen through a 25 cm (10 in) Meade LX200 telescope
Orbits of Jupiter's inner moons within its rings
The Galilean moons. From left to right, in order of increasing distance from Jupiter: Io; Europa; Ganymede; Callisto.