The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei. By far the largest of the many moons of Jupiter, they are visible even in a small telescope or binoculars. In fact, if the observing conditions are sufficient, it is possible to see Ganymede with the unaided eye.
They were first observed by Galileo on January 7, 1610. It is now claimed that Gan De, a Chinese astronomer, may have discovered the moons in 362 BC, nearly 2 millennia earlier. Galileo observed the moons' motion over several days and realized that they were in orbit around Jupiter. This discovery supported the heliocentric theory of Nicolaus Copernicus and showed that not everything revolves around Earth.
Galileo first called his discovery the Cosmica Sidera, in honour of Cosimo II de' Medici (1590–1621), grand-duke of Tuscany from 1609, whose patronage Galileo wanted to secure. At the grand-duke's suggestion, Galileo changed the name to Medicea Sidera ("Medici stars"), because the Medici were four brothers (Cosimo, Francesco, Carlo, and Lorenzo). The discovery was announced in the Sidereus Nuncius ("Starry Messenger"), published in Venice in March 1610, less than two months after the first observations.
Amongst the other names that were put forward, there is Principharus, Victipharus, Cosmipharus and Ferdinandipharus, for each of the four Medici brothers, proposed by Giovanni Batista Hodierna, a disciple of Galileo and author of the first ephemerides (Medicaeorum Ephemerides, 1656). Johannes Hevelius called them the Circulatores Jovis or Jovis Comites, and Jacques Ozanam called them Gardes or Satellites (from the Latin satelles, satellitis: escort). It would be the names proposed by Simon Marius (Simon Mayer), who claimed to have discovered the moons at the same time as Galileo, that would eventually prevail: Io, Europa, Ganymede and Callisto, published in his Mundus Jovialis in 1614.
Galileo steadfastly refused to use Marius' names and invented as a result the numbering scheme that is still used nowadays, in parallel with proper moon names. The numbers run from Jupiter outward, thus I, II, III and IV for Io, Europa, Ganymede and Callisto respectively. Galileo used this system in his notebooks but never actually published it.
The Galilean moons are, in order from closest to Jupiter to farthest away:
| Name | Image | Interior structure |
Diameter (km) |
Mass (kg) |
Semi-major axis (km) |
Orbital period (days) |
|---|---|---|---|---|---|---|
| Io | File:Io, moon of Jupiter, NASA.jpg | File:PIA01129 Interior of Io.jpg | 3643 | 8.93×1022 | 421,800 | 1.77 |
| Europa | 
|
File:PIA01130 Interior of Europa.jpg | 3122 | 4.8×1022 | 671,100 | 3.55 |
| Ganymede | File:Ganymede, moon of Jupiter, NASA.jpg | File:PIA00519 Interior of Ganymede.jpg | 5262 | 1.48×1023 | 1,070,400 | 7.16 |
| Callisto | File:Callisto, moon of Jupiter, NASA.jpg | File:PIA01478 Interior of Callisto.jpg | 4821 | 1.08×1023 | 1,882,700 | 16.69 |
The Galilean moons may have been known to the ancients: Babylonian Marduk (Jupiter) was said to be accompanied by four dogs (Jensen, Die Kosmologie der Babylonier, p. 131) and Egyptian Horus (Jupiter) had four sons (Mercer, Horus, the Royal God of Egypt, 1942). This is a conjecture, and not widely accepted as probable.
At their closest distance to Earth, the moons have a magnitude of 4.6 (Ganymede) to 5.6 (Callisto). Io at its apsis is separated from Jupiter by about two arc minutes. It is theoretically possible that dedicated and well-trained observers could manage to see the moons with the naked eye, but whether this was actually achieved by the Chaldeans remains a matter of speculation.