Galilean moons
The Galilean moons are the four moons of
Jupiter discovered by
Galileo Galilei. They are by far the largest of the many moons of Jupiter.
Encyclopedia
The
Galilean moons are the four moons of
Jupiter discovered by
Galileo Galilei. They are by far the largest of the many moons of Jupiter.
Visibility
The Galilean moons are visible from Earth with a small
telescope or binoculars. In fact, if the observing conditions are sufficient, it is possible to see
Ganymede with the unaided eye. At their closest distance to Earth, the moons have magnitudes of 4.6 to 5.6 . Io at its apsis is separated from Jupiter by about two arc minutes. It is theoretically possible that dedicated and well-trained observers could see the moons with the naked eye, but there is no evidence that this has ever been achieved.
Discovery
The Galilean moons were first observed by Galileo on January 7, 1610. It has been claimed that Gan De, a Chinese astronomer, may have seen Ganymede in 362 BC, nearly 2 millennia earlier, and it has been claimed that
Babylonian and
Egyptian astronomers were aware of the Galilean moons.
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.
Name
Galileo initially called his discovery the
Cosmica SideraOther names put forward include '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 .
Johannes Hevelius called the moons the 'Circulatores Jovis' or 'Jovis Comites', and Jacques Ozanam called them 'Gardes' or 'Satellites' .
The names that eventually prevailed were chosen by
Simon Marius, who claimed to have discovered the moons at the same time as Galileo: he named them after lovers of thr god Jupiter:
Io,
Europa,
Ganymede and
Callisto, in his
Mundus Jovialis, published 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
| Mass
| Semi-major axis | Orbital period |
|---|
| Io | | | 3643 | 8.93×1022 | 421,800 | 1.77 |
| Europa | | | 3122 | 4.8×1022 | 671,100 | 3.55 |
| Ganymede | | | 5262 | 1.48×1023 | 1,070,400 | 7.16 |
| Callisto | | | 4821 | 1.08×1023 | 1,882,700 | 16.69 |
Notes
See also