Galilean moons
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 | | | 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 |
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.
