The moon galiléenne

Discovered

One qualifies, in Astronomie, of the principal moons galiléennes the four satellite of Jupiter, discovered by Galileo in January 1610 which it names the Médicées stars. They are so brilliant that it is in fact just possible, if the conditions of observation and the vision of the observer lend themselves to it, to see Callisto (most external) with the naked eye.

The anteriority of their discovery was disputed in 1614, that is to say four years after, by the astronomer Simon Marius in a work entitled Mundus Iovialis where he claimed them to have also observed to him at the same date, and even front. It names them Io, Europe, Ganymède and Callisto. But the absence of observations consigned and gone back to this moment to support these claims let plane more than one doubt about their serious. Indeed the oldest Jupiter observation consigned by S. Marius goes back to December 1610 and the examples which it gives in his work go back to 1613.

These four celestial bodies, the first new objects discovered since mists of time thanks to the invention of the Telescope, were initially named by Galileo the Cosmica Sidera , in the honor of Cosimo II of Medicis (1590 - 1621), large-duke of Toscane starting from 1609, and whose Galileo sought patronage. Galileo will call them the Medicea Sidera (Médicées stars), because the Médicis were four brothers. The discovery was announced in the Sidereus Nuncius (stellar Messenger), published in Venice in March 1610, less than two months after the first observations. Their movement around Jupiter was going to be an weighty argument for the abandonment of the system of Ptolémée in favor of that of Copernic, then recently published (1543).

Among the other names suggested, one finds Principharus , Victipharus , Cosmipharus and Ferdinandipharus , in the honor of the four brothers Médicis, names that Hodierna, disciple of Galileo and author of the first éphémérides ( Medicaeorum Ephemerides , 1656), will use. Hévélius calls them Circulatores Jovis or Jovis Comites , and Ozanam calls them Gardes or Satellites (of satelles Latin , satellitis : escort). In fact the names suggested by Simon Marius (Simon Mayer) will be essential: Io , Europe , Ganymède and Callisto , published in the Mundus Jovialis of the author in 1614. Galileo refused to use the names suggested by Marius and invented consequently the system of classification which is still used nowadays, in parallel with the proper names. Classification starts with the moon nearest to Jupiter:

I: Io,

II: Europe,

III: Ganymède,

IV: Callisto.

Galileo used this system in his books of notes, but it did not on the occasion to use it in a printed version.

Internal structure

The data collected by the Probes To travel and Galileo about the density and of the nature of the crust of these four moons made it possible to work out a model of the interior of each one of them. Io (in top on the left), Europe (in top on the right) and Ganymède (in bottom on the left) have all a three core of dense rocks (and perhaps out of solid metal); Perhaps Callisto (in bottom on the right) has a small core but it is thought that its interior is formed mainly of water in the form of ice and rocks (largely undifferentiated).

The moons galiléennes, of nearest with most remote to Jupiter, are:

External bonds

Animation of the observation of Galileo, March 1613

Random links: