Galilean moons

The Galilean moons are the four moons of Jupiter Jupiter

Jupiter is the fifth planet [i] from the Sun [i] and the largest [i] within the solar system [i] ...

discovered by Galileo Galilei Galileo Galilei

Galileo Galilei was an Italian [i] physicist [i], astronomer [i], astrologer [i] and philosopher [i] ...

. They are by far the largest of the many moons of Jupiter.

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The Galilean moons are the four moons of Jupiter Jupiter

Jupiter is the fifth planet [i] from the Sun [i] and the largest [i] within the solar system [i] ...

discovered by Galileo Galilei Galileo Galilei

Galileo Galilei was an Italian [i] physicist [i], astronomer [i], astrologer [i] and philosopher [i] ...

. They are by far the largest of the many moons of Jupiter.

Visibility

The Galilean moons are visible from Earth with a small telescope Telescope

The word "telescope" usually refers to optical telescope [i]s, but there are telescopes for most of the ...

or binoculars. In fact, if the observing conditions are sufficient, it is possible to see Ganymede Ganymede

In Greek mythology [i], Ganymede, or closer to the Greek Ganymedes was a divine hero whose homelan ...

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 Babylon Babylon

Babylon was an ancient city in Mesopotamia, the ruins of which can be found in present-day Babil Province [i] ...

ian and Egypt Egypt

[i] country in [[North Africa]...

ian 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 Heliocentrism

In astronomy [i], heliocentrism is the theory [i] that the Sun [i] is at the center of the Universe [i]...

of Nicolaus Copernicus Nicolaus Copernicus

Nicolaus Copernicus was an astronomer [i] who provided the first modern formulation of a heliocentric [i] ...

and showed that not everything revolves around Earth Earth

Earth is the third planet [i] in the solar system [i] in terms of distance from the Sun [i], and the fi ...

.

Name

Galileo initially called his discovery the Cosmica Sidera
Other names put forward include 'Principharus, Victipharus, Cosmipharus and Ferdinandipharus', for each of the four Medici brothers, proposed by Giovanni Batista Hodierna Giovanni Battista Hodierna

Giovanni Battista Hodierna was an astronomer [i] at the court of the Duke of Montechiaro [i]. ...

, a disciple of Galileo and author of the first ephemerides .
Johannes Hevelius Johannes Hevelius

Johannes Hevelius, also called Johann Hewelke, Johannes Hwelcke or Johannes Hewel, or ...

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 Simon Marius

Simon Marius was a German [i] astronomer [i]. ...

, 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 Ganymede

In Greek mythology [i], Ganymede, or closer to the Greek Ganymedes was a divine hero whose homelan ...

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	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 Ganymede In Greek mythology [i], Ganymede, or closer to the Greek Ganymedes was a divine hero whose homelan ...	5262	1.48×10²³	1,070,400	7.16
Callisto	4821	1.08×10²³	1,882,700	16.69