Galilean moons

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Jupiter with its four largest moons (photo montage). From top to bottom: Io, Europa, Ganymede, Callisto
The orbits of the Galilean moons around Jupiter

The Galilean moons (also Galilean satellites or Galilean satellites ) are the four largest satellites on the planet Jupiter :

The designation as the Galilean moons honors the Italian astronomer and naturalist Galileo Galilei , who was the first to describe them in 1610.


The photomontage on the right, composed of individual images from the Galileo space probe , shows the Galilean moons in the correct proportions to each other and to Jupiter. However, their distances from the giant planet are much greater, they are between three and thirteen times the diameter of Jupiter.

The order of their distances from Jupiter is: Io, Europa, Ganymede, Callisto, where Io is closest to the planet and Callisto is farthest.

The Galilean moons are among the largest in the solar system . With a diameter of 5262 km, Ganymede is even larger (albeit less massive) than the planet Mercury . The surfaces of the moons are very different. The moon Io caused a sensation when active volcanoes were discovered on it while the Voyager 1 probe flew by (see volcanism on Jupiter's moon Io ). Europe has a furrowed surface under which there may be an ocean (see extraterrestrial ocean ). There are clear traces of tectonics on Ganymede , and Callisto has the second highest density of craters in the known solar system.

The mean density of the moons decreases with increasing distance from Jupiter (from 3.5 to 1.9 g / cm³), but the material from Callisto already contains more ice than rock .

All of the other moons of Jupiter - the fifth Amalthea with a diameter of around 150 km to be discovered in 1892 - are nowhere near the size of the Galilean moons. Despite 59 other satellites counted, their total mass is barely one per mille of the mass of Europe, the smallest Galilean moon.

Comparison of some properties with those of the Earth's moon and Mercury
Earth moon Mercury
for comparison
Diameter (km) 3,643.2 3,121.6 5,268 4,820.6 3,476 4,878
Mass (10 22 kg) 8.94 4.88 14.82 10.76 7.348 33.02
Density (g / cm³) 3.56 3.01 1.936 1,851 3.345 5.427
Mean acceleration due
to gravity on the surface (m / s²)
1.81 1.32 1.81 1.32 1.62 3.7
Medium spherical albedo
Medium geometric albedo
Mean orbit radius (km) 421,600 670,900 1,070,600 1,883,000 384,405
Rotation time (days) 1.76 3.55 7.16 16.69 27.32


Jupiter with its four largest moons in the telescope
The disk of Jupiter (bottom right) with the four (only at full image resolution Galilean moons Io, Ganymede, Europa and Callisto recognizable) (the latter at maximum elongation right above Jupiter) in relation to the full moon during the meeting on April 10, 2017 in the constellation Virgo

The observation of the Galilean moons is very popular with amateur astronomers. You are already in a good night binoculars , e.g. B. 7 × 50 mm, but it is advisable to stabilize the binoculars with a tripod, for example. They can then be seen as small points of light next to Jupiter and can be mistaken for stars. Since the moons change their position within hours, it is attractive to observe them regularly and to see coverages by Jupiter or passages in front of the planetary disk. Through a good telescope with a large opening, preferably from 20 cm, the moons can be seen as small discs, all of which differ in color and size; with high magnification and good seeing it is even possible to see rough structures.

As the moons cast shadows, it happens regularly that they cause a solar eclipse on Jupiter. In a telescope you can then see a small black shadow on Jupiter, which is slowly moving across the planetary disk. The Galilean moons can also cover or darken one another. Observing such phenomena is also possible with a good telescope.

History of science

Galileo Galilei

Galileo reported in 1610 in his Sidereus Nuncius that he had discovered the four moons on January 7th of the same year with the help of a telescope he had made himself . He called them Sidera Medicea  - the " Medicean stars". Their names in detail were propagated by Simon Marius , an astronomer from Gunzenhausen , (at the suggestion of Johannes Kepler ). Together, Marius called them Sidera Brandeburgica in honor of his margrave , when he claimed in a paper published in 1614 that he had observed them since 1609 ( The world of Jupiter, discovered in 1609 with the Flemish telescope  - details and scientific-historical discussions on the priority dispute that arose can be gathered from the article on Marius).

With the discovery of these satellites, it could be observed for the first time that there are celestial bodies that do not rotate directly around the earth . Since this was a contradiction to the official geocentric worldview of church and society, according to which all celestial bodies revolved around the earth, Galileo's research was opposed or not recognized by influential circles. Professors in Florence even refused to look through his telescope .

Galileo was the first to suggest using the orbit of the four moons as a world-wide observable clock . With tables and observations of the eclipses of the moons it is possible to determine the local time and thus the longitude . But in 1676, by comparing the table and observations in Paris , Ole Rømer proved for the first time that the speed of light is finite. Then the tables had to be corrected for the time of flight. Pehr Wilhelm Wargentin demonstrated another problem at the Uppsala observatory around 1740 . The moons do not revolve at a constant speed like a clock. He suspected that the mutual attraction of the moons was the cause. This was confirmed by Lagrange in 1766 and by Laplace in 1788 by perturbation calculations. Laplace also demonstrated that the three moons Io, Europa and Ganymede orbit in a stable time ratio of 1: 2: 4, the so-called Laplace or orbit resonance . He was also able to calculate the masses of the moons for the first time. Today, the rare mutual eclipse of the moons is closely monitored in order to be able to calculate the orbits of exploration probes such as Galileo more precisely.


The asteroid (697) Galilea was discovered and named around the 300th anniversary of the discovery of the moons.

See also


  • S. Debarbat, C. Wilson: The Galilean Satellites of Jupiter from Galileo to Cassini, Roemer and Bradley. In: R. Taton, C. Wilson (eds.): Planetary Astronomy from the Renaissance to the Rise of Astrophysics, Part A: Tycho Brahe to Newton. In: M. Hoskin (Ed.): The General History of Astronomy. Volume 2A. Cambridge University Press, New York 1989, pp. 144-158.
  • D. Morrison (Ed.): Satellites of Jupiter. University of Arizona Press, 1982.

Web links

Commons : Galilean Moons  - collection of images, videos and audio files

Individual evidence

  1. ^ JST Gehler: minor planets. ( Memento from September 18, 2009 in the Internet Archive ). In: Physical dictionary. 1798.
  2. Lutz D. Schmadel (ed.): Dictionary of Minor Planet Names . Fifth Revised and Enlarged Edition. 5th edition. Springer Verlag , Berlin / Heidelberg 2003, ISBN 978-3-540-29925-7 , p. 106 (English, 992 pp., Dictionary of Minor Planet Names, Volume 1 in the Google Book Search - Original title: Dictionary of Minor Planet Names . First edition: Springer Verlag, Berlin / Heidelberg 1992).