Galilean moons facts for kids

The Galilean moons are the four largest moons that orbit the giant planet Jupiter. They were discovered by the famous scientist Galileo Galilei. These four moons are named Io, Europa, Ganymede, and Callisto. They are much bigger than Jupiter's many other moons.

Jupiter's 4 Galilean moons, shown with Jupiter to compare their sizes. From the top, they are Io, Europa, Ganymede, Callisto.

How They Were Found

Galileo Galilei discovered the Galilean moons around late 1609 or early 1610. He had made great improvements to the telescope. Because of this, Galileo could see objects in space better than anyone before him. Using his new telescope, Galileo was the first to see four moons orbiting Jupiter.

On January 7, 1610, Galileo wrote a letter about Jupiter’s moons. At first, he only saw three of them. He thought they were just fixed stars close to Jupiter. He kept watching them from January 8 until March 2. During this time, he found a fourth object. He also realized that these four objects were not stars. Instead, they were orbiting Jupiter!

Galileo Galilei, who discovered the 4 Galilean moons.

Galileo’s discovery showed how important the telescope was for astronomers. It proved there were many things in space that people could not see with just their eyes. More importantly, finding moons orbiting something other than Earth was a big challenge to the Ptolemaic world system. This old idea said that Earth was the center of the universe. It claimed that all other space objects revolved around Earth.

The fact that Jupiter had four moons, while Earth only had one, further weakened the belief that Earth was the most important place in the universe. Galileo wrote about his discoveries in a book called Sidereus Nuncius (which means Starry Messenger). He did not mention Copernicus's idea that the Sun was the center of the universe. However, Galileo himself believed in the Copernican theory.

Some historians say that a Chinese astronomer named Gan De might have seen one of Jupiter's moons in 362 BC. But without a telescope, it would have been very difficult to do this.

Naming the Moons

Galileo first called his discovery the Cosmica Sidera, meaning 'Cosimo's stars'. He named them after Cosimo II de' Medici, who was the grand-duke of Tuscany. Galileo wanted his support. Later, at the grand-duke's suggestion, Galileo changed the name to Medicea Sidera, or 'the Medici stars'. This honored all four Medici brothers. Galileo announced his discovery in his book Sidereus Nuncius, published in Venice in March 1610.

However, the names we use today were chosen by Simon Marius. Marius claimed he found the moons at the same time as Galileo. He named them after lovers of the Greek god Zeus (who is like the Roman god Jupiter). These names were Io, Europa, Ganymede, and Callisto. He published these names in his book Mundus Jovialis in 1614.

Galileo did not want to use Marius's names. Instead, he created a numbering system that is still used today, along with the proper names. The numbers go from the moon closest to Jupiter outwards. So, Io is I, Europa is II, Ganymede is III, and Callisto is IV. Galileo used this system in his notes but never published it.

The Galilean moons, from closest to Jupiter to farthest away, are:

Name	Image	Interior structure	Diameter (km)	Mass (kg)	Mean Density (g/cm³)	Distance from Jupiter (km)	Orbital period (days)
Io			3643	8.93×1022	3.528	421,800	1.77
Europa			3122	4.8×1022	3.014	671,100	3.55
Ganymede			5262	1.48×1023	1.942	1,070,400	7.16
Callisto			4821	1.08×1023	1.834	1,882,700	16.69

Seeing the Moons from Earth

All four Galilean moons are quite bright. If they were farther away from Jupiter, you could see them without a telescope. Their brightness is between 4.6 and 5.6 when Jupiter is in opposition (meaning Jupiter is on the opposite side of Earth from the Sun). They are a bit dimmer when Jupiter is in conjunction (meaning Jupiter is on the same side of Earth as the Sun).

The main reason they are hard to see is that they are very close to Jupiter. Jupiter's bright light makes them hard to spot. Their widest separation from Jupiter is between 2 and 8 minutes of arc. This is close to what the human eye can barely see. Ganymede and Callisto, when they are farthest from Jupiter, are the easiest to try and see without a telescope.

The easiest way to observe them is to block Jupiter's light. You can do this by using an object like a tree branch or a power line. Make sure the object is straight across the path of the moons' orbits.

Related pages

• List of Jupiter's moons

The Solar System
Sun • Heliosphere	Planets ☾ = moon(s) ∅ = rings		Mercury	Venus	Earth ☾	Mars ☾
			Jupiter ☾ ∅	Saturn ☾ ∅	Uranus ☾ ∅	Neptune ☾ ∅
	Dwarf planets		Ceres	Pluto ☾	Haumea ☾	Makemake
			Eris ☾
	Small Solar System bodies	Asteroids (minor planets)	Groups and families: Vulcanoids · Near-Earth asteroids · Asteroid belt Jupiter Trojans · Centaurs · Neptune Trojans · Asteroid moons · Meteoroids · Pallas · Juno · Vesta · Hygiea · Interamnia · Europa
			See also the list of asteroids.
		Trans- Neptunians	Kuiper belt – Plutinos: Orcus · Ixion – Cubewanos: Varuna · Quaoar · Huya
			Scattered disc: Sedna
		Comets	Periodic comets and non-periodic comets Damocloids · Oort cloud
See also the list of solar system objects

Images for kids

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  Montage of Jupiter's four Galilean moons, in a composite image depicting part of Jupiter and their relative sizes (positions are illustrative, not actual). From top to bottom: Io, Europa, Ganymede, Callisto.

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  Two Hubble Space Telescope views of a rare triple transit of Jupiter by Europa, Callisto and Io (24 January 2015)

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  The Medician stars in the Sidereus Nuncius (the 'starry messenger'), 1610. The moons are drawn in changing positions.

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  A Jovilabe: an apparatus from the mid-18th century for demonstrating the orbits of Jupiter's satellites

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  Tupan Patera on Io.

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  Europa.

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  Recurring plume erupting from Europa.

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  Ganymede.

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  Callisto's Valhalla impact crater as seen by Voyager.

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  Comparison of (a part of) Jupiter and its four largest natural satellites

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  Surface features of the four members at different levels of zoom in each row

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  Galilean moons compared with moons of other planets (and with Earth; the scale is changed to 1 pixel = 94 km at this resolution).

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  The relative masses of the Jovian moons. Those smaller than Europa are not visible at this scale, and combined would only be visible at 100× magnification.

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  Jupiter and all of the Galilean moons as seen through an 25 cm (10 in) amateur telescope (Meade LX200).

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  Jupiter with the Galilean moons – Io, Ganymede, Europa, and Callisto (near maximum elongation), respectively – and the full Moon as seen around conjunction on 10 April 2017

See also

In Spanish: Satélite galileano para niños