Galilean moons are the four largest moons of Jupiter—Io, Europa,
Ganymede, and Callisto. They were first seen by
Galileo Galilei in
January 1610, and recognized by him as satellites of
Jupiter in March
1610. They are the first objects found to orbit another planet.
Their names derive from the lovers of Zeus. They are among the largest
objects in the
Solar System with the exception of the
Sun and the
eight planets, with a radius larger than any of the dwarf planets.
Ganymede is the largest moon in the Solar System, and is even bigger
than the planet Mercury. The three inner moons—Io, Europa, and
Ganymede—are in a 4:2:1 orbital resonance with each other. Because
of their much smaller size, and therefore weaker self-gravitation, all
of Jupiter's remaining moons have irregular forms rather than a
Galilean moons were discovered in either 1609 or 1610 when Galileo
made improvements to his telescope, which enabled him to observe
celestial bodies more distinctly than ever. Galileo's discovery
showed the importance of the telescope as a tool for astronomers by
proving that there were objects in space that cannot be seen by the
naked eye. More importantly, the incontrovertible discovery of
celestial bodies orbiting something other than
Earth dealt a serious
blow to the then-accepted Ptolemaic world system, or the geocentric
theory in which everything orbits around Earth.
Galileo initially named his discovery the Cosmica Sidera ("Cosimo's
stars"), but the names that eventually prevailed were chosen by Simon
Marius. Marius discovered the moons independently at the same time as
Galileo, and gave them their present names, which were suggested by
Johannes Kepler, in his Mundus Jovialis, published in 1614.
1.2 Dedication to the Medicis
1.4 Determination of longitude
3 Comparative structure
3.2 Latest flyby
4 Origin and evolution
6 Orbit animation
7 See also
9 External links
Galileo Galilei, the discoverer of the four moons
As a result of improvements
Galileo Galilei made to the telescope,
with a magnifying capability of 20×, he was able to see celestial
bodies more distinctly than was ever possible before. This allowed
Galilei to discover in either December 1609 or January 1610 what came
to be known as the Galilean moons.
On January 7, 1610, Galileo wrote a letter containing the first
mention of Jupiter's moons. At the time, he saw only three of them,
and he believed them to be fixed stars near Jupiter. He continued to
observe these celestial orbs from January 8 to March 2, 1610. In these
observations, he discovered a fourth body, and also observed that the
four were not fixed stars, but rather were orbiting Jupiter.
Galileo's discovery proved the importance of the telescope as a tool
for astronomers by showing that there were objects in space to be
discovered that until then had remained unseen by the naked eye. More
importantly, the discovery of celestial bodies orbiting something
Earth dealt a blow to the then-accepted Ptolemaic world
system, which held that
Earth was at the center of the universe and
all other celestial bodies revolved around it. Galileo's Sidereus
Nuncius (Starry Messenger), which announced celestial observations
through his telescope, does not explicitly mention Copernican
heliocentrism, a theory that placed the
Sun at the center of the
universe. Nevertheless, Galileo accepted the Copernican theory.
A Chinese historian of astronomy, Xi Zezong, has claimed that a "small
reddish star" observed near
Jupiter in 362 BCE by Chinese astronomer
Gan De may have been Ganymede, predating Galileo's discovery by around
Dedication to the Medicis
The Medician stars in the
Sidereus Nuncius (the 'starry messenger'),
1610. The moons are drawn in changing positions.
In 1605, Galileo had been employed as a mathematics tutor for Cosimo
de' Medici. In 1609, Cosimo became Grand Duke Cosimo II of Tuscany.
Galileo, seeking patronage from his now-wealthy former student and his
powerful family, used the discovery of Jupiter's moons to gain it.
On February 13, 1610, Galileo wrote to the Grand Duke's secretary:
"God graced me with being able, through such a singular sign, to
reveal to my Lord my devotion and the desire I have that his glorious
name live as equal among the stars, and since it is up to me, the
first discoverer, to name these new planets, I wish, in imitation of
the great sages who placed the most excellent heroes of that age among
the stars, to inscribe these with the name of the Most Serene Grand
Galileo asked whether he should name the moons the "Cosmian Stars",
after Cosimo alone, or the "Medician Stars", which would honor all
four brothers in the Medici clan. The secretary replied that the
latter name would be best.
On March 12, 1610, Galileo wrote his dedicatory letter to the Duke of
Tuscany, and the next day sent a copy to the Grand Duke, hoping to
obtain the Grand Duke's support as quickly as possible. On March 19,
he sent the telescope he had used to first view Jupiter's moons to the
Grand Duke, along with an official copy of
Sidereus Nuncius (The
Starry Messenger) that, following the secretary's advice, named the
four moons the Medician Stars. In his dedicatory introduction,
Scarcely have the immortal graces of your soul begun to shine forth on
earth than bright stars offer themselves in the heavens which, like
tongues, will speak of and celebrate your most excellent virtues for
all time. Behold, therefore, four stars reserved for your illustrious
name ... which ... make their journeys and orbits with a marvelous
speed around the star of
Jupiter ... like children of the same family
... Indeed, it appears the Maker of the Stars himself, by clear
arguments, admonished me to call these new planets by the illustrious
name of Your Highness before all others.
A Jovilabe: an apparatus from the mid-18th century for
demonstrating the orbits of Jupiter's satellites
Galileo initially called his discovery the Cosmica Sidera ("Cosimo's
stars"), in honour of
Cosimo II de' Medici
Cosimo II de' Medici (1590–1621). At
Cosimo's suggestion, Galileo changed the name to Medicea Sidera ("the
Medician stars"), honouring all four Medici 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.
Other names put forward include:
I. Principharus (for the "prince" of Tuscany), II. Victripharus (after
Vittoria della Rovere), III. Cosmipharus (after Cosimo de' Medici) and
IV. Fernipharus (after Duke Ferdinando de' Medici) – by Giovanni
Battista Hodierna, a disciple of Galileo and author of the first
ephemerides (Medicaeorum Ephemerides, 1656);
Circulatores Jovis, or Jovis Comites – by Johannes Hevelius;
Gardes, or Satellites (from the Latin satelles, satellitis, meaning
"escorts") – by Jacques Ozanam.
The names that eventually prevailed were chosen by Simon Marius, who
discovered the moons independently at the same time as Galileo: he
named them at the suggestion of
Johannes Kepler after lovers of the
Zeus (the Greek equivalent of Jupiter): Io, Europa, Ganymede 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 numbered names (
Jupiter x) were used until
the mid-20th century when other inner moons were discovered, and
Marius' names became widely used.
Determination of longitude
See also: History of longitude
Galileo was able to develop a method of determining longitude based on
the timing of the orbits of the Galilean moons. The times of the
eclipses of the moons could be precisely calculated in advance, and
compared with local observations on land or on ship to determine the
local time and hence longitude. The main problem with the technique
was that it was difficult to observe the
Galilean moons through a
telescope on a moving ship; a problem that Galileo tried to solve with
the invention of the celatone. The method was used by Cassini and
Picard to re-map France.
Some models predict that there may have been several generations of
Galilean satellites in Jupiter's early history. Each generation of
moons to have formed would have spiraled into
Jupiter and been
destroyed, due to tidal interactions with Jupiter's proto-satellite
disk, with new moons forming from the remaining debris. By the time
the present generation formed, the gas in the proto-satellite disk had
thinned out to the point that it no longer greatly interfered with the
moons' orbits. Other models suggest that Galilean satellites
formed in a proto-satellite disk, in which formation timescales were
comparable to or shorter than orbital migration timescales. Io is
anhydrous and likely has an interior of rock and metal. Europa is
thought to contain 8% ice and water by mass with the remainder
rock. These moons are, in increasing order of distance from
Model of interior
I E G C
Further information: Moons of Jupiter
Main article: Io (moon)
Tupan Patera on Io
Jupiter I) is the innermost of the four
Galilean moons of Jupiter
and, with a diameter of 3642 kilometers, the fourth-largest moon
in the Solar System. It was named after Io, a priestess of
became one of the lovers of Zeus. Nevertheless, it was simply referred
to as "
Jupiter I", or "The first satellite of Jupiter", until the
With over 400 active volcanos, Io is the most geologically active
object in the Solar System. Its surface is dotted with more than
100 mountains, some of which are taller than Earth's Mount
Everest. Unlike most satellites in the outer
Solar System (which
have a thick coating of ice), Io is primarily composed of silicate
rock surrounding a molten iron or iron sulfide core.
Although not proven, recent data from the Galileo orbiter indicate
that Io might have its own magnetic field. Io has an extremely
thin atmosphere made up mostly of sulfur dioxide (SO2). If a
surface data or collection vessel were to land on Io in the future, it
would have to be extremely tough (similar to the tank-like bodies of
Venera landers) to survive the radiation and magnetic
fields that originate from Jupiter.
Main article: Europa (moon)
Jupiter II), the second of the four Galilean moons, is the
second closest to
Jupiter and the smallest at 3121.6 kilometers in
diameter, which is slightly smaller than the Moon. The name comes from
a mythical Phoenician noblewoman, Europa, who was courted by
became the queen of Crete, though the name did not become widely used
until the mid-20th century.
It has a smooth and bright surface, with a layer of water
surrounding the mantle of the planet, thought to be 100 kilometers
thick. The smooth surface includes a layer of ice, while the
bottom of the ice is theorized to be liquid water. The apparent
youth and smoothness of the surface have led to the hypothesis that a
water ocean exists beneath it, which could conceivably serve as an
abode for extraterrestrial life. Heat energy from tidal flexing
ensures that the ocean remains liquid and drives geological
activity. Life may exist in Europa's under-ice ocean, perhaps
subsisting in an environment similar to Earth's deep-ocean
hydrothermal vents or the Antarctic Lake Vostok. Life in such an
ocean could possibly be similar to microbial life on
Earth in the deep
ocean. So far, there is no evidence that life exists on Europa,
but the likely presence of liquid water has spurred calls to send a
Recurring plume erupting from Europa.
The prominent markings that criss-cross the moon seem to be mainly
albedo features, which emphasize low topography. There are few craters
on Europa because its surface is tectonically active and young.
Some theories suggest that Jupiter's gravity is causing these
markings, as one side of Europa is constantly facing Jupiter. Also,
volcanic water eruptions splitting the surface of Europa, and even
geysers have been considered as a cause. The color of the markings,
reddish-brown, is theorized to be caused by sulfur, but scientists
cannot confirm that, because no data collection devices have been sent
to Europa. Europa is primarily made of silicate rock and likely
has an iron core. It has a tenuous atmosphere composed primarily of
Main article: Ganymede (moon)
Jupiter III), the third Galilean moon is named after the
mythological Ganymede, cupbearer of the Greek gods and Zeus's
beloved. Ganymede is the largest natural satellite in the Solar
System at 5262.4 kilometers in diameter, which makes it larger than
the planet Mercury – although only at about half of its mass
since Ganymede is an icy world. It is the only satellite in the Solar
System known to possess a magnetosphere, likely created through
convection within the liquid iron core.
Ganymede is composed primarily of silicate rock and water ice, and a
salt-water ocean is believed to exist nearly 200 km below
Ganymede's surface, sandwiched between layers of ice. The metallic
core of Ganymede suggests a greater heat at some time in its past than
had previously been proposed. The surface is a mix of two types of
terrain—highly cratered dark regions and younger, but still ancient,
regions with a large array of grooves and ridges. Ganymede has a high
number of craters, but many are gone or barely visible due to its icy
crust forming over them. The satellite has a thin oxygen atmosphere
that includes O, O2, and possibly O3 (ozone), and some atomic
Main article: Callisto (moon)
Callisto's Valhalla impact crater as seen by Voyager
Jupiter IV) is the fourth and last Galilean moon, and is the
second largest of the four, and at 4820.6 kilometers in diameter, it
is the third largest moon in the Solar System, and barely smaller than
Mercury, though only a third of the latter's mass. It is named after
the Greek mythological nymph Callisto, a lover of
Zeus who was a
daughter of the Arkadian King Lykaon and a hunting companion of the
goddess Artemis. The moon does not form part of the orbital resonance
that affects three inner Galilean satellites and thus does not
experience appreciable tidal heating. Callisto is composed of
approximately equal amounts of rock and ices, which makes it the least
dense of the Galilean moons. It is one of the most heavily cratered
satellites in the Solar System, and one major feature is a basin
around 3000 km wide called Valhalla.
Callisto is surrounded by an extremely thin atmosphere composed of
carbon dioxide and probably molecular oxygen. Investigation
revealed that Callisto may possibly have a subsurface ocean of liquid
water at depths less than 300 kilometres. The likely presence of
an ocean within Callisto indicates that it can or could harbour life.
However, this is less likely than on nearby Europa. Callisto has
long been considered the most suitable place for a human base for
future exploration of the
Jupiter system since it is furthest from the
intense radiation of Jupiter.
Comparison of (a part of)
Jupiter and its four largest natural
Fluctuations in the orbits of the moons indicate that their mean
density decreases with distance from Jupiter. Callisto, the outermost
and least dense of the four, has a density intermediate between ice
and rock whereas Io, the innermost and densest moon, has a density
intermediate between rock and iron. Callisto has an ancient, heavily
cratered and unaltered ice surface and the way it rotates indicates
that its density is equally distributed, suggesting that it has no
rocky or metallic core but consists of a homogeneous mix of rock and
ice. This may well have been the original structure of all the moons.
The rotation of the three inner moons, in contrast, indicates
differentiation of their interiors with denser matter at the core and
lighter matter above. They also reveal significant alteration of the
surface. Ganymede reveals past tectonic movement of the ice surface
which required partial melting of subsurface layers. Europa reveals
more dynamic and recent movement of this nature, suggesting a thinner
ice crust. Finally, Io, the innermost moon, has a sulfur surface,
active volcanism and no sign of ice. All this evidence suggests that
the nearer a moon is to
Jupiter the hotter its interior. The current
model is that the moons experience tidal heating as a result of the
gravitational field of
Jupiter in inverse proportion to the square of
their distance from the giant planet. In all but Callisto this will
have melted the interior ice, allowing rock and iron to sink to the
interior and water to cover the surface. In Ganymede a thick and solid
ice crust then formed. In warmer Europa a thinner more easily broken
crust formed. In Io the heating is so extreme that all the rock has
melted and water has long ago boiled out into space.
Surface features of the four members at different levels of zoom in
Galilean moons compared with other
Solar System bodies, although pixel
scale is not accurate at this resolution.
Jupiter and Io
Galilean moons circa 2007, imaged by
New Horizons during
flyby. (greyscale colour)
Origin and evolution
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×
Jupiter's regular satellites are believed to have formed from a
circumplanetary disk, a ring of accreting gas and solid debris
analogous to a protoplanetary disk. They may be the remnants
of a score of Galilean-mass satellites that formed early in Jupiter's
Simulations suggest that, while the disk had a relatively high mass at
any given moment, over time a substantial fraction (several tenths of
a percent) of the mass of
Jupiter captured from the Solar nebula was
processed through it. However, the disk mass of only 2% that of
Jupiter is required to explain the existing satellites. Thus there
may have been several generations of Galilean-mass satellites in
Jupiter's early history. Each generation of moons would have spiraled
into Jupiter, due to drag from the disk, with new moons then forming
from the new debris captured from the Solar nebula. By the time
the present (possibly fifth) generation formed, the disk had thinned
out to the point that it no longer greatly interfered with the moons'
orbits. The current
Galilean moons were still affected, falling
into and being partially protected by an orbital resonance which still
exists for Io, Europa, and Ganymede. Ganymede's larger mass means that
it would have migrated inward at a faster rate than Europa or Io.
Galilean moons seen with an amateur telescope.
Jupiter and its
Galilean moons Io, Ganymede, Europa, and Callisto (at
maximum elongation) juxtaposed with the full moon during their
conjunction on 10 April 2017.
Galilean moons are bright enough that they could be sighted
Earth without a telescope, if they were farther away from
Jupiter. (They are, however, easily visible with even low-powered
binoculars.) They have apparent magnitudes between 4.6 and 5.6 when
Jupiter is in opposition with the Sun, and are about one unit of
magnitude dimmer when
Jupiter is in conjunction. The main difficulty
in observing the moons from
Earth is their proximity to
they are obscured by its brightness. The maximum angular
separations of the moons are between 2 and 10 minutes of arc from
Jupiter, which is close to the limit of human visual acuity.
Ganymede and Callisto, at their maximum separation, are the likeliest
targets for potential naked-eye observation.
GIF animation of the resonance of Io, Europa, and Ganymede
The three inner
Galilean moons revolve in a 1:2:4 resonance.
Moons of Jupiter
Jupiter's moons in fiction
Colonization of the Jovian System
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Jupiter is about 750 times brighter than Ganymede and about 2000
times brighter than Callisto.
Ganymede: (5th root of 100)^(4.4 Ganymede APmag − (−2.8 Jup
APmag)) = 758
Callisto: (5th root of 100)^(5.5 Callisto APmag − (−2.8 Jup
APmag)) = 2089
Jupiter near perihelion 2010-Sep-19: 656.7 (Callisto angular
separation arcsec) − 24.9 (jup angular radius arcsec) = 631 arcsec =
Animation of Galileo's observation, march 1613
Telescope utility for identifying Galilean moons
Wikimedia Commons has media related to Moons of Jupiter.
Moons of Jupiter
Listed in approximately increasing distance from Jupiter. Provisional
designations in italics.
S/2003 J 12
S/2011 J 1
S/2003 J 16
S/2003 J 19(?)
S/2003 J 9(?)
S/2003 J 10(?)
S/2003 J 23(?)
S/2003 J 4(?)
S/2003 J 2
Rings of Jupiter
Natural satellites of the Solar System
≥ 100 km)
largest / 2634 km / 0.413 Earths
smallest / 106 km / 0.017 Earths
NOTE: Italicized moons are not close to being in hydrostatic
equilibrium; [bracketed] moons may or may not be close to being in
Outline of Jupiter
Great Red Spot
S/2003 J 2
S/2003 J 12
S/2011 J 1
Jupiter-crossing minor planets
Comet Shoemaker–Levy 9
Jupiter impact event
Jupiter impact event
Jupiter Icy Moons Explorer (2022)
Europa Clipper (2025)
Io Volcano Observer
Io Volcano Observer (2021)
The Solar System
S/2015 (136472) 1
Solar System objects
By discovery date
Gravitationally rounded objects
Possible dwarf planets
first discovered: Ceres
Planets beyond Neptune
List of crewed spacecraft
List of probes
Outline of the Solar System
Solar System → Local Interstellar Cloud → Local
Bubble → Gould Belt → Orion Arm → Milky
Milky Way subgroup → Local Group → Virgo
Supercluster → Laniakea Supercluster → Observable
universe → Universe
Each arrow (→) may be read as "within" or "part of".
Leaning Tower of Pisa experiment
Phases of Venus
De Motu Antiquiora
Dialogue Concerning the Two Chief World Systems
"Discourse on the Tides"
"Letter to the Grand Duchess Christina"
Two New Sciences
Vincenzo Galilei (father)
Michelagnolo Galilei (brother)
Vincenzo Gamba (son)
Maria Celeste (daughter)
Marina Gamba (mistress)
"And yet it moves"
Villa Il Gioiello
Tribune of Galileo
In popular culture
Life of Galileo
Life of Galileo (1943 play)
Lamp At Midnight
Lamp At Midnight (1947 play)
Galileo (1968 film)
Galileo (1975 film)
Starry Messenger (1996 book)
Galileo's Daughter: A Historical Memoir of Science, Faith, and Love
Galileo Galilei (2002 opera)