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Science Astronomy Moons Mars Jupiter Saturn Uranus Neptune Pluto Size

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Astronomy: Moons

A popular style of quiz question is, "Which planet has moons (or satellites) named ...?" This page aims to help with what can be a very confusing subject.

The number of known moons has risen steadily over the years, especially since the dawn of what used to be called "the space age". Last time I checked (July 2015), Jupiter had 67 known moons, Saturn had 62, Uranus 27 and Neptune 14. Mars has two, both of which were discovered in 1877.

The good news for quizzers, however, is that you only really need to know details of about ten of them – although there are about fifteen more that it might be useful to know the names of.

Mars

Mars has two known moons:

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22 km 12.5 km

Both Phobos and Deimos were discovered in 1877 by the American astronomer Asaph Hall. They are named after the sons of Ares, the Greek god of war, who accompanied their father into battle. They were, respectively, the personifications of panic (or fear) and terror (or dread).

Ares was known as Mars to the Romans.

Note that Phobos and Deimos are by some margin the smallest of the moons covered on this page, except for the smallest moons of Pluto. (All four of the giant planets have lots of other moons that are bigger than these two, but they are never going to come up in quizzes.)

Jupiter

Jupiter's six biggest moons, in descending order of size, are:

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5,262 km 4,821 km 3,643 km 3,122 km 170 km 167 km

The principal satellites of Jupiter are named after various lovers of Zeus (the Greek equivalent of Jupiter): Ganymede was the (male) cup–bearer to the gods, beloved of Zeus; Callisto was a nymph; Io was a priestess of Hera; Europa was a Phoenician noblewoman who became the queen of Crete.

The smallest of these four so–called Galilean satellites is Europa. Jupiter's fifth biggest satellite, Himalia (one of the four that orbit closer to Jupiter than the Galileans) is less than 1/20th of Europa's diameter.

Saturn

Saturn's eight biggest moons, in descending order of size, are:

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5,150 km 1,527 km 1,470 km 1,123 km 1,062 km 504 km 396 km 179 km

The principal satellites of Saturn are named after the Titans – including Titan itself (discovered by Huygens, in 1655), Iapetus, Rhea, Tethys, Dione (Cassini, 1671), Mimas, Enceladus (Herschel, 1789), Hyperion (1848), Phoebe (Pickering, 1899). Note however that Mimas and Enceladus were not actually Titans but Gigantes (see Titans in Mythology). Calypso (discovered in 1980) was also not a Titan; she was a nymph.

Uranus

The six biggest moons of Uranus are, in descending order of size:

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1,577 km 1,523 km 1,169 km 1,158 km 472 km 162 km

The two largest (Oberon and Titania ) were discovered in 1787 by William Herschel; the next two Umbriel and Ariel in 1851 by William Lassell, and the fifth largest (Miranda) in 1948 by Gerard Kuiper. Only these five were known until the visit of the Voyager 2 spacecraft in 1985-6. Puck (the biggest of the ones discovered by Voyager 2) orbits just inside the "big five".

The satellites of Uranus are all named after characters from Shakespeare – except for two or three that are named after characters from Alexander Pope.

Umbriel and Ariel are named after characters from Pope's The Rape of the Lock. A third moon – Belinda, which was one of the ones discovered by Voyager 2 – is also named after a character from The Rape of the Lock; Belinda is the unfortunate victim of the 'rape'.

Rather confusingly, Shakespeare (as well as Pope) has a character named Ariel, in The Tempest.

Neptune

The four biggest moons of Neptune are, in descending order of size:

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2,705 km 420 km 340 km 195 km

The principal satellites of Neptune are named after water nymphs from Greek mythology.

The largest one, Triton, was discovered in 1846, just 17 days after Neptune itself. Next to be discovered – just over 100 years later, in 1949 – was Nereid, with a diameter of just over one–eighth that of Triton. Larissa was discovered in 1981, and in 1989 five more were discovered – the largest of which was Proteus (second biggest of them all!).

Triton's orbit is retrograde to the rotation of Neptune, and is steeply inclined relative to the planet's equator. In both of these respects, Triton is unique among the larger moons of the Solar System.

Pluto

Pluto, although no longer classified as a planet, has five known satellites:

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1,207 km 50 km 50 km 12.5 km 10.5 km

Charon is the only satellite in the Solar System that's bigger, in comparison to its parent body, than our own Moon. It's so big that the barycenter (the centre of gravity, in layman's terms) of the Pluto–Charon system lies outside Pluto. Charon's mean diameter of 1,207 km is just over half that of Pluto (2,368 km).

In Greek mythology, Charon was the ferryman who carried the souls of the dead across the rivers Styx and Acheron to Hades (which was ruled by Pluto).

Hydra and Nyx were discovered by the Hubble telescope in 2005. Hydra is so called because the mythological creature of that name had nine heads, and Pluto was, at the time of its discovery, still the ninth planet (in both size and orbital radius). Nyx is named after the personification of night in Greek mythology.

Two more satellites were discovered in 2011 and 2012 respectively. Kerberos is named after Cerberus, the three–headed dog that guarded the endrance to Hades; and Styx is named after the river already referred to.

Size Order

When I was researching this page, I noticed that if you sort the biggest satellites into descending order of size, they fall into several well–defined groups:

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  Jupiter   Saturn   Jupiter
5,262 km 5,150 km 4,821 km

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  Jupiter   Earth   Jupiter   Neptune
3,643 km 3,475 km 3,122 km 2,705 km

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  Uranus   Saturn   Uranus   Saturn   Uranus   Uranus   Saturn   Saturn
1,577 km 1,527 km 1,523 km 1,470 km 1,169 km 1,158 km 1,123 km 1,062 km

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  Saturn   Uranus   Neptune   Saturn   Neptune
504 km 472 km 420 km 396 km 340 km

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  Neptune   Saturn   Jupiter   Jupiter   Uranus
195 km 179 km 170 km 167 km 162 km

Notice how the biggest gap in the first group is 329 km, then the gap between the smallest satellite in the first group and the largest in the next group is nearly 1,200 km. Similarly, the biggest gap in the second group is 417 km, then there's a gap of 1,128 km do the biggest in the third group.

The following diagram illustrates this:

Relative sizes of satellites in the Solar System

This diagram shows the relative diameters of all of the bodies that we've been discussing on this page, plus the minor planets, the biggest asteroids, and the planet Mercury (all of which are shown on the top row). The next smallest body in the Solar System (the smallest one not shown on the diagram) is Mars, with a mean diameter of 6,780 km – very nearly 40% bigger than Mercury, and well off to the right of the diagram on this scale.

The red dots are where there are two moons that are too close together in size to show separately on this diagram.

The white vertical bands represent the ranges of sizes where there are satellites, and the grey bands represent the ranges where there aren't. My point is that the grey bands are similar in size to the white ones. This suggests that the sizes of satellites in the Solar System may not be random.

I'm not sure if this is significant scientifically; but by breaking the list into groups like this it helps me to remember the order.

© Haydn Thompson 2017