Updated October 2019
To Ganymede and Titan, yes Sir, I've been around...
– Dave Lister singing in the opening episode of Red Dwarf
Moons are rocky worlds orbiting planets. They have their own gravity which can exert a noticeable influence upon the parent planet. This can even stabilise the planet's orbit as our Moon does to the Earth, keeping the axis tilted at 23° and preventing wild climate fluctuations. The only moons we know of are the ones in our own Solar System. There are, no doubt, other moons in orbit around extrasolar planets but we don't yet have precise-enough technology to detect them. Most of the extrasolar planets which have been discovered up to 2009 (over 400) are gas giants like Jupiter, which cannot harbour life (as we know it), but any moons big enough to support an atmosphere could, so long as the planet resides in that system's habitable zone.
The largest moon in our Solar System is Jupiter's moon Ganymede: at 5,268 km in diameter it is bigger than Mercury (diameter 4,878 km) and only a bit smaller than Mars (diameter 6,792 km). The second largest moon in the Solar System is Titan, which belongs to Saturn. The smallest moon is hard to define as there is no official cut-off point below which a lump of rock is no longer considered a moon. Two of Jupiter's moons, S/2003 J 9 and S/2003 J 12, each have a diameter of one kilometre. Saturn's rings are composed of tiny 'moonlets' which range in size from that of icy pebbles to small cars, but they don't count as official moons because they are part of the ring system.
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Moons and their Names
The International Astronomical Union (IAU) is a worldwide association of 6,000 professional astronomers and is based in Paris, France. The IAU has been responsible for the naming of newly-discovered planetary bodies since its formation in 1919.
The Earth's Moon is usually just called 'the Moon', but the Latin name is Luna. Another name is Selene, after the Greek goddess of the Moon. Astronomers who like to study, draw or map1 the Moon prefer to be known as selenographers rather than lunatics! The Moon is tidally locked to the Earth, meaning that it always shows the same face to us. Consequently we should only expect to ever see one-half of the Moon, but because it tilts and wobbles slightly on its axis, we actually get to see about 59%. It exerts gravitational influence causing the tidal motion on Earth. It orbits the Earth in 29.5 days2 and goes through phases from new (when it is invisible to us) to growing crescent shapes until half-full, then it gets bigger each night (waxes) until it is full. After this, it starts to get smaller (wanes) through half-full and crescent back to invisible again.
The monthly full moons all have names in folklore and are a joy to witness, especially when the Moon is reflected across water. Most full moons are bright enough to cast a shadow of the watcher - their 'moon-shadow' - but it's the Sun, not the Moon providing the light; the Moon acts like a mirror reflecting the Sun's light back at us on Earth3.
The moon occasionally provides us with awesome total solar eclipses when it completely covers the Sun's disc, and lunar eclipses when the Sun behind the Earth casts an Earth shadow across the face of the full moon. Very rarely4, we are treated to a fantastic event such as the Blue Moon and partial eclipse on New Year's Eve 2009, which nicely polished off the International Year of Astronomy. Other natural lunar phenomena are amazing things called moonbows and moon halos, although these are purely visual effects in the Earth's atmosphere.
Mars is the Roman god of war, and the red planet was named after him. Its two moons Phobos and Deimos, discovered in 1877 by American astronomer Professor Asaph Hall Snr (1829-1907), are captured asteroids the shape of potatoes. Phobos and Deimos have their own Edited Guide Entry.
The discovery of moons orbiting Jupiter caused a scientific revolution as the knowledge added weight to the argument that the Sun was central in the Solar System. In January 1610 Galileo Galilei studied Jupiter through his telescope over several nights and noted that four points of light moved around it. He dubbed his discovery 'the Medicean Planets' but the four moons (Callisto, Europa, Ganymede and Io) were eventually collectively called the Galilean moons in his honour. Since then, more moons have been discovered and up until 2019 there are 79 known Jovian moons. Over 50 of the moons are likely to be captured objects which flew too close to the giant planet sometime in the long-distant past. Their orbits are eccentric5, either highly inclined6 or retrograde7, or just plain irregular. The ones that bear names honour the Roman top god's many lovers and some of his offspring.
The Jovian system has provided a wealth of material for science fiction writers, eg Arthur C Clarke's 2001: A Space Odyssey film and novel series, and Isaac Asimov's Lucky Starr and the Moons of Jupiter. Robert A Heinlein favoured the moon Ganymede, choosing it as a possible place to terraform for human colonies in his novels. Philip K Dick's aliens from Ganymede were sentient worm-like creatures and intelligent slime mould. The Gentle Giants of Ganymede by James P Hogan feature in their own Edited Entry. The moon Io featured heavily in the film Outland starring Sean Connery, and the original Arnold Rimmer of the Red Dwarf universe hailed from Io.
Roman god Saturn (called Kronos by the ancient Greeks) is one of the oldest deities, ruling the agricultural tasks which gave rise to civilisation. For some time he was the ruler of the Universe, after he deposed his father. Among his children borne by his sister/wife Ops (Rhea), the goddess of abundance and wealth, were Jupiter, Neptune and Pluto, who themselves became major gods. The planet Saturn has over 80 moons, most of which are named after mythological figures associated with him. The seven largest are named after titans who were Saturn's siblings. Some of the smaller moons are so-called 'shepherding moons' as they 'lead' or 'guard' the magnificent rings, their gravitational influence nudging the ring particles into definite bands. Some astronomers think the rings were formed from past moon collisions.
The Saturnian moons display a diverse range of surfaces, for example, Enceladus has a striped mantle like the skin of a tiger. Saturn's 'E' ring is brightest when it coincides with the orbit of Enceladus, leading astronomers to believe there is a connection. It's thought that Enceladus is active, erupting fine ice particles into space which form the 'E' ring. Some of the moons provide us with multiple transit opportunities when the rings are edge-on.
Saturn's largest moon is icy Titan, which has an atmosphere of mostly nitrogen and small amounts of carbon dioxide and ethane in a double haze layer. The satellite Mimas boasts one of the largest impact craters compared with the size of the satellite that we know of. Named Herschel Crater (after the moon's discoverer Sir William Herschel), it measures approximately 130 km, which is about one third the diameter of the moon. Mimas was lucky to survive such an impact. With her surface unchanged for a billion years, Rhea has the oldest face known to mankind. Two moons, Epimetheus and Janus, share the same orbit - the only known occurrence of this phenomenon in the entire Solar System. Other Saturnian moons of note include:
- the cracked frozen Dione
- spongy Hyperion
- the mysterious walnut-shaped Iapetus half-covered in an unknown dark material
- the sooty captured centaur8 Phoebe which creates the Phoebe Ring
- the icy cliffs of Tethys
Until the deep-space exploration of the planets beyond Saturn by Voyager 2, it was thought Uranus had only five moons: Titania and Oberon, discovered in 1787 by Sir William Herschel; Ariel and Umbriel, discovered in 1851 by English astronomer William Lassell (1799-1880); and Miranda, discovered by Gerard Kuiper in February 1948. Voyager 2 identified another ten moons, including Ophelia, in 1986, and others were pinpointed later from photographs taken during the same mission. The Hubble Space Telescope identified new rings and two more moons in 2005, bringing the current tally to 27. All Uranian moons are named after Shakespearean characters, a precedent begun by Bard fan Herschel who called his own discoveries after the King and Queen of the Fairies in A Midsummer Night's Dream.
Neptune is the Roman god of the sea, so its moons were named after other underwater characters: Triton, Nereid, Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, Halimede, Psamathe, Sao, Laomedeia, Neso and an as-yet unnamed 14th satellite. In Greek mythology Triton is the son of Poseidon and Amphitrite, god and goddess of the sea. The moon Triton was discovered on 10 October, 1846, by William Lassell less than three weeks after the discovery of Neptune. The Lassell ring of Neptune is named in his honour. Triton orbits Neptune retrograde (backwards), and it was thought to be the only moon of Neptune until the discovery of Nereid by Gerard Kuiper in 1949. Voyager 2 raised the total of Neptunian moons to eight in 1989.
Neso is the furthest known moon at 48,387,000 km distance from its parent planet. At that range it takes 9,374 days (over 25 years) to complete one orbit.
When Pluto was discovered in 1930, it was declared a planet and retained that honour until 2006 when it was demoted to dwarf planet status. Some people will always consider this to be a miscarriage of justice and refuse to accept the official ruling. However, Pluto hasn't disappeared; it's still out there in an erratic orbit, 40 AU9 (about three billion miles) from the Sun, occasionally wandering across the orbital track of Neptune10. Pluto's name honours the Roman god of the Underworld and Pluto's main satellite Charon (over half the size of Pluto and pronounced 'kah-ron') was named after the mythological ferryman who delivered the souls of the dead across the River Styx to their final resting place.
Two satellites of Pluto were discovered in 2005 by the Hubble Space Telescope. They are about 45,000 km (27,000 miles) from Pluto and roughly two to three times further away than their sibling moon Charon. They were catalogued S/2005 P1 and S/2005 P2 until the IAU could assign them appropriate names, although it was practically a dead cert that their names would honour Pluto or death in some way. Predictably enough, they named one Nix after the Greek goddess of the night11 and mother of the demon Thánatos (personification of death), Charon the soul-transporter, and the demi-god Hypnos (personification of sleep). The other moon was named Hydra, after the multi-headed serpent which guarded the entrance to the Underworld. The Hydra was the stuff of nightmares - it had poisonous breath and its appearance was supposedly so hideous that some unfortunate souls who encountered it were frightened to death. Yet another two satellites were detected by the HST in 2011 and 2012 which brings Pluto's family of moons to five. The names that have been chosen for them are Kerberos (after the hound of Hades) and Styx (after the river patrolled by Charon).