Names from the sky

The gods of Greece and Rome named the planets, and through them the days of the week, half the months and a dozen chemical elements. Today the International Astronomical Union keeps the custom alive with a theme for each planet’s moons.

Gods and what carries their names

Greek	Roman	God of	Named after them
Zeus	Jupiter (Iuppiter)	king of the gods, sky and thunder	the largest planet; Thursday in Romance languages (jueves, jeudi)
Hera	Juno	queen of the gods, marriage	asteroid 3 Juno; probably the month of June
Poseidon	Neptune	the sea	the eighth planet; its moon Triton is Poseidon’s son
Hades	Pluto (Dis)	the underworld	the dwarf planet Pluto and plutonium; Pluto’s moons carry underworld names
Demeter	Ceres	grain and harvest	the dwarf planet Ceres, cerium, and the word cereal
Hestia	Vesta	hearth and home	asteroid 4 Vesta
Ares	Mars	war	the red planet, its moons Phobos and Deimos (his sons, Fear and Dread), the month of March, Tuesday in Romance languages
Aphrodite	Venus	love and beauty	the brightest planet; Friday in Romance languages (viernes, vendredi)
Hermes	Mercury	messenger, trade, travellers	the innermost planet, the metal, Wednesday in Romance languages
Athena	Minerva	wisdom, crafts	asteroid 2 Pallas (Pallas Athena) and, through it, palladium
Artemis	Diana	the hunt; linked with the Moon	NASA’s Artemis programme, named as Apollo’s twin sister
Apollo	Apollo	light, music, prophecy; linked with the Sun	the Apollo programme. Rome kept his Greek name; the Sun itself is Helios in Greek and Sol in Latin
Hephaestus	Vulcan	fire and the forge	Vulcan, a planet inside Mercury’s orbit predicted in 1859 and never found; Einstein’s relativity explained the motion it was invented for (1915). Also the word volcano
Kronos	Saturn	sowing and time; father of Zeus	the ringed planet and Saturday
Ouranos	Caelus	the sky; father of Kronos	the seventh planet (the only one with a Greek name) and uranium
Gaia	Terra (Tellus)	the Earth	tellurium; the Earth itself has an English name from Old English eorþe
Eros	Cupid (Amor)	love	asteroid 433 Eros, the first asteroid orbited and landed on by a spacecraft (NEAR Shoemaker, 2000–2001)
Helios	Sol	the Sun	helium (found in the Sun’s spectrum before it was found on Earth), heliocentric, solar
Selene	Luna	the Moon	selenium, selenography (mapping the Moon), lunar
Eos	Aurora	the dawn	the aurora, the polar lights
Prometheus	Prometheus	the Titan who stole fire	promethium, and Prometheus, a moon of Saturn that shepherds its F ring

Rome took most Greek gods by identifying them with its own (Zeus with Iuppiter, Hera with Juno); a few, like Apollo, came over with their Greek names. The Roman god of the Sun was Sol. Apollo and Artemis came to be linked with the Sun and Moon only later. For the planets themselves, see Planets and their names.

Moons: a theme for each planet

Planet	Moons	Theme
Mars	Phobos, Deimos	The sons of Ares who drove his chariot: Fear and Dread. Found by Asaph Hall in 1877.
Jupiter	Io, Europa, Ganymede, Callisto and dozens more	Lovers, favourites and descendants of Zeus. Galileo found the four large moons in 1610; their names were proposed by Simon Marius. Under the present convention the names of prograde moons end in -a and of retrograde moons in -e.
Saturn	Titan, Rhea, Iapetus, Dione, Tethys, Enceladus, Mimas, Hyperion…	Titans and giants, named by John Herschel in 1847; the many small outer moons are grouped by giants of Inuit, Gallic and Norse myth.
Uranus	Titania, Oberon, Ariel, Umbriel, Miranda…	An exception: characters from Shakespeare and Alexander Pope, not myth. Titania and Oberon rule the fairies in A Midsummer Night’s Dream.
Neptune	Triton, Nereid, Proteus…	Gods and spirits of the sea. Triton was found 17 days after Neptune itself, in 1846.
Pluto	Charon, Nix, Hydra, Kerberos, Styx	The underworld: its ferryman, river, guard dog and monsters.

Rules and themes: Gazetteer of Planetary Nomenclature; Jupiter’s convention: Jovian moon names.

Dwarf planets

Ceres: Roman goddess of grain; patron of Sicily, where Giuseppe Piazzi found it on 1 January 1801
Pluto: Roman god of the underworld; a planet from 1930 to 2006
Eris: Greek goddess of strife: its discovery in 2005 started the debate that led the IAU to define “planet” in 2006
Haumea: Hawaiian goddess of childbirth; its moons are her daughters Hiʻiaka and Namaka
Makemake: creator god of the people of Rapa Nui (Easter Island)

The week

English	Old English: whose day	Latin	Spanish	French
Sunday	sunnandæg: the Sun	dies Solis	domingo	dimanche
Monday	mōnandæg: the Moon	dies Lunae	lunes	lundi
Tuesday	Tīwesdæg: Tiw, a Germanic war god equated with Mars	dies Martis	martes	mardi
Wednesday	Wōdnesdæg: Woden, equated with Mercury	dies Mercurii	miércoles	mercredi
Thursday	Þunresdæg: Thunor (Thor), the thunder god, equated with Jupiter	dies Iovis	jueves	jeudi
Friday	Frīgedæg: Frig, equated with Venus	dies Veneris	viernes	vendredi
Saturday	Sæternesdæg: Saturn, the only Roman god kept in English	dies Saturni	sábado	samedi

The seven-day week named after the seven moving lights of the sky (Sun, Moon and the five planets seen by eye) spread through the Roman world in the first centuries AD. The Germanic peoples put their own gods in place of the Roman ones. Romance languages took the Christian “Lord’s day” (dies Dominica). Spanish and French took the Sabbath. Etymologies: OED.

The months

January	Janus, the two-faced god of doorways and beginnings
February	Februa, a Roman festival of purification held in that month
March	Mars; the first month of the oldest Roman year
April	uncertain; perhaps from Latin aperire, “to open”
May	Maia, a goddess of growth
June	probably Juno
July	Julius Caesar, renamed in 44 BC (before: Quintilis, “fifth”)
August	the emperor Augustus, 8 BC (before: Sextilis, “sixth”)
September to December	seventh to tenth (septem, octo, novem, decem), counting from March

September to December keep the numbers of a Roman year that began in March, which is why the ninth month is called the seventh.

Elements

Element	Symbol	Named after
Helium	He	Helios, the Sun: seen as a yellow line in the solar spectrum in 1868, found on Earth in 1895
Selenium	Se	Selene, the Moon: named in 1817 as the companion of tellurium
Tellurium	Te	Tellus, the Earth
Mercury	Hg	the planet and god; the symbol is Latin hydrargyrum, “water-silver”
Uranium	U	the planet Uranus, found eight years before it (1789)
Neptunium	Np	Neptune, the planet after Uranus, as element 93 comes after uranium (1940)
Plutonium	Pu	Pluto, the next “planet” out, as element 94 follows neptunium (made 1940–41)
Cerium	Ce	the dwarf planet Ceres, discovered two years earlier (1803)
Palladium	Pd	the asteroid Pallas, discovered the year before (1803)
Titanium	Ti	the Titans, sons of the Earth
Promethium	Pm	Prometheus, who stole fire from the gods
Thorium	Th	Thor, the Norse thunder god
Vanadium	V	Vanadís, a name of the Norse goddess Freyja

Name origins: RSC periodic table.