Naming The Planets, Part I

A question to the discussion forum this past week asked about the origins of the names of the planets. The "official" names of objects in the solar system are assigned by the International Astronomical Union (IAU), a global association of astronomers.

The IAU follows several conventions in naming planets and moons, the main ones being that planets are given names of Roman mythological beings and moons are given Greek mythological names associated with the Greek equivalent of the Roman god. Moons tend be named after goddesses, while planets, with the exception of Venus, are all male gods. Many of these names did not originate with the IAU, but have borne the names of these deities dating back into antiquity. The naming conventions are not rigid and there are exceptions. For example, Shakespearean names are assigned to moons of Uranus and in recent years the occasional Norse or Inuit mythological name has been assigned to newly discovered objects.

The largest and central object in the solar system is the sun, the nearest star to the Earth. Sun is an Old English word from a common Germanic root. Our local star is also known as Sol, the Latin name, which has been in English use since c.1450. The associated adjective is solar. Helio-, after helios, the Greek name for the sun, has been used as a combining form, as in heliocentric, since 1804.

Mercury, the planet closest to the sun and the fastest to orbit, is named after the Roman messenger god. The name in English usage dates back to Old English. The adjective is Mercurian, c.1576. The ancient Greeks called the planet by two names, even though they knew it to be one object, Apollo when seen in the morning and Hermes, the Greek counterpart to the Roman god, when seen in the evening.

Venus, the second planet from the sun and Earth’s nearest neighbor, is named after the Roman goddess of love. English usage of the name dates to c.1290. The adjective is Venusian, 1874. Early uses of the adjectival form are primarily found in science fiction, with astronomical use dating from 1913. The ancient Greeks called it by two names, Eosphorus, morning star, and Hesperus, evening star, although, like Mercury, they knew it was one body.

Earth dates back to Old English and is from a common Germanic root. The root is probably related to the Greek era, but no other non-Germanic cognates are known and the exact relationship with the Greek word is uncertain. Earth was not used in the sense of a planet, like the others, until c.1400. The adjective is Terran, 1953, and is most commonly found in science fiction. The adjective is after Terra, the Latin name for earth.

The earth is the first planet to have a natural satellite, called the moon. The word is found in Old English, from a common Germanic root. Moon has been used to denote a satellite of a planet other than earth since 1665. The earth’s moon is sometimes known as Luna, the Latin name in English use since before 1529. The adjective is lunar, 1626. Unlike the other astronomical names, earth and moon are often not capitalized.

Mars, the fourth planet from the sun, is named after the Roman god of war. The name has been in English use since c.1300. The adjective is Martian, c.1395. Mars has two small satellites, Phobos (fear) and Deimos (panic), the names of Ares’ sons, Ares being the Greek counterpart to the Roman diety.

Between Mars and Jupiter are the asteroids, named by astronomer William Herschel in 1802, from the Greek aster (star) + -oid (like). There are probably over a million of these rocks, of which several hundred thousand have been given alphanumeric designations by the IAU. Some asteroids are named. The largest is Ceres, after the Greek god of agriculture. At 933 km in diameter, Ceres contains 25% of the mass of all the asteroids combined.

Jupiter is the fifth and largest planet, named after the Roman king of the gods. The astronomical name has been in English use since c.1290. The planet is sometimes called Jove, c.1374, the poetical name of Jupiter. The adjective is Jovian, 1794

Most of Jupiter’s moons are named after paramours of Zeus, the Greek equivalent to the Roman god. It is well that Zeus enjoyed the ladies, because Jupiter has 27 moons. The Jovian moons are:

  • Metis, after first wife of Zeus, discovered in 1979 by astronomer Stephen Synnott of the Voyager 1 project.
  • Adrastea, a daughter of Jupiter, discovered in 1979 by David Jewett of the Voyager 1 project.
  • Amalthea, the nymph who nursed Jupiter, discovered by Edward Barnard in 1892.
  • Thebe, nymph daughter of river god Asopus, discovered in 1979 by Synnott.
  • Io is the innermost of the Galilean moons, so-called because they were independently discovered by Galileo the German astronomer Simon Marius in 1610. Named after a lover of Zeus who was turned into a heifer to hide her from Hera, his jealous wife, Marius named this and all the Galilean moons.
  • Europa, was another lover of Zeus, is the next Galilean moon.
  • Ganymede, the largest moon in the solar system, has a diameter larger than Mercury although smaller than that planet in mass. It is named for the Trojan boy who, because of his beauty, was made cupbearer to the gods.
  • Callisto is named after a nymph lover of Zeus who was turned into a bear by Hera. In the myth, Zeus placed her in the sky as the constellation Ursa Major. Callisto is the outermost of the Galilean moons.
  • Leda is the smallest of Jovian satellites. It is named for the queen of Sparta, who was the mother of Polydeuces (Pollux) and Helen of Troy by Zeus and of Castor and Clytemnestra by her mortal husband. Astronomer Charles Kowal discovered the moon in 1974. Leda and the next three satellites may be the remains of an asteroid captured by Jupiter and broken up.
  • Himalia is named for the mother of three of Zeus’ children. It was discovered by Charles Perrine in 1904.
  • Lysithea was another lover of Zeus. The satellite was discovered by Seth Nicolson in 1938.
  • Elara, was a lover of Zeus. Discovered by Perrine in 1904
  • Ananke, was yet another lover of Zeus, the mother of Adrastea. Nicolson discovered this one in 1951
  • Carme was also discovered by Nicolson, but in 1938. It was named for a lover of Zeus, of course.
  • Pasiphae was the wife of King Minos of Crete and mother of the Minotaur. It was discovered by Philibert Melotte in 1908
  • Even Zeus occasionally struck out. Sinope was a woman who spurned the god’s advances. Nicolson discovered this one too in 1914
  • Eleven small moons, recently discovered, were given names by IAU in 2004, a mix of daughters and lovers of the god:
    • Callirrhoe, stepdaughter
    • Themisto, lover
    • Megaclite, lover
    • Taygete, lover
    • Chaldene, lover
    • Harpalyke, lover
    • Kalyke, lover
    • Iocaste, lover
    • Erinome, lover
    • Isonoe, lover
    • Praxidike, lover
    • Autonoe, lover
    • Thyone, lover
    • Hermippe, lover
    • Aitne, lover
    • Eurydome, lover
    • Euanthe, lover
    • Euporie, daughter
    • Orthosie, daughter
    • Sponde, daughter
    • Kale, daughter
    • Pasithee, daughter

Our look at the names of the planets and moons will continue next week, starting with Saturn, the sixth planet.

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