The gods of
Olympus, after whom the Solar System's planets are named.
In ancient times, Greek astronomers noted how certain lights moved across the sky in relation to the other stars. These objects were believed to orbit the Earth, which was considered to be stationary. The "wandering" lights were called "πλανήτης" (planētēs), a Greek term meaning "wanderer", and it is from this that the word "planet" was derived.
In near-universal practice in the Western world, the planets in the Solar System are named after Graeco-Roman gods, as, in Europe, it was the Greeks who originally named them. However, because of the influence of the Roman Empire and, later, the Catholic Church, in most countries in the West, the planets are known by their Roman (or Latin) names, rather than the original Greek. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable. When the Romans studied Greek astronomy, they gave the planets their own gods' names. In ancient times, there were seven known planets; each presumed to be circling the Earth according to the complex laws laid out by Claudius Ptolemy in the 2nd century. They were, in increasing order from Earth: the Moon (called Luna by the Romans, and Selene by the Greeks), Mercury (called Hermes by the Greeks), Venus (Aphrodite), the Sun (called Sol by the Romans, Helios by the Greeks), Mars (Ares), Jupiter (Zeus), and Saturn (Kronos). Eventually, the Sun and Moon were removed from the list of planets in accordance with the heliocentric model. However, when subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptune (Poseidon). The Greeks still use their original names for the planets.
Some Romans, following a belief imported from Mesopotamia into Hellenistic Egypt,[4] believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth, in Ptolemaic orbit order listed inwards. As a result, a list of which god has charge of the first hour in each day came out as Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order.[5] Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, Wóden, Thunor, and Fríge, Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus.
Since Earth was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god. Many of the Romance languages (including French, Italian, Spanish and Portuguese), which are descended from Latin, retain the old Roman name of Terra or some variation thereof. However, the non-Romance languages use their own respective native words. Again, the Greeks retain their original name, Γή (Ge or Yi); the Germanic languages, including English, use a variation of an ancient Germanic word ertho, "ground," as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde. The same is true for the Sun and the Moon, though they are no longer considered planets.
Some non-European cultures use their own planetary naming systems. China, and the countries of eastern Asia subject to Chinese cultural influence, such as Japan, Korea and Vietnam, use a naming system based on the five Chinese elements.[5]
