Etymology

In ancient times, 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 lights were first called "πλανήτης" (planētēs), meaning "wanderer", by the Ancient Greeks, 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 first named them. However, the practice of naming planets after gods originated in the West with the Sumerians, who lived in modern-day Iraq in about 3000 BC. Successive Mesopotamian civilizations, such as the Babylonians, retained the Sumerian naming convention but adapted it to their own very different pantheons. The Greeks borrowed much of their astronomy, including constellations and the zodiac, from the Babylonians, and by 600 BC had already begun using Babylonian concepts in their calculations.^[4] The Greeks grafted the names of their own gods onto the Babylonian planet list, although there was some confusion in translation: for instance, the Babylonian Nergal was a god of war, and the Greeks, seeing this aspect of Nergal's persona, identified him with Ares, their god of war. However, Nergal, unlike Ares, was also a god of the dead and a god of pestillence.^[5]

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 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.

To the Greeks and Romans, there were five 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: Mercury (called Hermes by the Greeks), Venus (Aphrodite), Mars (Ares), Jupiter (Zeus), and Saturn (Kronos). Although strictly the term "planetes" referred only to those five objects, the term was often expanded to include the Sun and the Moon.^[6] 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,^[7] believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts began with Jupiter and worked inwards; as a result, a list of which god had charge of the first hour in each day became Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order.^[8] 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 respectively.

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. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. 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