Short answer: when people needed names for the lights that moved against the stars, they used the Roman gods. So the solar system is a family reunion: Zeus is Jupiter, Aphrodite is Venus, Ares is Mars. Then the week borrowed the same seven names, and the Germanic north swapped the Roman gods for local equivalents. That is why Thursday belongs to Thor.
The planets
| Planet | God | Why that name |
|---|---|---|
| Mercury | Hermes, Roman Mercury | Fastest orbit of all, 88 days. The messenger never stops moving |
| Venus | Aphrodite | The brightest point in the sky after the Sun and Moon, so it went to the goddess of beauty |
| Mars | Ares | Rust red to the naked eye, and red meant blood, which meant war |
| Jupiter | Zeus | The largest planet takes the king of the gods |
| Saturn | Cronus | Slowest of the planets visible unaided, about 29 years per orbit. The old titan, Jupiter's father |
| Uranus | Ouranos, the sky itself | Found in 1781, and the only planet that kept its Greek name. The family order is right too: Uranus fathered Cronus, Cronus fathered Zeus |
| Neptune | Poseidon | A deep blue disc in the telescope, so it went to the sea |
| Pluto | Hades | Dark, cold and far out. The lord of the unseen, named in 1930 by an eleven year old |
| Earth | nobody | The odd one out. From Old English eorthe, meaning simply the ground under your feet |
| The Moon | Selene, Roman Luna | Her Latin name survives in every "lunar" word we use |
| The Sun | Helios, Roman Sol | Same trick: "solar" is the Roman god still doing his job |
The week, twice
Rome named the seven days after the seven moving lights, gods included, and the Romance languages inherited that list almost intact. When the week reached the Germanic north the names were not translated word for word but god for god: whoever did the job of Mars locally got that day.
| Day | Romance name and god | Germanic name and god |
|---|---|---|
| Monday | lunes, lundi, lunedì, from dies Lunae, the Moon | Monday, the Moon. No swap needed |
| Tuesday | martes, mardi, martedì, from dies Martis, Mars | Tuesday, Tyr, Old English Tiw, the Norse war god |
| Wednesday | miércoles, mercredi, mercoledì, from dies Mercurii, Mercury | Wednesday, Woden, that is Odin, wanderer and guide of the dead |
| Thursday | jueves, jeudi, giovedì, from dies Iovis, Jupiter | Thursday, Thor. Thunder god for thunder god |
| Friday | viernes, vendredi, venerdì, from dies Veneris, Venus | Friday, Frigg, possibly Freya. Love for love |
| Saturday | sábado, sabato, from Hebrew shabbat: here Christianity replaced the god | Saturday, Saturn. The one Roman god English kept untranslated |
| Sunday | domingo, dimanche, domenica, from dies Dominica, the Lord's day | Sunday, the Sun |
Read the table across and the joke lands: Spanish jueves and English Thursday are the same day named after two different thunder gods who never met, and miércoles and Wednesday are Mercury and Odin, matched because both were travellers who escorted souls.
Quick questions
Why is Earth the only planet not named after a god?
Because it was never a light in the sky waiting to be named. The others wandered and needed labels. Earth was just the ground, and its name is the ordinary old word for soil: English earth, Spanish tierra, from Latin terra. There were earth deities, Gaia above all, but nobody had to name the ground after one.
Who named Pluto?
Venetia Burney, an eleven year old in Oxford, at breakfast in March 1930. Her grandfather passed the idea to an astronomer friend, it reached the Lowell Observatory by telegram, and the staff voted for it unanimously. It fit perfectly: the god of the unseen for a world nobody had ever seen. Pluto was reclassified as a dwarf planet in 2006, but the name stayed.
Why do Thursday and jueves mean the same thing?
Both mean "the thunder god's day". Jueves comes from dies Iovis, the day of Jove, that is Jupiter. Thursday comes from Thor's day. When the Roman week spread north each god was matched to the nearest local equivalent, and Jupiter's thunderbolt found Thor's hammer waiting.
Every god in these tables has an illustrated profile in our encyclopedia, and the two thunder gods get compared properly in Zeus vs Thor vs Ra.
