Mercury isn't named Mercury because it's fast

The planets from Mercury to Saturn got their names from Roman gods. How were the names chosen? Here are the explanations given by the International Astronomical Union’s Working Group for Planetary System Nomenclature:

• Mercury: ‘Named Mercurius by the Romans because it appears to move so swiftly.’
• Venus: ‘Roman name for the goddess of love. This planet was considered to be the brightest and most beautiful planet or star in the heavens.’
• Mars: ‘Named by the Romans for their god of war because of its red, bloodlike color.’
• Jupiter: ‘The largest and most massive of the planets was named Zeus by the Greeks and Jupiter by the Romans; he was the most important deity in both pantheons.’
• Saturn: no reasoning given (just ‘Roman name for the Greek Cronos, father of Zeus/Jupiter’.)

These explanations are entirely bogus. They’re made up.

The solar system bodies known in antiquity: the earth and the seven moving bodies (planētai, including moon and sun)

You might say it’s not a big deal, no one minds, it doesn‘t make a difference what the historical reasons for the names are. That’s all true. Still, here are some counter-points:

1. These are literally the people in charge of planets’ names. They had one job!
2. These explanations get repeated whenever anyone wonders how the planets got their names. If you make up something and it gets repeated as fact all over the world, that’s not OK.
3. It’s not as though it’d be hard to get it right. You just need to open a book written by someone who knows something about ancient astronomy. If anyone’s going to do make that minimal effort, you’d think it would be the people who are bloody well in charge of planets’ names.

The IAU has professional reasons to take an interest in the history of the names, sure. That doesn’t mean they’re experts. It’s painfully clear that they couldn’t care less what real experts have to say.

For reference, here’s a sample of people who have been misled, often introducing some new fictional material along the way: The Washington Post (7 October 2016), Universe Today (Mercury, Venus, Jupiter, Saturn; Mars omitted), Cornell University, Medium.com, Science ABC (along with the bizarre claim that Venus was first observed by the Maya), Planets for kids, Wonderopolis, Sporcle.com, Quora (Dec. 2020), StackExchange, and the Name Explain YouTube channel (with the bonus howler ‘Roman gods were based on Greek gods’). A number of social Q&A sites since 2013 have referred to an author called ‘Dustin Chiasson’ with similar explanations, but ‘Dustin Chiasson’ appears to be another fabrication.

Let’s consider some more detailed points.

1. Mercury.

It orbits the sun at a velocity of 50 km/s, so the Romans appropriately named it after their swiftest god, Mercury.

ScienceABC

No. Ancient astronomers had no way of measuring Mercury’s real orbital speed. They could only observe its apparent motion. And while Mercury bounces from one side of the sun to the other more frequently than Venus, their apparent speed isn’t much different.

Mercury’s real orbital speed is faster, but when they’re on the near side of the sun Venus is closer and that makes up nearly all the difference. In transits of the sun, for example, both planets transit at roughly the same speed, about an eighth of the sun’s diameter per hour. (Not that ancient astronomers observed transits of Mercury or Venus! This is just a convenient direct comparison.) For real information about what ancient astronomers thought about their motions, see Van der Waerden 1982.

If you’re choosing a planet to assign to a messenger god, you’d be better to choose the planet that travels the furthest. Mercury’s apparent position always stays within 28° of the sun; Venus ventures as far as 47° away, and the outer planets go all the way around the sky.

In ancient Babylonian astronomy, by the way, some planets’ names did reflect their apparent motion. The Akkadian name for Mercury, Šihṭu ‘attack, jump’, nicely matches its yo-yo-like motion around the sun; Saturn’s name, Kayyamānu ‘steady’, suits its slow motion. But there’s no indication of anything like that in connection with the gods linked to those planets.

2. Venus. Venus is the brightest planet (not counting the sun and moon, which ancient astronomers did count as planets). But who says brightness is ‘beauty’? Not anyone ancient, I can tell you that. Besides, Venus/Aphrodite’s field of interest wasn’t beauty, if anything it was lust, passionate sexual desire. The ‘brightness = beauty’ explanation isn’t just wrong, it’s also prudish.

At one time, some of the astronomers in the ancient past thought that Venus was actually two stars. This was due to the fact that it appeared as both the morning and the evening star.

Planets for kids

Not true. Ancient astonomical texts are perfectly clear that the ‘light-bringer’ and ‘evening star’, or rather Greek Phosphoros and Hesperos, were two names for the same planet (see e.g. Cleomedes, On the heavens 1.2). The same applies to most other ancient civilisations that had multiple names for the inner planets: for example, see Quark 2019 on ancient Egyptian astronomy. In Greek, Homer actually gives us three names for Venus: see below.

3. Mars. In some other ancient cultures Mars does have a name that probably reflects its redness, such as the Chinese name Huǒxīng ‘fire star’, or the late Egyptian form ‘Horus the red one’. That doesn’t impose an obligation on the Romans to do the same, and anyway those names denote ‘red’, not ‘bloody’. I haven’t found any Greco-Roman source that links Mars’ colour to blood.

4. Jupiter.

Jupiter shares a title with the king of the gods because it's the solar system's giant.

The Washington Post

Ancient astronomers certainly did not know Jupiter’s size. They had no way of measuring its radius or mass. This is from the science column, by the way: this writer wasn’t just ignoring ancient evidence, they were trying really hard to avoid imagining how the planets look when you don’t have a telescope.

5. Saturn.

Saturn is the last planet visible in the sky without any kind of aid, and named after the Roman god for agriculture–introducing agriculture to the people. The Greek equivalent to Saturn is Kronos — and both govern time (as well as the harvest we just established).

Sporcle.com

Saturn may have been an agricultural god, but it’s doubtful whether Kronos was. Conversely, in some contexts Kronos could indeed be imagined as having something to do with time (Greek chronos), in mystical forms of Greek religion that drew on name-magic. But that’s Kronos, not Saturn, and the mystical wordplay has no bearing on astronomy anyway.

The actual origins of the names

The names are simply translations. ‘Mercury’, ‘Venus’, etc. are romanised versions of the Greek names ‘star of Hermes’, ‘star of Aphrodite’, and so on. And the Greek links to various divinities were in turn borrowed from links to Babylonian divinities in Babylonian astronomy.

Addendum, an hour later: for maximum clarity, this is as far as we can push the explanation. The evidence trail ends with Nabu, Ishtar, Marduk, etc. We can’t know why Babylonian astronomers linked those gods to those planets: we can only point out that they weren’t gods of ‘speed’, ‘beauty’, and so on. Basically, the real explanation boils down to: ‘Because tradition.’

Old Babylonian cylinder seal depicting Ishtar/Inanna, with Venus shown as an eight-pointed star to the left (Oriental Institute Museum, Chicago; source: Wikimedia, CC BY 3.0)

For accurate accounts of planetary naming systems in antiquity, your top pick for an online source is the Oxford research encyclopedia of planetary science. For Roman and Greek names, see ‘The moon and the planets in classical Greece and Rome’, in the subsection ‘The planets’ (Hannah 2020); and for the Babylonian naming system, ‘The moon and planets in ancient Mesopotamia’, in the subsection ‘The moon, the sun, and the planets in religion, cult, and mythology’ (Ossendrijver 2020). The Encyclopedia covers several other ancient and non-European civilisations too. Neugebauer also has some good material on the Babylonians (1955: ii.498-503, ii.467-497), and the most detailed account of Greek naming systems is an older article by Franz Cumont (1935), who also covers regional variations.

Here are the planet naming systems side by side: English/Latin, Greek, and the Babylonian systems.

Latin, English	Associated Greek god(s)	Associated Babylonian god(s)	Akkadian name
Sol/sun	Hyperion, Helios (‘sun’)	Šamaš	Šanšu
luna/moon	Selene	Suen/Sin	Sīnu
Mercury	Hermes, Apollo	Nabu, Ninurta	Šihṭu (‘rising, attack, jump’)
Venus	Aphrodite, Hera	Ishtar	Dilbat (‘radiant’?)
Mars	Ares, Herakles	Nergal	Ṣalbatānu (meaning unknown)
Jupiter	Zeus	Marduk, Šulpaea	Peṣû (‘white’), Mulbabbar, Sagmegar, Nēberu, etc.
Saturn	Kronos	Ninurta, night-time version of Šamaš	Kayyamānu (‘steady’)

Now, there are a few catches.

• The Romans put a lot of work into linking their native gods to Greek gods. That’s how we got to having Mercury identified with the Greek god Hermes, Venus with Aphrodite, and so on. That isn’t the same things as Mercury being derived from Hermes. Only a handful of Roman gods were actual imports.
• With other pantheons things aren’t nearly as tidy. In particular, with the Babylonian gods there’s no real sense of qualities like ‘god of messengers’, ‘god of beauty’, and so on. When Greek astronomers borrowed the Babylonian set of links between gods and planets, Ishtar could be treated as an equivalent to either Aphrodite or Hera depending on context. Some places like Anatolia and Egypt had their own equivalences. (For details about regional variants, see Cumont 1935.)
• No one thought the planets actually were gods. Greek astronomers called them ‘star of Hermes’, ‘star of Aphrodite’, and so on. Planets could however metaphorically represent the gods in some poetic contexts, like when the Neo-Platonic Hymn to Ares (5th cent. CE?) refers to the god as ‘whirling [his] fiery sphere among the sevenfold courses of the aether’.
• The borrowed names were in use in the Greek world by the time of Plato (Hannah 2020). The borrowing from Babylonian astronomy probably took place in the 5th century BCE, a few decades earlier. Things are unclear because we don’t have any tracts written by astronomers in that period.
• Prior to that borrowing, we know almost nothing about homegrown Greek planet names. The only ones we know of are three names for Venus that appear in Homer: Eosphoros ‘dawn-bringer’, Eoios ‘morning (star)’, and Hesperos ‘evening (star)’.
• In Babylonian astronomy the planets had their own names, as well as being associated with a god. It’s only the divine names that survived translation into Greek and Latin.
• Egyptian astronomy doesn’t have anything much to do with the Greek naming system. The ancient Egyptians named all of the outer planets after Horus (Mars = ‘Horus of the horizon’, Jupiter = ‘Horus who bounds the two lands’, Saturn = ‘Horus bull of the sky’), and until relatively late their ‘morning star’ was Mercury, not Venus. (See Quack 2019.)

Alternate names: ‘shiny’, ‘shiny’, ‘shiny’, ‘shiny’, and ‘shiny’

Finally, there was an alternate set of Greco-Roman names based on words for ‘shiny’. The alternate names only pop up from Ptolemy onwards, and when they are mentioned they’re normally explained by referring to the divine names. Here’s how Martianus Capella introduces them (viii.851, trans. Stahl and Johnson):

Saturn is called ‘the Shiner’ (Phaenon), and Jupiter ‘the Blazer’ (Phaëthon), and Mars ‘the Fiery’ (Pyrois), Venus ‘the Light-Bringer’ (Phosphoros), and Mercury ‘the Twinkler’ (Stilbon).

In other words, the divine names were the older system, and it seems they were always more standard. Latin translations of the alternate Greek names could also be used.

Latin, English	Alternate Greek name	Alternate Latin name
Mercury	Stilbon	Scintillans
Venus	Phosphoros	Lucifer
Mars	Pyroeis	Rutilus
Jupiter	Phaethon	Splendidus
Saturn	Phainon	Lucidus

These alternate names aren’t very distinctive in meaning. I’d guess that in this system it was harder to remember which planet is which. It isn’t surprising that the older god names continued to stick.

References

• Cumont, F. 1935. ‘Les noms des planètes et l’astrolatrie chez les grecs.’ L’antiquité classique 4.1: 5–43. [Persée link]
• Hannah, R. 2020. ‘The moon and the planets in classical Greece and Rome.’ In: Oxford research encyclopedia of planetary science, online [DOI link]
• Neugebauer, O. 1955. Astronomical cuneiform texts, 3 vols. Princeton (reprinted New York, 1983).
• Ossendrijver, M. 2012. Babylonian mathematical astronomy: procedure texts. New York.
• —— 2020. ‘The moon and planets in ancient Mesopotamia.’ In: Oxford research encyclopedia of planetary science, online [DOI link]
• Quack, J. F. 2019. ‘The planets in ancient Egypt.’ In: Oxford research encyclopedia of planetary science, online [DOI link]
• Van der Waerden, B. L. 1982. ‘The motion of Venus, Mercury and the sun in early Greek astronomy.’ Archive for History of Exact Sciences 26.2: 99–113. [JSTOR link]