A few of us on Twitter fell into a discussion this morning about the names of days of the week. To summarize, here are the names of the days in English, along with their origins:
- Sunday: obviously named for the Sun.
- Monday: Moon-day. In French, the word is lundi, for la lune = Moon.
- Tuesday: for the Old English Tiw, a god of war. This was translated from mardi, or Mars-day in French.
- Wednesday: for Woden (or Odin or Wotan). I’m not sure how what the direct correspondence to Woden is, but the French is mercredi for Mercury-day.
- Thursday: for Thor, a thunder god. The French is jeudi or Jupiter-day; Jupiter or Zeus was known to hurl thunderbolts on occasion.
- Friday: for Frigga, a goddess with some fertility connotations. The French version is vendredi or Venus-day.
- Saturday: for Saturn-day.
For connoisseurs of musty astronomy and astrology, it should be clear that the origins of these names are the seven “classical planets” (from Latin through French, then translated to English and other Germanic tongues). A “planet” in ancient systems was any object that wandered across the apparently fixed background of stars: the Moon, the Sun, Mercury, Venus, Mars, Jupiter, and Saturn. You still see this outmoded planet definition in “modern” astrology (also evidenced in the song “Age of Aquarius”, when the Moon is in the Seventh House, etc.). It’s funny to think of how influential these astrological definitions have been over time, lingering on long after the rise of real astronomy. But I digress.
With the advent of the Sun-centered Solar System (how many more “s”-words can I stick into that phrase?), the definition of planet began to shift. Instead of being wanderers, the key property became objects in orbit around the Sun, so both Sun and Moon changed status; the discovery of moons around Jupiter and Saturn further refined the picture of the Solar System. Uranus and Neptune were relatively uncontroversial additions to the rank of planet, as were Ceres, Vesta, and other asteroids … and here is where things began to shift again. With so many planets being added to the Solar System, astronomers quietly moved these much smaller new objects into a separate category (though occasionally they are still referred to as “minor planets”, and people studying them are still planetary scientists). I’ve emphasized this before, and I’ll do so again I’m sure: no formal scientific definition of what a “planet” is existed at that time. Instead, it was more a pornography-style definition: you know one when you see one.
In fact, that kind of definition—which is called an operational or heuristic definition—is pretty common in science, and for good reason. Jupiter and Pluto and the Sun are there, whatever we call them. That’s a major reason why I don’t care if Pluto is classified as a planet “officially” or not: it’s simply not important. (It’s also why I’m not bothered by the “official” planet definition by the International Astronomical Union: it’s flawed, but it’s not important enough in the scheme of things to get my knickers in a twist about it.) The contrasting position is what philosopher Sir Karl Popper calls essentialist: the idea that some things “really are” planets. Essentialist definitions are very legalistic and concerned with getting all the details right (whatever that means), but that’s not how science progresses.
The precision in scientific practice is moved from semantics, which can and should change over time, to collecting evidence and making theoretical predictions. In an Earth-centered universe, it made perfect sense to regard the Sun as a planet under that definition; today it makes none, because both our cosmology and our definition of planet have changed. The discovery of planet-like objects unattached to any star (which we quasi-democratically on Twitter named “vagabonds”) means we must rethink again, and that’s just fine. In the end, what we name those objects doesn’t change anything about them; it may change how we think a little bit, but that’s normal and healthy. So, on this Saturn’s Day, celebrate the evolution of our conception of the cosmos, and (to quote The Decemberists), raise a glass to the turning of the seasons.