Saturn-Day and the Absurdity of Absolute Definitions

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.

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6 Responses to “Saturn-Day and the Absurdity of Absolute Definitions”


  1. 1 Laurel Kornfeld November 19, 2011 at 15:11

    No “formal scientific definition of what a planet is” exists today either. Instead, we have two concepts for understanding the solar system, the dynamical and the geophysical. Neither are wrong; they’re just different ways of viewing what is out there. What will be really exciting is how the many types of exoplanets will be incorporated into our future understanding of solar systems. It’s fascinating how scientists expected other solar systems to be just like ours only to find they’re not like ours at all. And we haven’t even begun to discover the smaller exoplanets, those Earth-sized and smaller.

  2. 2 Thony Christie November 19, 2011 at 15:58

    Woden like Mercury is a god of hunting and death therefore the association.

    The astrological week functions as follows. The first hour on Saturday (Saturn day) from midnight to 1am is the hour of Saturn, the second the hour of Jupiter, the third the hour of Mars, the fourth the hour of the Sun the fifth the hour of Venus, the sixth the hour of Mercury and finally the seventh the hour of the Moon. This is the Ptolemaic order of the planets. If you continue to count and name the hours in the same order the twenty-fifth hour, the first hour of the next day in the hour of the Sun and therefore the day in Sunday. The twenty-fifth hour on Sunday is the hour of the Moon leading to Monday and so on and so forth.

    I leave it to the mathematicians to work out the algorithm in modulo 7.

    • 3 Matthew R. Francis November 19, 2011 at 16:07

      Ah, that connection between Mercury and Woden makes sense. I admit my knowledge of Norse and Germanic mythologies is not that great.

      And I will definitely take your word for it that the math works. ;) Now that you mention it, I do remember all the stuff about the hours and days. It’s another great example of how our intellectual ancestors weren’t dumber than we are, just had different toys to play with.

  3. 4 Mike Wrathell November 19, 2011 at 21:12

    Had the International Astronomical Union demoted Pluto in a “clean” way, I would agree with you that it doesn’t matter; however, the way in which it went about it included having one voting member threatened with the destruction of his career (a fact he actually admitted to in a post on Facebook), along with a lack of notice and vetting of the absurd resolution (because Earth also does not “clear its orbit,” having 19,500 asteroids of September 2010 according to NASA), violating its own by-laws. Having seen footage of the session of the 2006 Prague General Assembly in which the said resolution was voted on by 424 of the 10,000 member IAU body (online voting is not allowed), (the vote done of the last day of the GA and was not on the agenda, so many pro-Pluto members had “checked out” of their hotel rooms and flown home) I was appalled that one pro-Pluto member who chose to speak was actually cut off by a member of the Executive Committee in a very rude manner. That matters. Also, I must note that the odds are now in Pluto’s favor that it is larger than Eris, and, confirmation of such will occur in 2015 when NASA’s New Horizons spacecraft reaches Pluto. Mike Brown, who somehow feels he can boast of killing Pluto because of his co-discovery of Eris, was sorely mistaken when initial data suggested that Eris was larger than Pluto. The data now suggests Pluto is 7 miles wider. It is time to kill that rancid resolution, if you ask me. A more rational definition of a planet would then make Ceres, Pluto, Haumea, Makemake, and even dinky Eris planets.


  1. 1 Chapter 3: Operationism and Essentialism | Cedar's Digest Trackback on November 22, 2011 at 14:49
  2. 2 Moonday: Double World « Galileo's Pendulum Trackback on June 11, 2012 at 12:13
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