Pseudo-history and the definition of a planet

[mmcirvin]

This Planetary Society article about the proposed redefinition of "planet" says:

In the past, no term in astronomy had seemed as clear cut as a "planet." There were nine, six of which had been known for several millennia, and while no formal definition was available, none seemed necessary. Planets seemed intuitively obvious, and if more were discovered somewhere in the universe, both professionals and lay people confidently assumed that they will know one when they see one.

factitiouslj points out that, at least in 1828, the definition was different:

The book First Steps to Astronomy and Geography, from 1828, listed the planets as "Mercury, Venus, the Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, and Herschel." Apparently the name "Uranus" hadn't caught on yet.

So at least one textbook was listing four asteroids (three of which may be planets under the proposed IAU definition) as planets, 27 years after astronomers started finding asteroids and before Neptune was discovered. I suppose the demotion happened once they started to realize how many there were. If the definition of a planet was ever considered intuitively obvious, it was an obviousness that was subject to change.

Comments

[mmcirvin.livejournal.com]

...So I think I sort of like the hydrostatic-ellipsoid part of the definition (though it needs to be more precise: how big a mountain will you tolerate? I get the impression that Pallas and Vesta are probably right on the edge), but I don't like the moon-barycenter part of the definition at all. I say if there's a bound multiple body consisting of potential planets, the biggest one is the only one that can be a planet, and the others are moons. It's just as simple, takes no more astronomical data, and is probably no more prone to tough calls.

[mmcirvin.livejournal.com]

...oh, and we'll say "biggest" by mass, just to make it easy on the astronomers.

[reverendluke.livejournal.com]

Yeah, I've been thinking the same thing. The whole barycenter-based distinction seems needlessly complex, and if not more prone to tough calls, it's certainly more prone to weirdness.

[mmcirvin.livejournal.com]

I first heard of the IAU proposal from a picture of the new planet lineup on the front page of a newspaper. My initial reaction was "2003 UB313 is a planet? Fine. Ceres is a planet? That's really surprising, but I guess I'm OK with it. Charon is a planet? They're on crack!"

[factitiouslj.livejournal.com]

Does this make Hydra and Nix moons of both Pluto and Charon?

[mmcirvin.livejournal.com]

If they insist that the barycenter rule distinguishes moons from non-moons, then I suppose they would have to be. We just got through learning that Pluto has three moons; now Pluto and Charon would be a double planet that share two collective moons.

This is all about words, but, still, the actual complexity of the situation doesn't seem to justify the complexity of the nomenclature.