17 August 2006

Greetings From A Double Planet

Brit over at Daily Duck posted on the new definition of "planet" just promulgated by the International Astronomers Union (IAU) which save Pluto as a planet at the cost of letting some awfully dodgy members into the Club. (And when I'm down South it usually only takes a few minutes before I'm saying "Y'awl.")

I wondered whether anyone could tell me whether the Moon is actually a moon or, as I once heard, not a moon because the Sun's gravity acted on it more strongly than the Earth's gravity. Well, according to this Wikipedia article, I'm on the cutting edge of astronomical knowledge. At the same time the IAU saved Pluto from non-planethood, it also clarified that the Moon is a moon. Whether an otherwise planetary body is a moon now depends on whether, with its putative primary, it rotates around a spot in space (in which case it's a double planet) or a spot within the primary (it which case it is a moon). Under this definition, the Moon is a moon.

Clearly, the IAU has got it wrong both times. Pluto is not a planet. That's obvious; and certainly when it comes to matters of definition, the definition should follow the obvious.

Except, of course, when the obvious is mundane, but the definition is really cool. Hence, I reject the definition of the Moon as a moon and adopt the definition (taken from the Wikipedia article linked to above) first proposed by Isaac Asimov:
The late Isaac Asimov suggested a distinction between planet-moon systems and double-planet systems based on what he called a tug-of-war (TOW) value that describes whether the presumed satellite is more firmly under the gravitational influence of the presumed planetary primary or the Sun. In the case of the Moon, the Sun "wins" the tug of war, i.e., its gravitational hold on the Moon is greater than that of Earth. The opposite is true for other presumed satellites in the Solar System (with a few exceptions), including the Pluto-Charon system. By this definition, the Earth and Moon form a double-planet system, but Pluto and Charon represent a true primary with a satellite.
From here on out, the Earth and Moon form a double planet system. There are still nine planets: Mercury, Venus, Earth, Moon, Mars, Jupiter, Saturn, Uranus and Neptune. So says me.

4 comments:

Susan's Husband said...

As I pointed out, Asimov's definition is wrong because it drags in irrelevant information, such as distance from the Sun. Why should that make a difference?

P.S. I also eagerly await your definition of "planet" that excludes Pluto but includes the other eight.

David said...

sh:

From here on out, the Earth and Moon form a double planet system. There are still nine planets: Mercury, Venus, Earth, Moon, Mars, Jupiter, Saturn, Uranus and Neptune. So says me.

It's arbitrary, but it's not at all ambiguous.

joe shropshire said...

I like the center-of-mass definition better, because it takes us back to February 17, 1930 when there were only eight planets. Conservatives: rolling back the 20th century, one planet at a time.

Brit said...

I like your definition, David, and will henceforth subscribe to it.

And now that there are two of us using it, we don't fall foul of Wittgenstein's Private Language Argument against the impossibility of meaningfully defining terms on your own.

Nine planets, including the Moon, it is.