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Posted by Jonathan Silverlight on August 18, 2005, 6:21 pm
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>-> So what? If we want to exclude those we must define a criterium that
>-> the five original planets have but which they don't have.
>
>Actually, there were seven original planets, and the days of the week
>were named after them. Sunday and Monday are named after the two we
>don't consider to be planets nowadays.
>
>Nobody disputes that Mercury is a planet. But suppose its orbit were
>between those of Jupiter and Saturn. It would not be visible to the
>naked eye, and it would be far smaller than the other planets *in its
>vicinity*. Would we call it a planet then? I suspect not. So our
>intuitive sense of what a planet is includes some reference to the
>other planets at similar distances from the sun.
>
It's obvious that the definition of a planet is highly artificial - so
much so that it's probably no longer worth taking seriously. It makes
for nice discussions, though.
For instance, if Mercury was between Mars and Jupiter it would be
clearly visible to the naked eye, and it would fit "Bode's Law", which a
body between Jupiter and Saturn wouldn't. It would still be a planet
,even though it's the size of Jupiter's Galilean moons.
And on the subject of definitions, is Chiron a comet, or an asteroid, or
a planet? No wonder some people are calling it a centaur, and putting it
in a class by itself. Defining things is a human trait, and taxonomists
in particular are divided into "lumpers" and "splitters" :-)
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