On Sep 20, 2007, at 2:10 PM, Zev Sero wrote:
Marnen Laibow-Koser wrote:
This was not a "mistake". The term "planet" was not given an
official definition by the IAU until 2006 (see http://astro.cas.cz/
nuncius/nsiii_09.pdf , page 8, resolution 5A; this is apparently
the final text of the resolution that passed). Before that, there
was no reason to say Pluto wasn't a planet.
Dictionaries had no problem defining it before 2006.
You're right. And the definition included Pluto, and properly so
based on the de facto standards of planethood at the time. It could
perhaps reasonably have included Chiron too, though for some reason
it never seems to have done so.
Without some sort of official definition, it is preposterous to
definitively say that some piece of rock orbiting the sun "was never"
a planet.
And there was a
reason Ceres was de-planetted back in the mid-19th century.
At the time Pluto was reclassified as not a planet in 2006, there was
an alternate proposal on the table to reclassify Ceres as a planet
again (keeping Pluto, of course). This suggests that the difference
between "planet" and "non-planet" has been less clear-cut than you
are implying.
It didn't
need a resolution by the IAU. People just realised that it was not
really different than the other asteroids,
Ceres *is* different from the other asteroids, sort of. It is by far
the largest object in the asteroid belt (952 km in diameter, where
the next largest [Pallas] is 531 km), and it is now classified as one
of the three "dwarf planets", along with Pluto and Eris.
and since they couldn't all
be called planets none of them should be.
But among the asteroids, only Ceres is called a dwarf planet. Since
they can't all be called dwarf planets, should none of them be?
Exactly the same thing has
now happened to the Kuiper belt. Pluto is no different than all the
other Kuiper belt bodies ("plutinos") and either they're all
planets or
none of them are.
But among the Kuiper belt bodies, only Pluto is called a dwarf
planet. See above.
And of course, the third dwarf planet, Eris, is larger than either
Ceres or Pluto and, like Pluto, has a moon.
Ceres was considered a planet for almost as long as
Pluto, too, so it's not as if tenure in the textbooks is that
important.
True. Ceres was reclassified when the asteroid belt was recognized.
However, Pluto is larger than Ceres, and most of the other Kuiper
belt objects are far smaller than the asteroids, so it was perhaps
less easy to make the decision to reclassify Pluto as just another
Kuiper belt object once the Kuiper belt was discovered.
--
Zev Sero Something has gone seriously awry with this
Court's
zev@xxxxxxxxx interpretation of the Constitution.
Best,
--
Marnen Laibow-Koser
marnen@xxxxxxxxxx