Back in the summer of 2006 the great "Is Pluto a Planet" debate raged about the halls of astronomy and - surprisingly - about the more normal news as well. It went so far as to have New Mexico pass (or try to, I don't know if it passed) legislation officially declaring Pluto a planet after the IAU "declassified" it. In all, it mostly
just highlighted the fact that there is no good scientific definition of a "planet" and that almost everyone just defaults to "I know it when I see it."
So in this book, Weintraub takes us on a little historical journey covering the ever changing meaning of the word "planet," from back when there were just the seven - and the Sun and the Moon were two of them - to definitions that had to keep repeatedly morphing as man both redefined what a planet was
and just kept finding more
Along the way we find that thanks to those changing definitions, we've had a "Ninth Planet" four times now - even though officially we only have eight at this point - and Neptune was once the thirteenth, rather than the eighth and last.
We see a time when what we'd now call "asteroids" were classed as planets - and so were moons.
Nicely, Weintraub himself seems to feel that - by any not horribly contrived and convoluted definition - Pluto still classes as a planet...as does Sedna, Ceres, Vesta and a flock of other "Plutinos," cranking the total up to at least twenty-three planets (this week). Since I also have no trouble with double (or triple, or sextuple) digit planet counts, it's kinda nice to have an astronomer agree with me that our apparent dislike of counting to more than nine or ten is just that - as dislike - and not a reason to skew the definition so much just so the number comes out that low (probably just so they won't have to buy too many styrofoam balls for their kids' Solar System models in school..).
It's an interesting book and I highly recommend it - and not just 'cause he agrees with me...;)