Margo Schulter wrote:

wrote:


(2) A dwarf planet is a celestial body that (a) is in orbit around
the Sun, (b) has sufficient mass for its self-gravity to overcome
rigid body forces so that it assumes a hydrostatic equilibrium
(nearly round) shape[2], (c) has not cleared the neighbourhood
around its orbit, and (d) is not a satellite.{[3]}

Sounds good to me, though we still need to quibble about what 'mostly
round' means... at some level everything is nonspherical.

Hi, and of course I'd agree -- a point also made by people discussing
the "nearly round" test before the IAU adopted it. A rotating body will
undergo oblation -- and the small deviations of the Earth from sphericity
have been one interesting topic of the space age.

Similarly, people who propose that an object considered a major planet in
our system should have a "near-circular" orbit (a criterion sometimes
proposed in discussions on this topic, although not used in the IAU
resolution) likely are aware of Kepler's discovery that planets indeed
orbit in ellipses, not circles.


My first pass: The difference between the high and low point should be
under X% of the rotation-adjusted radius, and then we can debate what X
should be. 5 may disallow Ceres but 10 probably permits it, and I see
no real a priori reason to favor any particular value.... set X=.1 and
we even rule out Earth.

The near-roundness test has been discussed a bit, and there's a general
consensus that Ceres is a bit more than large enough to meet it -- IAU
footnote 2 accompanyinng the text you quote (see my original post)
says that there will be a process to sort out borderline objects, possibly
an appreciable number of Kuiper Belt Objects or KBO's in coming years,
into dwarf planets or other categories.

So we're still confused, but on a deeper level 8)

I'm not sure if the roundness test is "confusing" -- it does have a
borderline where the judgment might be a bit arbitrary, but that's true
of lots of taxonomic schemes in various disciplines.

Of course, it never hurts to be aware that there _are_ ambiguities.

Most appreciatively,

Margo Schulter


Wouldnt snake charming be more personally 'engaging' for you ?