# Re: OT: One more for the deniers

RS <RS@xxxxxxxxxx> wrote:
On Sat, 17 May 2008 21:59:37 GMT, "RichL" <rpleavitt@xxxxxxxxx> wrote:

My recollection is that the oblate spheriodal shape is understandable
purely in pre-relativistic physics in terms of the equilibrium shape
achieved by a rotating blob of fluid.

Yeah, formation should be primarily Newtonian, though you're the
authority here. There may be a tiny bit of relativistic physics in
play in that massive bodies can pull space-time as they travel ("Frame
dragging", Lense, Thirring). Very small factor by comparison though,
even for something as large as the Earth.

http://www-history.mcs.st-andrews.ac.uk/HistTopics/Newton_bucket.html

Interesting article!
Turns out that my suggestion about the rotating fluid held together by
gravitational force is qualitatively correct, but to get the "right"
answer you have to account for the variation in density with depth into
the earth:
http://en.wikipedia.org/wiki/Figure_of_the_Earth
(Yeah, I cheated and looked it up.)

"Determining the exact figure of the Earth is not only a geodetic
operation or a task of geometry, but is also related to geophysics.
Without any idea of the Earth's interior, we can state a "constant
density" of 5.515 g/cm³ and, according to theoretical arguments (see
Leonhard Euler, Albert Wangerin, etc.), such a body rotating like the
Earth would have an flattening of 1:230.
"In fact the measured flattening is 1:298.25, which is more similar to a
sphere and a strong argument that the Earth's core is very compact.
Therefore the density must be a function of the depth, reaching from
about 2.7 g/cm³ at the surface (rock density of granite, limestone
etc. - see regional geology) up to approximately 15 within the inner
core. Modern seismology yields a value of 16 g/cm³ (iron or hydrogen) at
the center of the earth."

.

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