At 3:54 PM -0500 9/11/08, Boyd, Todd M. wrote:
> Considering that my other profession is Geophysicist, I'm kind of up
on those sort of things. The Earth is an oblate spheroid and the
computation to include the curvature of the earth would be a bit more
involved.
---8<--- snip
But it's also NOT an oblate spheroid! :)
http://blogs.discovermagazine.com/badastronomy/2008/09/08/ten-things-you
-dont-know-about-the-earth/
Todd:
LOL -- Discover magazine is hardly the definitive authority for
Geophysical terms. This is more an article written to make
controversial claims for laymen to ponder rather than to provide
anything of real substance for people who work in the Geophysical
field.
Note, while the article claims that the Earth is NOT an oblate
spheroid, it does NOT provide a real alternative. However, it does
say "it would be smoother [billiard ball]" but it also claims that
"The Earth is more complicated than an oblate spheroid." So, which is
it? Is it as simple as a billiard ball or more complicated than an
oblate spheroid?
You see, it's one of those "Hit and run -- You can't prove me wrong
because I didn't say anything" articles.
While technically the Earth isn't ANY standard/formal static size, an
oblate spheroid is a pretty good fit.
Perhaps an example may be of assistance -- if the Earth was pure
water (a fluid medium having an equipotential surface) then its
surface could be described as a spheroid -- imagine a drop of water
in space without any external forces applied to it.
If you introduce rotation, then the radius from the center of the
Earth to the equator (perpendicular to the axis of spin) expands and
the radius at the poles (axis of spin) reduces. As such, this surface
IS known and IS defined as an oblate spheroid -- and with respect to
Earth, this IS upon which the geodetic latitude determinations are
made.
Now, to be more exact in describing the Earth's surface, requires
more attention to detail.
First, the Earth consists of more than just water and it's content
are not uniform with respect to distribution and density -- not to
mention numerous other physical properties (i.e., elasticity);
Second, the Earth has rotational, orbital, and even processional
forces applied to it;
Third, the Earth has external gravitational forces exerted on it from
the Sun, Moon, Planets, and et all;
Considering all, the surface of the Earth becomes very complex to
compute, let alone to define for the general population.
So, the problem of what term to use to define the surface of the
Earth really depends upon the audience you're addressing. If I were
talking to a group of preschoolers, I would use the term "Ball", but
if I was addressing a group of Geophysical Scientist, then I would
use the term "oblate spheroid." Incidentally, I have done both
successfully without objection.
An "oblate spheroid" is good enough for me, but the author of the
article likes the term "Ball" -- each has their audience of believers.
Cheers,
tedd
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