Cause for Oblate Spheroid
Why is the distance from the north pole to the south pole
smaller than the distance around the equator?
Think of the Earth as a collection of rocks, water, gases, and so on -
held together by its own gravity. Anything that tends to counter this inward
gravitational pull will have the effect of stretching the Earth out. For
example, the gravitational pull of the moon can cause the waters of the Earth
to stretch out in the direction of the moon.
Now, think of the Earth rotating about its axis. This spinning motion, much
like, say the way a spinning skater's hair will stand away from the person's
body, can cause the collection of rocks, water, gases, etc. to bulge out in
the direction of the spin.
As such, the Earth is wider around the equator (and it would be a longer
distance to circumnavigate the Earth in that direction) than it is from pole
Greg (Roberto Gregorius)
This is caused by the rotation of the Earth. Remember that, although you are
sitting in front of a computer, you are actually moving at a rate of
approximately 400 metres per second, by virtue of the Earth's rotation on its
axis. That is a lot of centripetal force on a planet 12700 km in diameter. The
Earth, therefore bulges about 11 km at the equator.
Did you know you weigh more at the north or south pole than you do on the
equator? You will in fact weight about 0.5 % less at the equator (a savings
of say 3.5 newtons for many people). The reason is much the same as why the
earth is bulging at the equator. Ask yourself this: What is different between
the equator and the poles that can change weight?
In essence the answer to both has to do with the rotation of the earth. At
the poles you find no rotation and at the equator you have the maximum in
rotation. While the acceleration due to gravity is the same in both locations,
the outcome is slightly different. Near the pole you can reasonably neglect
the rotation of the earth about its axis and all of the acceleration of
gravity is manifest in the force you feel pushing up (i.e. weight). At the
equation you still experience most of this force, but it is compensated to a
small extent by the fact that you are rotating (along with the earth) at a
significant velocity. Part of the force you feel at the poles is lost at the
equator as your are "pulled" around in an orbit by what you would feel as a
A couple of examples of numbers may make this more concrete. Think about it
this way, everyday at the equator you travel around 40,000 kilometers while at
the poles you go nowhere (ignore for a minute that the earth is also zipping
around the sun). To put it another way, if Earth were rotating fast enough,
you would have no weight at all at the equator while still having full weight
at the poles (though I imagine Earth would cease to be "Earth" as we know it
at this point too).
So in essence things get packed a little bit more tightly at the poles than
they do at the equator leading to a narrower diameter from pole to pole than
across the equator.
PS : To avoid any nitpicking over numbers, weight at the equator is also less
due to being farther from the center of Earth than at the poles. Wikipedia
places this at around 70% due to your orbit while 30% is due to the being
further from the center of mass of the earth. However, to be fair, all of the
is due to the rotation as without it, you would not find a significant bulge.
PPS : Digressing further... you could still expect some further (but much
smaller) deviation from even this shape due to tidal effects from the moon
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Update: June 2012