Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Cause for Oblate Spheroid
Name: Chris
Status: other
Grade: other
Location: NV
Country: N/A
Date: N/A


Question:
Why is the distance from the north pole to the south pole smaller than the distance around the equator?


Replies:
Chris,

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 to 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.

Howard Barnes,
Astronomer


Hello Chris!

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 centripetal force.

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.

Cheers,

Michael Pierce

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 and sun.



Click here to return to the Astronomy Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory