Earth - an Oblate Spheroid

Date: 05/17/99 at 17:06:08
From: Sharon Paul
Subject: Ellipsoid

I'm looking for real-life examples of an ellipsoid for elementary 
school children. I think that a football and an egg are not true 
examples, but I'm not sure why. I've looked in every print resource I 
have and have searched the Internet. Any help you could lend would be 
appreciated! Thanks.

Date: 05/19/99 at 11:41:00
From: Doctor Fwg
Subject: Re: Ellipsoid

Dear Sharon,

You are right about a football and egg not being the best examples of 
an ellipsoid. I think a better example might be the earth - it is 
technically called an oblate spheroid which is a fancy way to say 
ellipsoid flattened along the spin axis. 

Because large spinning objects (like the earth) are sort of plastic-
like, they tend to have larger equatorial diameters than polar 
diameters due to the centrifugal spinning forces, which are always 
perpendicular to the spin axis. You can look up values for the earth's 
polar and equatorial diameters (or the corresponding radii) to 
illustrate just how large (or small, depending on one's point of view) 
this effect really is. I believe the current figures are 6357 km and 
6378 km for the earth's polar and equatorial radii, respectively. 

Isaac Newton was the first simultaneously to predict that the earth 
had this shape and to estimate the dimensional characteristics. 
Another theory (incorrect), during the time of Newton, held that the 
earth had the shape of a prolate spheroid, which is an ellipsoid that 
has a polar radius that is longer than its equatorial radius.

I hope this has been helpful.

- Doctor Fwg, The Math Forum
http://mathforum.org/dr.math/