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/
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