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© 1999-2010 by Francis Edward Su
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From the Fun Fact files, here is a Random Fun Fact, at the Easy level:

Hugging the Equator

Figure 1
Figure 1

Suppose that you have a rope around the equator of a basketball. How much longer would you have to make the rope so that it is 1 foot from the surface of the basketball at all points? The answer is 2*Pi feet.

Now suppose you have the rope around the equator of the earth. (Yes, a rope about 25,000 miles long!) How much longer would you have to make the rope so that it is 1 foot off the ground all the way around the equator?

Surprise answer: it's 2*Pi feet also!

Presentation Suggestions:
Ask quickly so that students respond by intuition rather than calculation. Students are always amazed that the answer is independent of the radius of the sphere!

The Math Behind the Fact:
Many will think that since 25,000 miles is so large, you will need to add lots of rope to lift it off of the equator. But extra circumference only scales as 2*Pi times the extra radius, which is 1 foot... a somewhat surprising consequence of simple geometry. See also Football Field.

How to Cite this Page:
Su, Francis E., et al. "Hugging the Equator." Math Fun Facts. <http://www.math.hmc.edu/funfacts>.

The Link for this Fun Fact: is directly accessible here.

Keywords:    simple geometry
Subjects:    geometry
Level:    Easy
Fun Fact suggested by:   Tommy Ratliff
Suggestions? Use this form.
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