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About a month ago, I posted a challenge problem for calculating the radius of the Earth.  Some of my summer AP Physics students attacked it and did pretty well.

I wanted to show the solution to this practical problem that men like Newton used to determine the radius of the Earth.  And once that was known, he knew the mass of the Earth (without ever putting it on a scale).  Then he knew the distance to the sun as well as the distance to the moon.  He could even find the radius of your anus…I mean the radius of Uranus.  All this done before calculators and all this from simple geometry and trigonometry.  So without further adieu, here’s the solution.