#### Koveras00

May I know how scientist measure the radius of the earth?

And how accurate it is??

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#### Guybrush Threepwood

http://opal.physik.uni-bonn.de/~mkobel/pos/deleg/zapp/zapp1.htm [Broken]

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#### russ_watters

Mentor
The first person to measure the circumference (and thus by calculation the radius) of the earth was a Greek by the name of Erosthothenese (spelled horribly wrong). He traveled between two cities in Italy and measured the angle of the sun in a well. From there its simple geometry.

#### LURCH

Originally posted by russ_watters
The first person to measure the circumference (and thus by calculation the radius) of the earth was a Greek by the name of Erosthothenese (spelled horribly wrong). He traveled between two cities in Italy and measured the angle of the sun in a well. From there its simple geometry.
YES! His name is spelled "Eratosthenes", and he (in any inertial frame of reference) da man!!

A more modern method used to independently verify this measurement is satellite VLBI. "VLBI" stands for Very Long Baseline Interferometry, and involves two satellites looking at a common object. The two satellites can be placed at a known altitude (using radio altimeters), with a known difference in global position. For example, they could be placed 90o from one another so that when one is directly over the North Pole, the other is directly over the equator. For this example, let us suppose there is a Quasar perfectly lined up with the earth's axis, so that both satellites can see it by looking directly "North".

Now, quasars have a signature EM wave pattern. Both satellites will see this distinctive pattern, but the satellite directly over the pole will see it first. By measuring the difference between the two wave patterns (how they "interfere" with one another) we can determine how far apart the two satellites are. This gives us an extrasolar source for our measurement. Submitting the information from this alternate source to the same geometry used by Eratosthenes, we can get independent verification of his measurement.

#### Integral

Staff Emeritus
Gold Member
A few years back Sci. Am. had an article which told of a method to measure the radius of the earth from a promintory above the sea.

I tried it from Yaquina Head on the Oregon coast.

We must have made some canceling errors because we came up with a result much closer then I believed possible. It's been 20+ yrs so I really can't remember the specifics.

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