MadSci Network: Earth Sciences

Re: How do scientists measure the circumference of earth today?

Date: Mon Aug 7 21:14:06 2006
Posted By: James Holliday, Grad student, Physics Department, University of California, Davis
Area of science: Earth Sciences
ID: 1145495437.Es


Although it is over 2000 years old, Eratosthenes' method is still basically how we measure the earth's circumference. What we do now, however, is to independently verify the measurement using satellite based Very Long Baseline Interferometry (or VLBI).

We start with two different satellites looking at the same far away object. Using altimeters, the two satellites can be placed at a known altitude above the earth and with a known difference in global position. For example, they could be placed so that one is directly over the North Pole while the other is directly over the equator. Let us suppose that both satellites are looking at a distant quasar.

Quasars have a distinct EM wave pattern. Both satellites will see this distinctive pattern, but one of the satellites will see it first. By measuring the interference between the two wave patterns we can determine how far apart the two satellites are. Submitting the information from this alternate source to the same geometry used by Eratosthenes, we can get independent verification of his measurement.

It's fun to also figure out a rough estimate for the earth's circumference using only logic. The Italian physicist Enrico Fermi was known for using intuition by itself to guess the solutions to problems. For example, to estimate the earth's circumference:

  1. How many time zones do you pass through when you fly from New York to Los Angeles? Answer: 3.
  2. How many miles is it, about, over that same distance? Answer: about 3000.
  3. How many miles per time zone, on average? Answer: about 1000.
  4. How many time zones must there be around the world? Answer: 24 because there are 24 hours in a day.
  5. How many miles around the world? Answer: 24 time zones x 1000 miles per time zone = 24000 miles.
Yes, it is about 24000 miles around the world!

I hope this has helped


James R Holliday

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