Researchers at the University of Bonn (Germany) announced Thursday, July 5, 2007, that a two-year international project—using 34 organizations from 17 countries—has been completed. They have finished measuring the diameter of the Earth with  the use of 70 radio telescopes positioned around the world.

The new diameter of the Earth is 7,926.3812 miles (12,756.274 kilometers)—and that is just an estimation used for the public announcement on Thursday.

The difference in diameter—0.2 inches—is about half the width of an average human finger. Not much for a human body, but the change in length is drastically important for scientific studies performed on the Earth.

For example, the accurate positioning of satellites that measure changes in sea level is very important. If not accurately positioned within one millimeter (0.04 inch), satellite measurements of sea level are not accurate, too. This problem could cause inaccurate conclusions in climate changes, such as the world’s concern with global warming.

The 70 telescopes were able to measure the diameter of the Earth to 0.07 inches per 621 miles (about two millimeters per 1,000 kilometers). The international collaborative effort used the procedure called Very Long Baseline Interferometry (VLBI). It is a type of astronomical interferometry that uses radio astronomy, or the use of radio waves, which are part of the electromagnetic spectrum.

Distance data was received from each of the 70 telescopes. Each pack of distance data, from individual telescopes, was paired with time data from a local atomic clock. There was a slight time delay as the distance data is transmitted at the speed of light. The data, both distance and time, was then correlated among the group of telescopes. Based on this method, the scientists were able to come up with the new measurement for Earth’s diameter. VLBI has been used with such measurements as tracking of spacecraft and imaging of cosmic radio sources (such as quasars), along with the movements of tectonic plates here on the Earth.

Axel Nothnagel headed the project with the German researchers. The results were published in the Journal of Geodesy.

Author note: Measurement conversions corrected due to sharp eye of one reader.

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