IN 240 BC, it took all of Eratosthenes鈥檚 ingenuity to estimate the circumference of the Earth within 10 per cent of the correct value. But students today can achieve a comparable level of accuracy using the Internet, a globe and a piece of string.
Key to the trick are the optical fibres along the seabed between major Internet nodes such as Seattle and Hawaii. Signals travel along these fibres at roughly two-thirds the speed of light, so if you know the time taken, you can deduce the distance travelled. If you then take a globe and stretch a piece of string between Seattle and Hawaii, and compare its length with a piece of string stretched round the globe, you can estimate the circumference of the Earth.
But how do you find the travel time of the optical signals in the first place? Simple: you use 鈥渢raceroute鈥�, a small Windows program built into every PC that lets IT people troubleshoot computer networks.
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The program bounces a sequence of data packets to and from 鈥渘odes鈥濃�� computers or network switches鈥� along the route from your computer to a destination machine. The output of the program is the time taken for each packet to go between each of the nodes along the route and back to your computer.
Suppose you choose a destination computer at the University of Hawaii. In Windows you open MSDOS and type 鈥渢racert 鈥�. The program will output a list of the nodes along the route, including ones in Seattle and Hawaii. Taking half the round-trip time between Seattle and Hawaii gives you the one-way travel time鈥� about 24 milliseconds. This corresponds to about 4800 kilometres, giving the radius of the Earth as about 7200 kilometres. That鈥檚 only 13 per cent out.
This oddball way to measure the size of the Earth has been devised by Snowflake Kicovic, Loren Webb and Michael Crescimanno at Youngstown State University in Ohio. 鈥淭he idea is that students can figure it out themselves rather than accept the word of the textbook,鈥� says Crescimanno.
