Miércoles 11 de Julio de 2007, Ip nº 198

Honey, we shrunk the Earth
Por Damian Carrington

The world is smaller than we thought – by five millimetres. That is the conclusion of an international project to measure the diameter of the Earth. The last such measurement was made in 2000.

However, the new data does not mean the Earth has shrunk. The new figure is simply more accurate, thanks to more accurate measurements, more data and better geophysical models.

The reduction makes no tangible difference to everyday life – it will not noticeably shorten your journey to work. But it is crucial in other ways, such as in the detection of sea-level rises, says Axel Nothnagel, at the University of Bonn in Germany who led one part of the project.

"It is essential for the positioning of the satellites that measure rises in sea level – they must be accurate to the millimetre," says Nothnagel, who led the German team. "If the positions of the ground stations tracking the satellites are not accurate to the millimetre, then the satellites cannot be accurate either."

The scientists round the number up to 12,756.274 kilometres (7,926.3812 miles) for the general use.

Global network
The new measurement was made by combining data obtained via three different techniques. The first is a method known as Very Long Baseline Interferometry, which exploits the radio waves that arrive on Earth from distant astronomical sources such as quasars.

"A network of more than 70 radio telescopes worldwide receives these waves, but because the stations are so far apart from each other, the signals are received at slightly different times," explains Nothnagel, who led the VLBI work. "From this difference we can measure the distance between the radio telescopes with an accuracy of 2 mm in 1000 kilometres."

The second method is called satellite laser ranging, which uses lasers to measure the distances between the Earth and orbiting satellites. The final method makes precise measurements between the Earth and GPS and DORIS satellites.

The synthesis of all the data collected up to 2005 took two years. It was organised by the International Earth Rotation and Reference Systems Service, who combined the data into a single common framework, called the International Terrestrial Reference Frame.

  06/07/2007. New Scientist Magazine.