The characters in Jules Vernes' science-fiction classic of 1864 A Journey to the Centre of the Earth encounter raging waters, erupting volcanoes and giant dinosaurs as they travel deep underground. In truth, even if someone managed to cross our planet's molten core, which begins halfway down from the Earth's surface, the centre of the planet is so hot — more than 3000 °C — that our intrepid traveller would burn up before they reached there. Thank goodness that we do not actually need to go to the centre of the Earth to work out where it is or how fast it is moving.
We now know that the centre of the Earth lies about 6370 km from the surface. In fact, because the Earth is shaped like a flat ellipsoid — not a perfect sphere — the radius at the Equator (6378 km) is slightly longer than the radius at the poles (6357 km). However, the Earth is constantly changing shape as tectonic plates move, ice sheets melt and volcanoes erupt. Scientists would therefore like to determine accurate values for the position and velocity of the centre of the Earth, which provide the frame of reference for measuring how fast locations on the Earth's surface are moving.
In the December issue of Physics World, Donald Argus outlines the different approaches scientists are using to locate the centre of the Earth.
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