Planetary puzzle
Is the Earth a perfect sphere or flattened at each pole like an orange?
This was the question scientists of the early 1800s were trying to answer. French physicist Jean-Baptiste Biot and British scientist Henry Kater journeyed to Unst to test out their theories. Britain and France were working together to try and crack this planetary puzzle.
With simple but precise tools, the two scientists calculated two things:
1) Their position on the surface of the Earth. They worked this out by measuring the angle of the stars from the horizon.
2) Their distance from the centre of the Earth, using the measurement of the local force of gravity. This was worked out by using a pendulum and a clock.
Their conclusion? The gravity in Unst was different to that at the Equator or North Pole. To swing at the same speed, a pendulum swinging here needed to be longer than one nearer the Equator. This told them that the Earth was not a perfect sphere but flattened out at the poles, more like an orange.
The findings were very important as the metre (the basic unit for measuring distance) was, for a long time, based on the size and shape of the Earth.
Modern mission
Scientists in more recent times have made the same calculations using satellites to achieve a far greater degree of accuracy. From 2009-13, a European Space Agency mission set out to measure the Earth’s gravitational field and model the shape of the Earth, or ‘geoid’. Data from the GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) mission has been useful for tracking changes in sea level and other indicators of climate change.