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How do we know the Earth spins?

Prior to Copernicus everybody believed that the heavens and the Sun moved around the Earth once every 24 hours. But putting a static Sun at the centre of the Solar System meant that the only explanation for day and night had to be that the Earth spins.

Until 1851, there was no direct evidence to justify the belief that the Earth spins. In that year, Foucault built a 67-metre, massive pendulum.

During the day, the plane in which the pendulum swung, turned - and the only explanation was that the ground beneath the pendulum was turning. This was the first direct evidence that the Earth turns.

Modern evidence comes from taking a long exposure photograph of the stars (see the cover illustration). Point the camera at the Pole Star and leave the shutter open for two hours, and all the stars will appear as blurred semicircles around the Pole Star. Either the stars are turning around the Pole Star or the camera, which is fixed to the Earth, is turning.

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