Foucault's pendulum (1851) |
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The slow swing of a very large (~100 kg) long (~30 m) pendulum slowly changes direction because of the rotation of the Earth. At the poles the Earth rotates under the pendulum. At other latitudes the swing rotates by (360 sinq) degrees per day. No rotation occurs at the equator.  |
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With ring lasers and gyroscopes we can now routinely measure the rotation rate directly with great accuracy but Foucault's effort is remembered as the first direct visual evidence that the Earth is in fact rotating. If you have a place to put it you can buy one. |