"The only stable point in the cosmos" - Foucault and his pendulum

“Idiot. Above her head was the only stable point in the cosmos, the only refuge from the damnation of the panta rei, and she guessed it was the Pendulum's business. A moment later the couple went off -- he, trained on some textbook that had blunted his capacity for wonder, she, inert and insensitive to the thrill of the infinite, both oblivious of the awesomeness of their encounter -- their first and last encounter -- with the One, the Ein-Sof, the Ineffable. How could you fail to kneel down before this altar of certitude?” (Umberto Eco, “Foucault's Pendulum")

3 January 1851, 162 years ago in his cellar, the French physicist Jean Bernard Léon Foucault set a 2 metres long pendulum in motion. Swinging short of the ground, the application seemed to change its direction by and by. Since every other force could be excluded, it was obvious that the floor and not the pendulum had moved – a simple but brilliant prove for Earth’s rotation.

The Pendulum described in Eco’s novel in the Musée des Arts et Métiers in Paris

Foucault repeated the experiment four weeks later with a 12 metre and in March with a 67 metre pendulum swinging from the Panthéon’s dome in Paris in public. A point on the pendulum’s body tracked every in a sandpit on the floor – the corolis effect was plain to see for everyone.

While the effect with a seemingly clockwise turning pendulum works quite obvious in the Northern hemisphere, the poles or the equator yield quite different results – since the pendulum does not rest on a fixed plane aligned to the distant stars, “the only stable point in the cosmos” moves indeed with respect to an inertial frame. Panta rei.

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