PenduluxFoucault Pendulum / Earth’s RotationSticker

Pendulux swings at the North Pole, and everyone insists she is turning. She isn’t.

Once set swinging, a pendulum keeps swinging in the same plane relative to the stars. At the pole, the Earth simply rotates underneath it. The floor, the ice station and the whole planet turn 360° relative to the stars once every sidereal day of 23.93 hours, so the swing plane appears to turn the other way at about 15.04° per hour, clockwise seen from above, the last two lines on this page. She knocks over the ring of pegs one by one to prove it.

Away from the pole it gets messier. Only the part of Earth’s spin around the local vertical turns the floor under her, so the rate is the rotation rate times the sine of the latitude, the first line. At the equator the plane does not turn at all.

Léon Foucault proved it in Paris. After first succeeding in his own cellar, on 3 February 1851 he invited scientists to the meridian room of the Paris Observatory to see the Earth turn. In March 1851 he hung the famous public version from the dome of the Panthéon: a 28 kg bob on a 67 m wire. At Paris’s latitude the plane takes about 31.8 hours, using the sidereal day, to turn full circle, about 11.3° per hour. Foucault reported the result to the Academy of Sciences that year. It showed the Earth’s rotation without anyone looking at the sky.

Real pendulums need a little help, a drive or a ring at the top, to keep swinging cleanly for hours. Pendulux has the pole, the patience and a perfectly clear conscience.

I’m not turning. The planet is.