At the Houston Museum of Natural Science they recently made a disturbing discovery: their Foucault pendulum had stopped swinging for the first time since its installation in the 1970s. (Video, embedded below.)

While some might take this as yet another sign of the end times, here it is simply a sign that the electromagnetic system that kicks the pendulum developed a fault and will need to be fixed.

Their explainer video of this Herzstein Foucault pendulum is also worth watching, as it explains both the underlying physics and this particular pendulum’s construction. Every 48 hours the 81.6 kg heavy pendulum completes a full rotation, like clockwork, with pins along the circumference being tipped over one by one as the pendulum precesses.

Overview of the Foucault pendulum system, with the electromagnets that sustain the movement on top. (Credit: Houston Museum of Natural Science)

Invented by [Léon Foucault] in 1851 as an experimental demonstration of Earth’s rotation using a swinging pendulum, the Foucault pendulum remains a popular physical demonstration in museums and elsewhere. Although the pendulum seems to just follow the same line, it gradually shifts its plane of oscillation, making it seem that it rotates around its attachment point.

The effect differs per region of the Earth globe, making it both a fascinating experiment and a sore point for those who insist that the Earth is a flat, unmoving disc.

Not only does it measure the rotation of the Earth, but also its shape due to how the effect changes depending on the pendulum’s position on the globe, with the north and south poles showing it will precess in exactly 24 hours, while at the equator the pendulum will not precess at all.

We hope that the repair of the mechanism behind the Houston museum’s pendulum progresses smoothly, and would love to see a video of its innards and repair.