Earth’s Rotation Day

What the day is

Earth's Rotation Day is observed on January 8. On that date in 1851, Léon Foucault first ran his pendulum experiment in the cellar of his house in Paris, before the public demonstrations later that year that made him famous.

Origin and background

That everything turns was accepted science long before 1851, but it was an inference drawn from astronomy rather than anything a person could go and look at. What Foucault produced was a demonstration you could stand in a room and watch.

The principle is simple. A pendulum, once set swinging, keeps swinging in the same plane, because nothing is pushing it sideways. Fix one to a long wire so it swings freely in any direction, and the plane it swings in stays put relative to the distant stars while the building, the ground and the observer rotate underneath it. Over hours, the line it traces across the floor visibly turns.

The elegant detail is that the rate depends on where you are. At a pole the floor completes a full turn beneath the pendulum in a day. At the equator the plane does not appear to rotate at all. Everywhere in between it goes round at a rate set by the latitude, so the pendulum also tells you where on Earth you are standing.

The rotation is not quite constant, either. Tidal interaction with the Moon is very gradually slowing the planet, lengthening the day by a couple of milliseconds a century while the Moon drifts a few centimeters further away each year. Timekeepers have inserted leap seconds to keep clocks aligned with the actual sky. Recently the Earth has run slightly fast, raising the prospect of removing a second rather than adding one.

How to take part

• Visit a Foucault pendulum. Many science museums keep one running.
• Watch it long enough to see the plane move, which takes patience and is the point.
• Work out the rotation rate for your own latitude.
• Look up why leap seconds exist and why timekeepers want to stop using them.
• Watch a sunrise and notice you are the thing that is moving.