Plate 22 · Space

A Wire That Proves the Earth Turns

In 1851, Léon Foucault hung a 28-kilogram brass-coated lead ball on a 67-metre wire under the dome of the Panthéon in Paris. Visitors watched it swing, and within minutes saw that the swing had begun to turn. It was not the pendulum that turned. It was the Earth under their feet.

Try it

Chapters · 6

01A Very Long Pendulum
02If It Hung at the North Pole
03Slower in Paris
04Pegs Falling One by One
05A Proof Without the Sky
06Your Turn

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01

A Very Long Pendulum

The longer a pendulum, the slower it swings. Foucault’s was 67 metres long and took about 16 seconds to swing there and back. Long and heavy, it could swing for hours — and as long as nothing pushes it, nothing makes it veer off on its own.

Foucault’s pendulum, schematic · swing sped up; the Earth’s turning sped up far more

02

If It Hung at the North Pole

Picture the pendulum hanging at the North Pole. It keeps swinging in the same direction while the Earth turns beneath it, once every 23 hours 56 minutes.

People standing on the floor turn with the Earth and can’t feel it, so what they see is the swing slowly turning clockwise, all the way round in a day.

03

Slower in Paris

The farther from a pole, the more slowly the floor turns beneath the pendulum. In Paris the swing takes about 32 hours to go all the way round; on the equator it doesn’t turn at all; south of the equator it turns the other way, counterclockwise.

How long a full turn takes depends only on your latitude — the Earth’s spin signing its name.

04

Pegs Falling One by One

Many museums today set a ring of little pegs around the pendulum. Each time the swing turns a little further, the bob knocks over the next one. Watch them fall one after another and you are watching the Earth turn.

05

A Proof Without the Sky

Before Foucault, people already believed the Earth turned, but most of the evidence came from the stars and the Sun. A few experimenters dropped balls down towers and mine shafts and found that they landed a hair to the east — but the shift was a few millimetres, and hard to measure. Foucault’s pendulum was the first simple, direct demonstration: indoors, without looking at the sky, you could watch the Earth turn.

06

Your Turn

Move the pendulum to different latitudes and see how fast its swing turns. Switch between “From the floor” and “From the stars” to see who is really turning.

Try thisMove the pendulum to the equator: how long does its swing take to turn full circle?

What the model leaves out

The swinging is sped up several times and the Earth’s turning more than a thousand times, or you would be watching all day. A real pendulum is also slowed by the air and disturbed by how it hangs, so it has to be started carefully and given a small push now and then. The time for a full turn is a sidereal day divided by the sine of the latitude — the answer for an ideal pendulum making small swings.

A common misreading

The pendulum doesn’t change direction by itself. At the North Pole it keeps swinging the same way relative to the stars; it is the Earth that carries the floor and the watchers round beneath it. At other latitudes the geometry is more complicated, but the idea is the same: the Earth turns, not the pendulum. The “From the stars” view is exact only at the poles; elsewhere it shows just the part of the Earth’s spin that turns the floor around its own centre.