The center of the earth can be found at the intersection of 20th Street and Benjamin Franklin Parkway in Center City, Philadelphia. The exact center is inside The Franklin Institute, in the stairwell. A long cord hangs from the ceiling above the stairway. A giant metal sphere swings from the bottom of the cord.

There is a name for this contraption: Foucault's Pendulum. Léon Foucault was a French physicist born in 1819. He had a knack for designing experiments. One day he realized that no one had yet come up with a simple demonstration of how the earth rotates on its axis. He hit upon the idea that a large and heavy pendulum could do the trick. When the pendulum is given a push, it swings back and forth on its own plane. Since the earth is spinning on its axis, it will rotate around the plane created by the swinging pendulum. Get a big enough pendulum, thought Foucault, and a human being watching the pendulum swing will actually be able to see the earth rotating beneath. That's why on many Foucault's Pendulums, like the one at the Franklin Institute, a circle of metal pins is placed in the path of the swinging pendulum. If you stand at the stairwell of the Franklin Institute for a few minutes, you can see the pendulum knock over one of those pins and thus be a direct witness to the rotation of the earth.

The first pendulum that Foucault exhibited in Paris was the talk of the town in 1851. Foucault sent out cards to all the scientists and prominent persons of the city. "You are invited," the cards read, "to come to see the Earth turn, tomorrow, from three to five, at Meridian Hall of the Paris Observatory." Everybody wanted to see the earth turn. Of course, everybody on earth is a witness to the earth rotating every day. But it is hard to know that the earth is moving, since you don't normally have the standpoint from which to see it. Foucault's pendulum was a brilliant way to create that standpoint. Anybody can watch the pendulum swing and knock down those pins and then understand the rotation that makes this happen.

Still, it can be an eerie experience to watch the earth turn. It transforms the location where the Pendulum is to be found into the center of the earth. Given that a Foucault's Pendulum can be set up anywhere on this, or any other, rotating planet, the center created by the pendulum is only relative. But the idea of being at the center of motion, an unmoved mover, is amusing. It was amusing enough to Umberto Eco that he made Foucault's Pendulum the fulcrum of his famous novel of the same name. In that delightful book, one of Eco's characters says that Foucault's Pendulum is, "the only stable point in the cosmos." That's not true of course. In reality, every point in the cosmos is just as stable as every other point. But that is not how it feels standing in the stairwell of the Franklin Institute. From that point in the stairwell, you alone are stable and fixed while the world spins crazily on its axis beneath. So, anytime you're feeling a little out of control, just make your way to 20th Street and Benjamin Franklin Parkway. There's a little bit of cosmic stability to be found, hanging from a cord.