Thinkers from Ptolemy to your moderately curious 21st century grade-schooler have wondered the same thing: If the Earth is spinning, how come I can't feel it?

	John Beale, Post-Gazette The Foucault pendulum in the Children's Museum, North Side. The refurbished pendulum is back at its original site in the part of the museum that was once the Buhl Planetarium. Click photo for larger image.

Plenty of people have tackled the question, but it was a 19th-century French toymaker-turned-physicist who found a way to make the truth of that rotation visually graspable.

In 1851 Jean Bernard Leon Foucault replicated on a grand scale in the Paris Pantheon an experiment he'd first done in his basement: He hung an iron ball about 1 foot in diameter from a wire more than 200 feet long inside the dome of the Pantheon. Just under it he built a ring circled with a ridge of sand. A pin attached to the ball would scrape sand away each time the ball swept by. The ball was drawn to the side and held in place by a cord until it was absolutely still. The cord was burned to start the pendulum swinging in a perfect plane.

Swing after swing, the plane of the pendulum appeared to turn slowly because the floor of the Pantheon was moving under the pendulum.

Foucault, who had started out making mechanical toys and then studied medicine, became well-known for the pendulum. Though many scientists had long since explained that the rate of the Earth's rotation is so slow that one would not expect to feel it, it was still intuitively difficult to grasp without such a concrete model.

Many science centers and planetariums had a Foucault pendulum installed.

When officials of the Buhl Foundation decided in 1935 to knock down the former Allegheny City Hall to build a state-of-the-art planetarium on the North Side, they wanted one.

So when the Buhl Planetarium and Institute of Popular Science was dedicated in 1939, it had a 150-pound bob fixed to a 35-foot wire swinging over an elegant pit of florentine marble and brass. A circle of 108 pins lined the pit beneath the swinging pendulum.

From October 1939 through February 1994 (when the Buhl building was closed) children leaned over the rail of the Foucault watching its bob move back and forth over the marble-floored pit, knocking over a pin every 20 minutes or so.

In the mid-1990s, the Foucault pendulum was placed on public display in a new exhibit in the Carnegie Science Center: the original pendulum pit remained at Buhl Planetarium.

In October 2002, the Foucault pendulum was returned to the Great Hall of Buhl Planetarium. It has now been reconditioned and reinstalled in its original location.

Each day the pendulum is set swinging directly on the north-south line using the bronze compass points set into the floor of the pit. Though the pendulum swing appears to rotate about 10 degrees an hour throughout the day, the pendulum does not twist. It "remembers" the direction in which it was started and swings in the same plane. The Earth, however, turning counterclockwise from west to east, moves the building beneath the pendulum.

The small silver pegs circling the pendulum pit are set up to 2.5 degrees apart, and as they cross the path of the pendulum are knocked over three times an hour.

At 10:30 a.m. Wednesday 100 fourth- and fifth-graders from Pittsburgh city schools will visit the Children's Museum of Pittsburgh for a performance by actor and storyteller Tim Hartman to celebrate the reinstallation of the Foucault pendulum.