Understanding the Foucault Pendulum

Global View of Precession

The simulation below provides a global view of the precession of the plane of oscillation for a Foucault Pendulum on Earth. The display shows the Earth (with equator shown in red) with a green horizon plane at the selected location. A magenta arrow points toward the North along this disk. A vertical gray disk shows the plane of oscillation for a pendulum initially moving north-south. A black line indicates where this oscillation plane intersects the horizon plane.

Controls let you play/pause the simulation, advance the simulation by one time step, initialize the simulation using current settings, or reset the simulation to the default settings. A drop-down menu lets you select from several locations on Earth while a checkbox can be used to control whether or not the Earth's rotation is shown. Displays show the elapsed time and the precession period, in hours. You can click and drag to rotate the 3D view.

Explore!

Precession at the Poles

• Set the location to the North Pole and DO NOT show Earth's rotation so that the view shown corresponds to what an observer standing on the Earth would see. (Note: the direction of the north arrow is arbitrary at this location since from the North Pole every direction is southward.) Play the simulation. Which way does the plane of oscillation precess, clockwise or counterclockwise? What is the period of the precession? Let the plane of oscillation precess 360 degrees to verify the precession period shown. How does the precession period compare to the length of a SIDEREAL day (23.93 hours)?
• Click the checkbox to show Earth's rotation. This gives the view of an observer floating in space above the North Pole. Play the simulation. Does the plane of oscillation move? What is the Earth doing? How does this view correspond to what you saw when the Earth's rotation was not shown?
• Now switch the location to the South Pole and repeat the steps above. What is the difference in the precession betweeen the North and South Poles? Why is it different?

Precession at Other Locations

• Use the location selector to move to other locations on Earth, excluding the poles and the equator. Watch the simulation with the Earth's rotation not shown, as well as with the rotation shown. Pay attention to both the direction of the precession and the period of the precession. What is the difference in precession between the northern and southern hemispheres? What is the difference in precession between lower latitudes (like Bangkok) and higher latitudes (like Paris)?
• Now set the location to the equator. Watch the simulation with Earth's rotation not shown as well as with the rotation shown. What happens to the precession at the equator?