PH-112 SPACE, ASTRONOMY and OUR UNIVERSE LABORATORY LAB # 3 Understanding Celestial Motions OVERVIEW If you look at the sky regularly, you will discover that the Sun, Moon, and planets appear to move with respect to the stars. Are the motions we see real or are they due to the motion of the Earth? In this lab, we will examine these motions in some detail and study some evidence that has enabled us to distinguish fact from fiction. Part I: The Diurnal Motion of the Earth Every 24 hours, the entire celestial sphere appears to make one complete rotation. Does this mean that each celestial object is moving as if it were attached to a physical sphere surrounding the Earth, or is this apparent motion due to the Earth’s rotation? Is there any clear evidence of the Earth’s rotation? There certainly is. In the 1800s, the French scientist J.B.L. Foucault suspended a solid metal sphere from a long filament in the Pantheon building in Paris. He pulled the mass to one side and released it very carefully, allowing it to swing in one plane. As the hours passed, he noticed that the plane in which the mass had been swinging changed gradually, returning to its original plane almost 32 hours later. The swinging mass is popularly called the Foucault Pendulum. The pendulum’s plane, Foucault explained, appeared to rotate because it was being observed from a rotating platform. The rotation of the pendulum’s plane is an illusion. Pendulum suspended over North Pole Imagine you are at the center of a rotating platform (at the North Pole of the Earth) illustrated in the diagram at right: Would the plane of a Foucault pendulum appear to rotate clockwise or counterclockwise at the Earth’s North Pole? What would be its direction of rotation at the Earth’s South Pole? Direction of platform rotation
The Correct Answer and Explanation is:
Here are the correct answers to the questions based on the provided text and diagrams, along with a detailed explanation.
Answers:
- At the Earth’s North Pole, the plane of a Foucault pendulum would appear to rotate clockwise.
- At the Earth’s South Pole, its direction of rotation would be counterclockwise.
Explanation
The behavior of a Foucault pendulum is a direct and compelling demonstration of the Earth’s rotation on its axis. The key principle is that once it is set in motion, the heavy pendulum bob will continue to swing back and forth in the same fixed plane relative to the distant stars. This is due to the conservation of momentum. The apparent rotation of the pendulum’s swing plane is an illusion created because the Earth, along with the observer and the building housing the pendulum, is rotating beneath it.
At the North Pole, the concept is most straightforward. If you were to view the Earth from space above the North Pole, you would see it spinning from west to east, which is a counterclockwise direction. As the Earth rotates counterclockwise under the pendulum, the pendulum’s swing plane, which remains fixed in space, appears to an observer on the ground to be rotating in the opposite direction. Therefore, at the North Pole, the pendulum’s plane appears to turn clockwise, completing a full circle in about 24 hours.
Conversely, at the South Pole, the perspective is reversed. Viewing the Earth from a point in space below the South Pole, the same west-to-east rotation appears as a clockwise motion. With the ground rotating clockwise beneath the fixed pendulum, an observer at the South Pole would perceive the plane of swing as rotating in the opposite direction, which is counterclockwise. This opposing behavior in the Northern and Southern Hemispheres is powerful evidence that the Earth is a rotating sphere. The rate of this apparent rotation is maximized at the poles and diminishes to zero at the equator, which is why the original pendulum in Paris, at a mid-latitude, took approximately 32 hours to complete a full rotation rather than 24.
