How did scientists determine that the Earth bulges at the equator?

Scientists once believed that the Earth is a perfect sphere. Proof of the bulge at the equator was the result of a scientific expedition sponsored by the French Academy of Science in 1672. Jean Richer, a French astronomer, went to French Guiana to observe the planet Mars. During his astronomical studies of the stars, Richer noticed that his pendulum clock lost about 2.5 minutes per day. Pendulum clocks depend on gravity, and they are accurate unless the force of gravity varies. A pendulum is a mass suspended on a long wire. The wire hangs from a point so that the mass can swing freely in a circular arc. Gravity pulls the pendulum toward its rest position until gravity pulls it back toward its rest position. Thus, the pendulum swings back and forth in an arc. The time required for the complete swing of the pendulum is called its period. The period of the pendulum depends upon the length of the pendulum wire and the gravitational attraction of the Earth.

Jean Richer knew that the length of the pendulum had not changed. Nor had the clock been damaged in shipment. Therefore, the difference in period must have been related to a change in the gravitational attraction of the Earth. Richer concluded that the force of gravity was weaker in French Guiana, near the equator, than in Paris, nearly 5,900 kilometers north of the equator.

Later Isaac Newton suggested a reason for the weaker gravitational attraction at the equator. He said that the Earth bulges at the equator. Newton reasoned that near the equator the rotational speed of 1,600 kilometer per hour particularly overcomes the pull of gravity at the center of the Earth. This force pulling matter away from the center of the Earth is called centrifugal force. Consequently, matter tends to bulge outward around the equator.

In Newton’s law of gravitation, gravitational attraction between two objects becomes less as the distance between the two objects becomes greater. This is another reason that the gravitational attraction for bodies on the Earth’s surface is least at the equator and greatest at the poles.

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