Earth and the Moon

Orbit and Rotation

How did scientists prove that Earth rotates?

James Bradley (1693–1762), England’s Astronomer Royal from 1742 to 1762, first provided evidence of Earth’s orbit and rotation. When Bradley tried to measure the parallax of stars—the observed angular motion of a star due to Earth’s motion around the Sun—he noticed that all of the stars in the night sky shifted their positions by exactly the same amount throughout the year, and in the same direction that Earth moved. In 1728, it became clear to Bradley that the apparent movement of the stars he observed was caused by to Earth’s forward motion toward the starlight as it came toward Earth. This effect, called the aberration of starlight, is similar to the sensation that makes it seem like raindrops are falling slightly toward an observer as he or she walks through a rainstorm, causing one to tilt an umbrella forward. It showed clearly that Earth was moving, suggesting strongly that Earth was rotating, too.

In 1852, the French scientist Jean-Bernard-León Foucault (1819–1868) confirmed Earth’s rotation by hanging a large iron ball on a two-hundred-foot (sixty-meter) wire from the domed ceiling of the Pantheon monument in Paris. A small pointer at the bottom of the ball scratched the ball’s path into a flat layer of sand. Over the course of an entire day, the path of the ball remained constant as it swung under the Pantheon; but the line etched out by the pointer slowly and continually shifted to the right. Eventually, the line came full circle, showing a full loop that corresponded with half the length of the day. Foucault’s pendulum was a simple, Earth-bound way of proving that Earth’s rotation is real, and not an optical illusion caused by the Sun and stars revolving around it.

This famous photo of Earth was taken by astronauts aboard Apollo 17. (NASA)


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