Kepler's Laws

StarDate: February 19, 2009

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Two planets bookend the sky around 8 o'clock the next few evenings. Venus, the "evening star," is in the west, while fainter Saturn is in the east.

Until a few centuries ago, most people thought that these and the other planets -- plus the Sun and Moon -- circled around Earth. But a book published 400 years ago demonstrated otherwise. It not only showed that Earth and the planets orbit the Sun, but it explained important details about their orbits.

The book was called New Astronomy. It was written by Johannes Kepler.

Kepler had spent years poring over the observations of Tycho Brahe, who'd carefully plotted the positions of the planets. Kepler used these observations to mathematically model the motions of the planets. From these equations, he devised laws of planetary motion.

The first says that the orbits of the planets are ellipses -- like circles that have been flattened. The Sun is offset from the center of the ellipse, so the distance between the Sun and a planet is constantly changing.

Kepler's second law says that a planet's orbital speed constantly changes, too. The planet moves fastest when it's closest to the Sun, and slowest when it's farthest from the Sun.

Astronomers still use these laws today. In fact, they'll use them to calculate the orbits of planets discovered in other star systems by a spacecraft that's scheduled for launch as early as next month -- a craft named Kepler.

Script by Damond Benningfield, Copyright 2008

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