Moon and Company
Mars and Saturn are staging a pas de deux early this month, dancing close together for several nights. Look for them low in the west at nightfall. Tonight, they're to the upper left of the crescent Moon, with the star Regulus lining up between the Moon and the two planets.
The combined orbital motions of Earth and the other planets make Mars, Saturn, and their sister worlds appear to loop through the sky. Most of the time they move eastward, but sometimes they stop, reverse direction, then stop again before resuming their easterly trek.
Generations of skywatchers tried to explain these loops, without success. They believed that the Sun and planets all orbited Earth in perfect circles. But in the 16th century, Nicolas Copernicus demonstrated that the Sun is at the center of the solar system, with Earth and the other planets orbiting it. As Earth and the other planets pass by each other, the planets appear to change direction.
Using this layout, Johannes Kepler finally calculated the paths of the planets. Starting with Mars, he found that the paths aren't circles. Instead, they're ellipses -- like circles that've been stretched out. Kepler used his findings to compose three laws of planetary motion -- laws that perfectly describe the "loopy" paths of the planets. And starting next year, astronomers will use the same laws to plot the orbits of planets in other star systems -- using data from an orbiting telescope named for Kepler.
Script by Damond Benningfield, Copyright 2008
For more about the Kepler Mission, visit NASA's website at http://kepler.nasa.gov.
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