The planet Mars marches past the heart of the lion the next few mornings. They’re high in the east at first light. Mars looks like a fairly bright orange star. The true star Regulus is quite close to its lower right tomorrow, but Mars will move down past the bright star over the next few days.
It’s this motion against the background of “fixed” stars that inspired long-ago skywatchers to call Mars and its kin “planets” — from a Greek word that means “wanderers.”
Mars and the other planets are much closer than the stars, and they move around the Sun, just as Earth does. Most of the time, we see that motion as a steady eastward trek across the sky.
But the situation is complicated by the fact that Earth moves around the Sun, too. The combined motions of Earth and the other worlds create a changing perspective — like one car overtaking another on the highway. So each of the planets periodically appears to stop for a while, then reverse direction. This “retrograde” motion continues for a few weeks or months before the planet once again stops, then resumes its normal eastward trek.
These loops on the sky gave ancient skywatchers fits. They thought that all the objects in the sky circled around Earth, so they just couldn’t figure out why any object would change direction. They didn’t figure it out until they deduced the true configuration of the cosmos: Earth and the “wanderers” all orbit the Sun, with the true stars far beyond them.
Script by Damond Benningfield, Copyright 2013
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