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Mars and Regulus

June 6, 2010

The planets of the solar system whirl around the Sun at thousands of miles an hour. Yet they're so far away that their motion is tough for the untrained eye to follow -- even for our closest planetary neighbors.

For the next few nights, though, that won't be the case for Mars. The planet is scooting past a stellar signpost, making its motion across the sky easy to track.

The signpost is Regulus, the brightest star of Leo, the lion. It's well up in the western sky as darkness falls. Tonight, bright orange Mars is just a little to the upper right of Regulus. Tomorrow night, it'll stand directly above the star. And by Tuesday night, it'll be to the upper left of Regulus.

Mars takes almost two years to orbit the Sun. For most of that time, the planet appears to move eastward against the background of stars, as it's doing now.

But every couple of years, Earth catches and passes Mars in our smaller, faster orbit around the Sun. When that happens, the planet moves backwards against the starry background for a while.

Mars doesn't actually reverse course, it's just a matter of perspective. It's like passing a car on the highway. For a while, the other car appears to move backward against the background of trees and buildings. When you pull far enough ahead of it, though, the other car appears to resume its normal forward motion.

So follow Mars as it tracks past Regulus the next few nights. More about Mars and Regulus tomorrow.

Script by Damond Benningfield, Copyright 2010

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