Backwards Mars

StarDate: December 27, 2009

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Mars is marching backwards right now. It's moving westward against the background of stars, not eastward as it usually does. That's not because the planet has reversed course, though. Instead, it's because Earth is about to pass Mars by, which changes our viewing angle.

Mars rises in mid-evening, and looks like a brilliant orange star. The true star Regulus follows it into the sky about an hour later.

Regulus is the leading light of Leo, the lion. Mars is near the constellation's western edge. In just a couple of weeks, though, it'll move out of Leo and into Cancer, the next constellation over.

All of this is because Earth is moving closer to Mars in our smaller, faster orbit around the Sun. Late next month, in fact, we'll overtake our neighbor world and pass right on by.

Because of the relative motions of the two planets, our viewing angle is constantly changing. As we close in on Mars, it appears to stand still for awhile, then reverse its normal easterly course through the stars.

The effect is like passing another car on the highway. When you get close enough, it appears to move backwards against the background of distant trees, buildings, or mountains. As you pull ahead, though, the other car resumes its normal motion against that same background.

Mars will resume its normal motion against the background of stars in March -- and march back into Leo in May.

We'll have more about Mars tomorrow.

Script by Damond Benningfield, Copyright 2009

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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