Close Encounters of a Red Kind: Retrograde Motion of Mars
There's something very strange going on in our night skies now, something which seems to defy the very order of all cosmic things. We have talked about the August 2003 perihelion opposition and how Mars will be closer to us than in about 73,000 years, but there is something else strange going on. Mars seems to have shed the very foundation of cosmic clockwork order and reversed its course across the heavens! This strange reversal of direction is called retrograde motion, and though we know what it is now, it has in the past caused some real problems for astronomers.
We see the stars and planets rise in the east, drift across the sky and set in the west. It appears this way because the of the Earth's daily rotation. The actual motion of the planets is from west to east, or it is most of the time. You can easily detect that eastward motion by watching a planet like Mars over a few months, as it moves from one constellation to the constellation east of it (planets appear to move faster than stars.)
All of the outer planets display retrograde motion at one time or another, but Mars has the best show. Mars is closer and so it is easier to detect it's changing motion against the background stars more easily than with a more distant planet.
During June and July Mars was gliding swiftly eastward through the stars of the constellation Aquarius. Towards the end of July, Mars began to slow its movements until on July 29th, Mars showed no eastward motion at all. Then, strange as it may seem Mars began moving westward through Aquarius and it will continue to do so until about September 27th, when Mars will once again slow down, halt and then begin moving eastward.
Now your common senses would tell you that planets just don't decide they want to go round the Sun the other way and you would of course be right. What you're seeing is a sort of celestial illusion of sorts. What's really happening is that Earth is catching up to Mars (as it must for an opposition) and then passing the red planet. An analogy to understand this is when you are driving down the freeway and pass a slower car. From the faster car's perspective the other car appears to be moving the opposite direction, but of course that's not what really happens.
Earth is moving faster than Mars and so we overtake Mars and pass it. Mars appears to be moving the opposite direction, a motion called retrograde motion. When Mars displays retrograde motion, it appears to etch out a looped path. The loop is because Earth and Mars' orbits are not in exactly the same plane.
This retrograde loop of Mars is a terrific display of celestial geometry. The loops vary from opposition to opposition. This year, the smallest of the decade, is a tight five degree wide oval. In 2005, the retrograde motion produces a strange zigzag loop. The 2010 retrograde loop is about twice the size of this one in 2003. The pattern of the loop changes depending on where Mars happens to be in its' orbit when Earth passes it.
If you have a tough time understanding all this about planetary motions, don't feel bad! It drove the early astronomers nuts trying to understand the motions of the heavens. They didn't understand the planets were moving around the Sun, and they concocted all sorts of elaborate models to try and explain this backwards motion! It was only after Copernicus, Kepler, Newton, Gallileo, and others discovered the various laws relating to motion and gravity that the mystery was solved. We understand the motions of the planets now, and so we can just sit back and watch Mars do this celestial loop-the-loop!
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Kathy Miles, Author, and Chuck Peters, Systems Administrator
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