Monday, September 25, 2006

Retrograde Motion

Today I had an email from a reader asking about retrograde motion of planets. The question is probably a homework question, so I didn't answer it outright, but sent some hints. But the question touches on one of the toughest concepts in astronomy -- understanding motion.

Everything in the universe is in constant motion relative to most everything else. But it doesn't feel like we are in motion. When you are in a car, plane or boat, you can "feel" the motion. On a motorcycle, you can feel the wind rushing past you. During an earthquake, you feel the ground moving under your feet.

But when you look up at the night sky, there is no feeling of motion. If you stand still for a long time, you might notice that the stars have moved a bit. If you look night after night, you might notice that, over time, the stars are in a slightly different place every night, and that some of the "stars" (actually planets) move with respect to the other "fixed" stars. But you certainly don't feel like you are spinning about Earth's axis at 700 miles per hour, or orbiting around the sun at 19 miles a second, or whizzing through the galaxy at 140 miles per second. You have to watch for an entire month to see the moon go around the Earth once. The sun takes a year to make a full circuit through the skies. This is hard to visualize!

In retrograde motion, a planet such as Mars stops its normal west to east motion through the skies and starts moving east to west for a few months before resuming its normal motion. Something happened, but what?

Mars didn't change directions in its orbit. The amount of energy that would be required to stop Mars, make it orbit backwards, and then switch directions again would be mind-boggling. It just doesn't happen.

On my move to Austin from Tucson, Arizona, I drove for several hundred miles along railroad tracks. At one point, I was passing a train, and I thought it was moving the opposite direction that I was -- it looked like it! But then I passed the front of the train, and saw it was moving east, like me. I was just moving somewhat faster than the train, and against distant mountains, it looked like the train was moving backwards.

The same thing happens with the retrograde motion of the planets. We all go the same direction around the sun, but the closer a planet is to the sun, the faster it moves. We take one year to orbit the sun, Mars takes almost two, Jupiter takes eleven. So, every couple of years, we catch up to Mars and pass it. And as we pass it, it appears to move "backward" with respect to the background stars. Once we are well past Mars, we get to a vantage point where we can see Mars's "real" motion again. The same thing happens with all planets further away from the sun than the Earth.

Confused? Probably. Just remember, we are moving through space, along with all the other planets; we just don't feel the motion. And that lack of perception of motion makes concepts like this all the harder to understand.

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