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Retrograde Motion. What is Retrograde Motion?. If you were to watch Mars in the night sky over a period of many months, you would notice something unusual. At a certain point in time it would appear as if Mars had stopped and was backing up!
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What is Retrograde Motion? • If you were to watch Mars in the night sky over a period of many months, you would notice something unusual. At a certain point in time it would appear as if Mars had stopped and was backing up! • Why might this be? What is happening in the night sky? Scientists call this phenomenon RETROGRADE MOTION
But what is happening? • It's an illusion, caused by the ways that Earth and Mars orbit the sun. • The two planets are like race cars on an oval track. Earth has the inside lane and moves faster than Mars -- so much faster, in fact, that it makes two laps around the course in about as much time as it takes Mars to go around once.
About every 26 months, Earth comes up from behind and overtakes Mars. While we're passing by the red planet this year, it will look to us as though Mars is moving up and down. • Then, as we move farther along our curved orbit and see the planet from a different angle, the illusion will disappear and we will once again see Mars move in a straight line.
Another Example • The same appearance of retrograde motion appears when people pass cars on the road. The faster car passes the slower car, which seems to move backwards in relation to it. • Just as the slower car does not actually move backwards, planets with larger orbits also do not move in a different direction than the rest of the planets.
