Retrograde Motion Study

If you have trouble understanding retrograde motion, you aren't alone. The planets seem to move forward for a while, and then some may seem to stop, turn around and begin to go backwards. Eventually, they hit the brakes again, and turn around and go forward again. It certainly looks confusing. If you forget for a moment about bodies in space and use examples right here on Earth, the strange motion of the planets will be a little easier to understand. It all depends upon your "point of view."

If you were to ride in an automobile that drove so smoothly that you had no sense of motion, you might experience the sensation more fully, but you can experience enough in any vehicle.

When your car is parked alongside another vehicle at a stop light, you can check this out for yourself. Look at the car next to you. If, as you watch the other car, your car moves forward sooner at the green light than they do, they will seem to move backwards. They didn't put their car in reverse, but from your "point of view," they moved backwards.

Consider the solar system as a very large example of an automobile race track. All of the cars are in their various lanes...some were lucky, and got inside lanes, closer to the sun. They will be able to go around the sun in much less time than those planets that had to take the farthest, outside lanes. You are in the third lane out from the sun. The cars in the first two lanes, a Lincoln Mercury and a Toyota Venus will travel much faster than you will. You will travel a little faster than the Jeep Mars Rover, and those cars in the outside lanes, such as the Pontiac Saturn and the Chevrolet Jupiter will travel much slower than you.

As you drive around the track to complete your first lap, you see the Jeep Mars Rover up ahead. Since it is travelling more slowly...it has farther to go to make a lap...you catch up to it. It doesn't stop in the middle of its lane, but it seems to stop for a moment as you come up directly alongside of it. Both of you pace each other for a bit...if you couldn't feel motion or see the scenery passing by, you'd think you had stopped. Gradually, however, you pull forward. The Jeep Mars Rover appears to go into reverse, when, in fact, it is merely moving at its slower pace. As you round the sun, you look over and see the Jeep Mars Rover across the track, way ahead of you. It seems to be speeding backward as you rush forward toward it. Since your lane is shorter, you catch up to the Jeep Mars Rover from behind. As you pull abreast of its position across the expanse of the track, it seems to stop and go backwards. From the Jeep Mars Rover driver's point of view, you are doing the same thing. You pull forward, stop, and move backwards, before appearing to move forward again.

 

You can demonstrate this with your family or friends. You will need two people for the demonstration. One of them will be the sun; the other will be a planet. You will be another planet from your place as a member of an audience.

Have the "planet" walk in a counterclockwise circle around the "sun." From your point of view, when the "planet" is at the far side of its orbit, it will appear to be stopped for a moment. The "planet" seems to move from left to right in front of the "sun," and from right to left when it goes behind the "sun." But, in fact, it is not stopping and moving to the right or the left, but in a circle. It is merely your "point of view" that the "planet" is moving from right to left or from left to right with pauses at either end as it turns to go the other way.

If you can understand these models, you can understand what you are seeing when a planet enters a retrograde pattern. It all depends on your position--your point of view.

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