A common analogy for the principle of apparent retrograde motion is that of turning on a circle. If two cars are travelling at the same constant speed from the same point on concentric circles "next to each other" such that one car is on the "outside" and the other on the "inside", then the car on the inside will complete its circle before the car on the outside, because it has a shorter distance to travel. Thus, it will seem to the driver of the car on the inside, that, although the speed of the cars is the same, the car on the outside is slower. The outside car will appear to "fall behind" the inside car. The same concept is the basis of retrograde motion. 