Whether we realize it or not, we judge the motion of an airplane in the sky by its relation to common things on the ground, such as trees, telephone poles and houses.
That’s the only way motion can be detected: in relation to something else. There’s no such thing as absolute motion; it’s all relative to something else. So the faster the plane appears to be passing the trees and houses, the faster we judge the plane to be moving.
But when you yourself are moving in relation to the trees and houses, you upset this simple association because the trees and houses appear to be moving also. As you walk forward, they appear to be moving backward, don’t they? Of course, you know that they’re not really moving backward because your daddy told you so when you were two years old.
So as you walk forward (which, I trust, is your customary direction of locomotion), but in the opposite direction from the airplane’s, the trees and houses also appear to be moving backward with respect to your direction; that is, they appear to move in the same direction as the plane.
It appears, then, that the airplane and the houses are moving together; the plane doesn’t seem to be overtaking them. And any airplane that can’t even pass a house would seem to be one very slow airplane.
Want to do the passengers a favor and get them to their destination sooner? Just walk in the same direction as the plane. As the trees and houses “move backward” it’ll look as if the plane is passing them even faster.
