The height of the orbit is an exact distance, because the speed of the orbit depends on how far it is from the center of the Earth. An orbit is a balance between centripetal force against the Earth's gravity. Objects closer to Earth feel more gravity. This is why objects in low orbit (like the International Space Station orbit the Earth very quickly, about 90 minutes or so for each orbit. Objects farther away take longer for each orbit. The moon for instance, takes about 29 days for each orbit.