A salesman travelled due west from city A to city B. The distance he travelled was X km. He returned from B to A and found that he had travelled half the distance i.e. 1⁄2X. How can that be?
The frame of reference for measuring the distances is the Solar System. Taking into account rotation of the Earth and its orbit of the Sun, journey A-B started and concluded at fixed points in the Solar System that were twice the straight-line separation of those of journey B-A; because journey A-B took roughly twice the duration.