On several homework questions I have encountered the following type of scenario: Two clocks are perfectly synchronized. One is placed in a spaceship, and the other is left on the ground. The spaceship flies to some distant location at some high speed, and then returns to earth. When the clocks are compared, the spaceship has recorded that less time has passed. Mathematically, this is because the spaceship's clock records the "proper time". Why though, is the spaceship the one with the proper time? Proper time is defined to be the time interval between two events that occur at the same point. Does not the observer on Earth record both events from the same location in his inertial reference frame? The reference frame of the spaceship does not even stay inertial during the turn-around. Thanks all!