I have just started reading Landau and Ligarbagez' fantastic work Classical Theory of Fields. It has got me thinking about some simple scenarios involving moving clocks.... Moving clocks run slower - but which clock is moving? A spaceship passes the earth say at a constant velocity, the time that passes on the spaceship is less according to the people on earth, than the time thats passed on earth. Yet, can the people on the ship not say that according to them, the time that has passed on earth is less than the time on the ship, since in their frame, the ship is standing still and all else is passing it by??