I'm getting quite stuck on this problem here. Galileo said that Xb = Xa - V*Ta. (This follows from dv = dx/t --> Xa - Xb = t*dv --> the above formula) Thus, it is concluded Xa = Xb + V*Ta, but why? In my thought experiment the objects are moving relative to each other, thus if A is moving away from B, then B is moving away from A. (I also read that it is convention for the Doppler-effect that V is + for source and observer approaching each other and - for source and observer receding each other) So should V not be positive or negative for both equations?