I was reading through my textbook and it said that the angle between the axes of two inertial frames, one stationary and one moving at velocity v is supposed to be tan^-1(v/c). I assumed this would be easy to show, but after spending a couple of hours on this probably trivial problem, I can't for the life of me get the right answer. I'm using the transformation: ct' = ctcosh(a) -xsinh(a) x' = -ctsinh(a) + xcosh(a) y' = y z' = z where a is the 'rapidity function', tanh^-1(v/c) (not that I really understand what that is) What I've done is choose a value of ct' and kept x' = 0 i.e. (0, ct'), because I'm supposed to be looking at the axes. I'm supposed to convert this into the other inertial frame, I think, but I'm really confused... Anyone know the answer?