A first stage of the determination. We have a body of length L = AB, which moves along the x-axis with velocity v, say that coming to us. A -------- B v <--- | | h - vertical distance | ./ - a light converges to us with the speed eq. c | / O ---------> x At some time t = 0, we see the point A at the x = 0; which means, of course, that the point A is actually in a different place, because the time of flight of light is: dt = h/c, that is, the body moved to the x = -vdt - it's obvious? But we are interested how we see at time t = 0 the whole body, which is where we see the other end - the point B? x_B (t = 0) = ?