I'm working through the derivation found on this site: http://maths.dur.ac.uk/~dma0cvj/mathphys/supplements/supplement5/supplement5.html And though most of it is clear to me, I stumbled upon a little problem. My question is about the section that starts with "This is all according to A. What about B?", it states that x'_r = 0 when the light is received. I'm not sure I understand that. Does this simply say that the observer doesn't move relative to himself? Secondly, could this entire section not just be substituted by an explanation involving time dilation? Or even, isn't it just time dilation? It comes to the same result, doesn't it? Well, even posting this, it's starting to dawn just a little more, but still, if someone could confirm my thoughts on this matter, I'd appreciate to know for sure.