Say there are two observer S and S'. Let's assume that their frames coincide at the instant t=0 and that observer S' is moving to the right at a speed 'v' with respect to the observer S along the X axis. Two lightning strikes occurs at the points A and B in the S frame at t=0. The corresponding points in the S' frame are A' and B', which coincide with A and B at t=0. My textbook says that the two events are simultaneous for the observer S, but not for the observer S'. This is what I don't get. The events are simultaneous for the observer S because the light pulses from the two events hit the midpoint O at the same time. Now, in determining whether or not the event is simultaneous in the S' frame, shouldn't we analyze the scenario form the S' frame, in which case, the light pulses would arrive simultaneously at O', and the two lightning strikes would be simultaneous in the S' frame as well. Isn't it erroneous to analyze the scenario from the S frame and then claim that the two events are not simultaneous in the S' frame. Shouldn't simultaneity of events in a frame be determined by clocks in that frame. In this case, the events would be simultaneous for both observers.