The example our lecturer uses to illustrate the relativity of simultaneity is a light clock, where a photon is bounced between a reflector and a detector. a tick in the clock means the light has made a round trip in the clock. There are two clocks, one is orientated perpendicular to the motion and the other parallel to the motion, both going at the same speed, it is shown that the photons hit the two reflectors not simultaneously, even though they were synchronized when stationary. However they will still measure the ticks at the same pace (so the photons return to the detectors at the same time, despite the time taken to reach the reflector is different), hence not violating the first postulate. But if we define a tick as the time the photon bounced off the reflector, then the two clocks will not measure the same time, even though going at the same speed, so the first postulate is violated?? How do I make sense of this? Help please?