Hello PF. A stationary observer generates two pulses of light. The pulses are 2 seconds apart according to his watch. What is the distance between the two pulses? Assume he marks out 10 metres and places himself half way at the 5 metre mark. If the light pulses go past and align with the 10 metre marks, the distance between the light pulses is 10 metres. Being equidistant from the 10 metre marks he records simultaneous measurements. The odds are the light and the 10 metre marks won't al;gn. But the 10 metres can be altered. ( 9,8,11,12 etc. ) until they align with the light pulses. thereby giving the distance between the pulses. With distance and time we have speed. One way speed?