1. The problem statement, all variables and given/known data 2. Relevant equations How do you come up with 2ud/(c^2-u^2) 3. The attempt at a solution Here's my issue with this problem: I would expect that for the light from A1 to reach Earth it would be d/u-c, and the time for the light of A2 to reach earth would be d/u-c + T/2 = d/(u-c) + pir/u Therefore Delta A would be (pi)(radius)/u This isnt the case however, am I missing something? Shouldn't the difference in times be equal to half the period of rotation?