I am having difficulty understanding the rotation in the michelson-morley experiment. I tried reading from several sources, but I think I am missing something each source regards as obvious. I can understand why the travel times involved in each light path must differ by T = Lv2/c3 (approximately). But I don't understand how rotating the apparatus by 90 degrees would change anything other than which path is labeled what. In particular I don't see how this causes a fringe shift.