Hi, I think I have a good grasp of the principles underlying the Michelson-Morley experiment, but I am not sure about a detail. In basic textbooks a simplified situation is presented whereby the ether wind is along one of the two arms of the interferometer. Simple classical physics calculations allow to determine the delay between the two light beams. This delay determines the feature of the interference fringes on a screen. The interferometer is then rotated about a vertical axis, and the fringe shift is measured during this rotation. That would be the change in position of one of the interferenge fringes, right? I expect that when the rotation angle is 90° the fringe pattern is the same as at angle 0°. Indeed rotating by 90° simply swaps the role of the two beams. But the relative delay should stay the same. This seems to be in agreement with the original data I have seen here and there (e.g. wikipedia page), in which the reading at 0° and 90° is the same (and at 180° of course). Could you confirm this? I wanted to create a simulation of the experiment, but I found a nice one here. However it seems to me that it is in contradiction with the above discussion. The interference patterns at 0° and 90° are different I have not had the time to check their calculations, but something seems off... I'd say that both waves (the blue and red one) should move when the interferometer is rotated. Can anybody comment on this?