Michelson and Morley used an interferometer to show that the speed of light is constant, regardless of Earth's motion through any purported luminiferous aether. An analogy can be made with the different times it takes a rowboat to travel two different round-trip paths in a river that flows at a constant velocity (M) downstream. Let one path be for a distance D directly across the river, then back again; and let the other path be the same distance D directly upstream, then back again. Assume that the rowboat travels at a constant speed v (with respect to the water) for both trips. Neglect the time it takes for the rowboat to turn around. Find the ratio of the cross-stream time to the upstream-downstream time, as a function of the given constants.