Page "Luminiferous aether" Paragraph 26

Thus at any point there should be one special coordinate system, " at rest relative to the aether ".

Maxwell noted in the late 1870s that detecting motion relative to this aether should be easy enough — light traveling along with the motion of the Earth would have a different speed than light traveling backward, as they would both be moving against the unmoving aether.

Even if the aether had an overall universal flow, changes in position during the day / night cycle, or over the span of seasons, should allow the drift to be detected.