linguistic:paraphrasing
Observer comoving with the interferometer

If the same situation is described from the view of an observer co-moving with the interferometer, then the effect of aether wind is similar to the effect experienced by a swimmer, who tries to move with velocity c {\textstyle c} against a river flowing with velocity v {\textstyle v}.

In the longitudinal direction the swimmer first moves upstream, so his velocity is diminished due to the river flow to c − v {\textstyle c-v}. On his way back moving downstream, his velocity is increased to c + v {\textstyle c+v}. This gives the beam travel times T 1 {\textstyle T_{1}} and T 2 {\textstyle T_{2}} as mentioned above.

In the transverse direction, the swimmer has to compensate for the river flow by moving at a certain angle against the flow direction, in order to sustain his exact transverse direction of motion and to reach the other side of the river at the correct location. This diminishes his speed to c 2 − v 2 {\textstyle {\sqrt {c^{2}-v^{2}}}}, and gives the beam travel time T 3 {\textstyle T_{3}} as mentioned above.