''Ether drag” was among the suggestions made to explain the null result of the Michelson-Morley experiment (see the More section). The phenomenon of stellar aberration refutes this proposal. Suppose Earth moves relative to the ether at velocity v and a light beam (e.g., from a star) approaches Earth at an angle u with respect to v. 

(a) Show that the angle of approach in Earth’s reference frame θ' is given by

(b) θ' is the stellar aberration angle. If θ = 90°, by how much does θ' differ from 90°?

tan 0' = sin 0 cos 0 + v/c