Since line E ′ -v ′ is a transversal in the same (approximately Euclidean) plane as parallel lines E-v and E ′ -v, it follows that the corresponding angles of intersection of these parallel lines with this transversal are congruent: the angle θ between lines of sight E-v and E ′ -v ′ is equal to the angle θ between E ′ -v and E ′ -v ′, which is the angle θ between observed positions of S in relation to its apparently unmoving stellar surroundings.