A relativistic jet coming out of the center of an active galactic nucleus is moving along AB with a velocity v, and is observed from the point O. At time t 1 a light ray leaves the jet from point A and another ray leaves at time t 2 = t 1 + δ δ t from point B. An observer at O receives the rays at time t 1 ′ ′ and t 2 ′ ′ respectively. The angle ϕ ϕ is small enough that the two distances marked D L can be considered equal.