Figure 4

Figure 4: Past-oriented lightlike geodesic $λ$ from an observation event $pO$ to an emission event $pS$ . $γO$ is the worldline of the observer, $γS$ is the worldline of the light source. $UO$ is the 4-velocity of the observer at $p O$ and $U S$ is the 4-velocity of the light source at $p S$ .