  In figure 15 the stars and observer are represented by their world  lines, and the past light cone \(K\) issuing from the observation  event \(O\) intersects the world lines of the stars \(\Sigma\) and  \(\Sigma^{*}\) in \(E\) and  \(E^{*}\) respectively. The light rays emitted at  \(E\) and \(E^{*}\), which arrive at the  observation event \(O\), are null geodesics laying on the past  light cone and are respectively denoted by \(\Lambda\) and  \(\Lambda^{*}\). This construction of the numerical  quantity of the angle \(\delta\) observed by \(B\) at \(O\), which is describable in the form of purely arithmetical relations, is  invariant under arbitrary coordinate transformations and constitutes  an objective fact of the   world.[111] 