The apparent size of distant objects
A formula in terms of the exponential map is derived, which reveals for any observer in an arbitrary pseudo-Riemannian space-time the apparent diameter (i.e., the angular diameter) of any distant object. The formula shows immediately the existence of the effect of apparent distortion of the objects. It is proved that this distortion is absent if and only if the space-time is conformally flat.
scientific article; zbMATH DE number 3994496