Figure 22: Apparent shape of a Kerr black hole for an observer at radius in the equatorial plane. (For the Schwarzschild analogue, see Figure 14.) The pictures show the celestial sphere of an observer whose 4-velocity is perpendicular to a hypersurface . (If the observer is moving one has to correct for aberration.) The dashed circle is the celestial equator, , and the crossing axes indicate the direction towards the center, . Past-oriented light rays go to the horizon if their initial direction is in the black disk and to infinity otherwise. Thus, the black disk shows the part of the sky that is not illuminated by light sources at a large radius. The boundary of this disk corresponds to light rays that asymptotically approach a spherical light ray in the region of Figure 21. For an observer in the equatorial plane at infinity, the apparent shape of a Kerr black hole was correctly calculated and depicted by Bardeen [16] (cf. [54], p. 358). Earlier work by Godfrey [141] contains a mathematical error.