Celestial Sky

If you look up at the sky on a clear cloudless night, you appear to see a hemispherical dome above you, punctuated by myriads of stars. In fact, you are realistically picturing the family of light rays that constitute the light cone centred at the event O that is occupied by your eye at the moment that you perceive the celestial scene.

(Roger Penrose, The Road to Reality, 2005)