A light day (also written light-day) is an astronomical unit of length. It is defined as the distance light travels in an absolute vacuum in one day (of 86,400 seconds) or 25,902,068,371,200 meters (~26 Tm).

Note that this value is exact, since the metre is actually defined in terms of the speed of light. The light day isn't very frequently used at all since there are few astronomical objects or distances of that magnitude; the Oort cloud, for example, is thought to extend between 290 and 580 light-days out from the Sun.

Every known objects in the solar system is less than a light-day distant. A light-day is about 173 astronomical units. Sedna is currently 90 AU from the sun, though it goes out as far as 975 AU. While 'Obi-Wan' (2003 UB313), claimed as the 10th planet, is 97 AU distant and currently the most distant known object in orbit around the sun. The existence of the Oort cloud has been deduced from comets, no object in the Oort cloud has yet been seen.

See also