A light year is the distance light travels in one year, or more specifically, the distance that a photon would travel, in free space and infinitely far away from any gravitational? or [magnetic field]?s, in the time it takes the Earth to complete on orbit of the Sun. It is approximately equal to 9.46 * 1015 meters ("10 million million kilometers"), because the speed of light in vacuum is 299,792,458 meters per second. |
A light year is the distance light travels in one year, or more specifically, the distance that a photon would travel, in free space and infinitely far away from any gravitational? or [magnetic field]?s, in one Julian year (365.25 days of 86400 SI seconds each). It is approximately equal to 9.46 * 1015 meters ("10 million million kilometers"), because the speed of light in vacuum is 299,792,458 meters per second. |
Note that the speed of light is used to measure large distances, like the distance from the solar system to nearest other star. A light year is not a unit of time. In astronomy, the parsec is nowadays the preferred unit for large distances; one parsec equals 3.26 light-years.
Interesting facts:
see also parsec, astronomical unit