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 fields, 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.
- It takes 8 minutes for light to travel from the Sun to the Earth.
- Our galaxy, the Milky Way, is 100,000 light years in diameter.
- No material object can travel faster than light. See Theory of relativity.