A light year measures approximately 9.5 trillion kilometers, which is the distance that light travels in one year. It is calculated by taking the speed of light, about 300,000 kilometers per second, and multiplying it by the number of seconds in a year, which is 31,557,600.
Using light years is a practical way to measure large distances in astronomy. For example, the nearest star, Proxima Centauri, is 4.2 light years away, and the distance from the Earth to the center of the Milky Way galaxy is 26,000 light years.
Similar units, such as light-minutes and light-seconds, are used for shorter distances. The distance from the Earth to the sun is around eight light-minutes, which means it takes about eight minutes for the light from the sun to reach Earth.