Definition of Light-Year

A light year, abbreviated ly, is the distance light travels in one year: roughly 9.46 × 10¹² kilometres (9.46 petametres, or about 5.88 × 10¹² miles). More specifically, a light year is defined as 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 seconds each).

The light year is often used to measure distances to stars: A light year is not a unit of time. In astronomy, the preferred unit of measurement for such distances is the parsec which is defined as the distance at which an object will generate one arcsecond of parallax when the observing object moved one astronomical unit. This is equal to approximately 3.26 light years. The parsec is preferred because it can be more easily derived from, and inter-compared with, observational data. However, outside scientific circles, the term light year is more widely used by the general public.