Light year

A light year (or light-year, or lightyear), abbreviated ly, is the distance light travels in one year: about 9.461 × 10¹⁵ metres (9.461 petametres), or about 5.879 × 10¹² (nearly six trillion) miles. More specifically, a light year is defined as the distance that a photon would travel, in free space and infinitely distant from any gravitational or magnetic fields, in one Julian year (365.25 days of 86,400 seconds each). Since the speed of light in a vacuum is exactly 299,792,458 m/s (670,616,629 mph) by the definition of metre, one light year is exactly equal to 9,460,730,472,580,800 m (5,878,625,373,184 mi).

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 perpendicular to the line of sight to the observer. 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.

A light year is also equal to about 63,241.077 astronomical units (AU). For a list of lengths on the order of one light year, see the article 1 E15 m.

Units related to the light year are the light minute and light second, the distance light travels in a vacuum in one minute and one second, respectively. A light minute is equal to 17,987,547,480 m. Since light travels 299,792,458 m in one second, a light second is 299,792,458 m in length.