A light-year or light year (symbol: ly) is a unit of length, equal to just under 10 trillion (i.e. 1013) kilometres. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year.
The light-year is often used to measure distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The preferred unit in astrometry is the parsec, because it can be more easily derived from, and compared with, observational data. The parsec is defined as the distance at which an object will appear to move one arcsecond of parallax when the observer moves one astronomical unit perpendicular to the line of sight to the observer, and is equal to approximately 3.26 light-years.
The related unit of the light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.
A light-year is equal to:
- exactly 9,460,730,472,580.8 km (about 10 Pm)
- about 5,878,630,000,000 miles (about 6 trillion miles)
- about 63,241.1 astronomical units
- about 0.306601 parsecs
The figures above are based on a Julian year (not Gregorian year) of exactly 365.25 days (each of exactly 86,400 SI seconds, totalling 31,557,600 seconds) and a defined speed of light of 299,792,458 m/s, both included in the IAU (1976) System of Astronomical Constants, used since 1984. The DE405 value of the astronomical unit, 149,597,870,691 m, is used for the light-year in astronomical units and parsecs.