Light Year

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A light-year or light year (symbol: ly) is a unit of length, equal to just under ten trillion kilometres. As defined by the International Astronomical Union (which is the body which has the jurisdictional authority to promulgate the definition), 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. In astronomy, the preferred unit of measurement for such distances is the parsec, which 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. This is equal to approximately 3.26 light-years. The parsec is preferred because it can be more easily derived from, and compared with, observational data. However, outside scientific circles, the term light-year is more widely used.

A light-year is equal to:

  • exactly 9,460,730,472,580.8 km (about 10 Pm)
  • about 5,878,625,373,183.61 international miles
  • about 63,241.0770881 astronomical units
  • about 0.306601393806 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.