Light-year (ly)

AstronomicalCurrent unit

The distance light travels in one Julian year: exactly 9,460,730,472,580,800 meters (about 9.46 trillion km).

1 light-year = 9.460730473×10¹⁵ meters (approximately).

About the light-year

Despite the name, a light-year measures distance rather than time, and its coinage is a little later than most people assume. Friedrich Bessel measured the distance to the star 61 Cygni in 1838 but did not use the unit himself. The light-year appeared in print in 1851, in a German article by Otto Ule, who explained its odd use of a time word by comparing it to saying that a place is an hour's walk away.

The International Astronomical Union defines it as the distance light travels in a vacuum in one Julian year of 365.25 days, about 9.46 trillion kilometers. NIST lists it as 9.460 73 times 10 to the 15th meters. Light-years dominate popular science, while professional astronomers mostly prefer the parsec, about 3.26 light-years. For scale, Proxima Centauri, the nearest star to the Sun, is about 4.24 light-years away, and the Milky Way is roughly 100,000 light-years across.

Sources: Wikipedia: Light-year; NIST Guide to the SI, Appendix B.8: conversion factors

Light-year conversion chart

1 light-year expressed in every other length unit:

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