The distance light travels in one year, used to measure astronomical distances.A light-year is the distance that light travels in one year in a vacuum, approximately 9.46 trillion kilometers (5.88 trillion miles). It is a unit of distance, not time, despite the word 'year' in its name. Astronomers use it to express the vast separations between stars and galaxies, where kilometers or even astronomical units become unwieldy.
The term 'light-year' was first used in 1838 by the German astronomer Friedrich Bessel, who measured the distance to the star 61 Cygni and expressed it in light-years.
The light-year is defined by multiplying the speed of light (299,792,458 meters per second) by the number of seconds in a Julian year (31,557,600 seconds). Because light has a finite speed, looking at an object one light-year away means seeing it as it was one year ago. This makes the light-year both a distance measure and a look-back time.
Light-years are the standard unit for distances to stars and galaxies in popular science and astronomy textbooks, and they underpin the cosmic distance ladder used to map the universe.
The nearest star to the Sun, Proxima Centauri, is about 4.24 light-years away — so the light you see from it tonight left that star over four years ago.
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