light-year

Convert between light years and other major units of astronomical distance.

A unit of distance used, especially informally, to express astronomical distances. It is the distance light travels in a vacuum in one year. Since 1983, by definition (see meter) light travels exactly 299,792,458 meters per second in a vacuum. The Julian year of 365.25 days contains 31,557,600 seconds, so a light-year is 9,460,730,472,580,800 meters, or about 9.5 petameters. (Some authorities use the tropical year, instead of the Julian year, for calculating the light-year, which makes it a bit shorter.) Symbol, LY.

The star currently nearest our sun, Proxima Centuri, is about 4.22 light-years away. The most distant galaxy observed (up to 2013) is 13.1 billion light-years away.

Expressing astronomical distances in light-years has the advantage of emphasizing the temporal nature of such measurements. We see Proxima Centuri, not as it is now, but as it was 4.22 years ago, and the most distant object observed so far as it was 13 billion years ago (ignoring relativistic effects, which preclude a universal now). By current thinking, the universe was then only a billion years old.

astronomical photograph

The distant object is the reddish streak indicated by the outline. The cluster of galaxies in the foreground (CL1358+62) acts as a gravitational lens, enlarging but smearing the image of the much more distant galaxy behind them.

Photo credit: Marijn Franx (University of Groningen, The Netherlands), Garth Illingworth (University of California, Santa Cruz) and NASA

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