A light-year, alternatively spelled light year (ly or lyr[3]), is a unit of length used to express astronomical distances and is equal to exactly 9460730472580.8 km, which is approximately 5.88 trillion mi.

As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days).

"[7][8] and localized abbreviations are frequent, such as "al" in French, Spanish, and Italian (from année-lumière, año luz and anno luce, respectively), "Lj" in German (from Lichtjahr), etc.

[9] The product of Simon Newcomb's J1900.0 mean tropical year of 31556925.9747 ephemeris seconds and a speed of light of 299792.5 km/s produced a light-year of 9.460530×1015 m (rounded to the seven significant digits in the speed of light) found in several modern sources[10][11][12] was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work,[13] which was updated in 2000, including the IAU (1976) value cited above (truncated to 10 significant digits).

In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be 660000 astronomical units (9.9×1013 km; 6.1×1013 mi).

[23] He recognized that his readers would enjoy the mental picture of the approximate transit time for light, but he refrained from using the light-year as a unit.

He may have resisted expressing distances in light-years because it would reduce the accuracy of his parallax data due to multiplying with the uncertain parameter of the speed of light.

Distances expressed in light-years include those between stars in the same general area, such as those belonging to the same spiral arm or globular cluster.