Light-year

A light-year is a unit of distance, not time, used to express astronomical distances. It is defined by the International Astronomical Union (IAU) as the distance light travels in a vacuum over one Julian year of 365.25 days. This equals exactly 9.46 trillion kilometres, or about 5.88 trillion miles. The unit is common in non-specialist contexts and popular science, while professional astronomy typically uses the parsec, which is roughly 3.26 light-years.

The IAU’s definition relies on the Julian year and a defined speed of light, both part of the 1976 System of Astronomical Constants. Before 1984, the tropical year and a measured speed of light were used, producing slightly different values. The abbreviation “ly” is standard, though “l.y.” and localized forms like “al” (French, Spanish, Italian) or “Lj” (German) appear. Multiples include the kilolight-year (kly), megalight-year (Mly), and gigalight-year (Gly).

The unit first appeared in 1851 in a German popular astronomy article by Otto Ule, who compared its odd name to a “walking hour.” It emerged after Friedrich Bessel’s 1838 measurement of the distance to the star 61 Cygni, which he noted light traverses in 10.3 years, though he avoided using the light-year as a unit due to the then-uncertain speed of light. Eddington later called it inconvenient. Distances expressed in light-years span from stars within a spiral arm to galaxies thousands to hundreds of thousands of light-years across, and intergalactic distances reach billions of light-years. Related units include the light-second, light-minute, light-hour, and light-day, with the light-month sometimes used for approximate measures.