The astronomer who first measured a star's distance avoided light-years
In 1838 Friedrich Bessel became the first to measure the distance to another star, and noted that its light takes about 10.3 years to reach us. Yet he never used that as a unit. Light's speed was still uncertain, and multiplying by it would have blurred his precise measurements.
A light-year measures distance, not time: how far light travels in a vacuum during one Julian year of 365.25 days. The International Astronomical Union fixes it at exactly 9,460,730,472,580.8 kilometres, roughly 9.46 trillion. Before 1984, the IAU's constants used the tropical year and a measured, rather than defined, speed of light, which is why slightly different values still circulate in some references.
Bessel's target was 61 Cygni, observed with a 160-millimetre heliometer designed by Joseph von Fraunhofer. The biggest cosmic yardstick at the time was the astronomical unit, the radius of Earth's orbit, about 150 million kilometres. From a parallax of 0.314 arcseconds he worked out a distance of some 660,000 of those units. He clearly enjoyed the image of light's travel time, but kept his figures in units he could trust. Light's speed was revised by Fizeau in 1849 and by Foucault in 1862.
The term itself first appeared in 1851, in a German popular astronomy article by Otto Ule, who explained the odd year ending by comparison with a walking hour, a distance described by the time it takes to cover. An English journal in 1868 still called it a German unit, and Eddington later dismissed it as inconvenient and irrelevant, grumbling that it had slipped from popular writing into technical work.
Professionals mostly prefer the parsec, about 3.26 light-years, but the light-year remains the popular choice for describing space. Stars within the same spiral arm or cluster sit a handful of light-years apart. A galaxy's diameter can range from thousands to hundreds of thousands of light-years, neighbouring galaxies are millions of light-years apart, and quasars and the Sloan Great Wall lie billions of light-years away. The IAU abbreviates it ly; French, Spanish and Italian use al, and German writes Lj.
Source: Light-year
