lightyears

A light-year is a unit of distance used in astronomy to express how far light travels in vacuum in one year. It is not a measure of time. In practical terms, one light-year equals about 9.461 × 10^15 meters, 9.461 × 10^12 kilometers, or 5.879 × 10^12 miles. This value assumes a Julian year of exactly 365.25 days and the constant speed of light in vacuum, about 299,792,458 meters per second.

The light-year provides a convenient scale for describing distances to stars, galaxies, and other objects beyond the Solar System. It is commonly used alongside the parsec; 1 parsec is approximately 3.26156 light-years. Thus, distances expressed in light-years can be converted to parsecs by dividing by 3.26156.

Typical distances in the Milky Way span a few to tens of thousands of light-years, such as Proxima Centauri at about 4.24 ly from Earth. Nearby galaxies are millions of light-years away, with the Andromeda Galaxy around 2.5 million ly, while distant galaxies and the observable universe measure billions of light-years.

Because a light-year reflects the distance light travels in a year, it is a convenient, intuitive unit for astronomical scales. It should be noted that at cosmological distances, discussions often employ additional distance concepts (such as comoving or luminosity distances) and units (megaparsecs), but the light-year remains a standard colloquial and scientific distance unit in astronomy.