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The light-year, also lightyear or light year, is a unit of distance equal to approximately 9.461 × 1015 meters (1 ly ≈ 9.461 × 1015 m), the base unit of length in the International System of Units (SI).
One light-year (ly) is also equal to about 9.461 × 1012 kilometers (km) (an SI unit of length), or 5.879 × 1012 miles (mi) or 31.04 × 1015 feet (ft), which are US customary / Imperial units of length.
The International Astronomical Union (IAU) defines the light-year as the distance that light travels in free space (or vacuum) in one Julian year. In astronomy, the Julian year is defined as having exactly 365.25 days of 86,400 SI seconds each.
The light-year is used in astronomy. Common astronomical measurement units include the parsec (1 ly = 0.306601 pc), the astronomical unit (1 ly = 63.2411 × 103 AU) and the light-second (1 ly = 31,557,600 light-seconds).
Also known as: lightyear or light year