As defined by the IAU, the light-year is the product of the Julian year[note 2] (365.25 days as opposed to the 365.2425-day Gregorian year) and the speed of light (792458 m/s). 299[note 3] Both of these values are included in the IAU (1976) System of Astronomical Constants, used since 1984. From this, the following conversions can be derived. The IAU recognized abbreviation for light-year is ly, although other standards like ISO 80000 use "l.y." and localized symbols are frequent, such as "al" in French (from année-lumière) and Spanish (from año luz), "Lj" in German (from Lichtjahr), etc.
Before 1984, the tropical year (not the Julian year) and a measured (not defined) speed of light were included in the IAU (1964) System of Astronomical Constants, used from 1968 to 1983. The product of Simon Newcomb's J1900.0 mean tropical year of 556925.9747 31ephemeris seconds and a speed of light of 792.5 km/s produced a light-year of 299530×1015 m (rounded to the seven 9.460significant digits in the speed of light) found in several modern sources was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work, which was updated in 2000, including the IAU (1976) value cited above (truncated to 10 significant digits).
Other high-precision values are not derived from a coherent IAU system. A value of 536207×1015 m found in some modern sources 9.460 is the product of a mean Gregorian year (365.2425 days or 556952 s) and the defined speed of light ( 31792458 m/s). Another value, 299528405×1015 m, 9.460 is the product of the J1900.0 mean tropical year and the defined speed of light.
Abbreviations used for light years and multiples of light years are
- "ly" for one light year
- "kly" for a kilolight-year (1,000 light years)
- "Mly" for a megalight-year (1,000,000 light years)
- "Gly" for a gigalight-year (1,000,000,000 light years)