section_path: Definitions
section_name: Definitions
paragraph_text: Definitions
  
  As defined by the International Astronomical Union (IAU), the light-year is the product of the Julian year (365.25 days, as opposed to the 365.2425-day Gregorian year or the 365.24219-day Tropical year that both approximate) and the speed of light (299 792 458 m/s). 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  The abbreviation used by the IAU for light-year is "ly", International standards like ISO 80000:2006 (now superseded) have used "l.y." and localized abbreviations are frequent, such as "al" in French, Spanish, and Italian (from année-lumière, año luz and anno luce, respectively), "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 31 556 925.9747 ephemeris seconds and a speed of light of 299 792.5 km/s produced a light-year of 9.460 530 × 10 15 m (rounded to the seven significant 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 9.460 536 207 × 10 15 m found in some modern sources is the product of a mean Gregorian year (365.2425 days or 31 556 952 s) and the defined speed of light (299 792 458 m/s). Another value, 9.460 528 405 × 10 15 m, 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: