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A light-year (abbreviation: ly), sometimes written light year,〔ISO 80000-3, Quantities and Units - Space and Time〕 is a unit of length used informally to express astronomical distances. It is approximately 9 trillion kilometres (or about 6 trillion miles).〔One trillion here is taken to be 1012 (one million million).〕 As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year. Because it includes the word ''year,'' the term ''light-year'' is sometimes misinterpreted as a unit of time. The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The unit usually used in professional astrometry is the parsec (symbol: pc, approximately 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc).〔 ==Definitions== As defined by the IAU, the light-year is the product of the Julian year (365.25 days as opposed to the 365.2425-day Gregorian year) and the speed of light ().〔The speed of light is exactly by definition of the metre.〕 Both these values are included in the IAU (1976) System of Astronomical Constants, used since 1984.〔"(Selected Astronomical Constants )" in ''Astronomical Almanac'', p. 6..〕 From this the following conversions can be derived. The internationally recognized abbreviation for light-year is "l.y."〔ISO 80000-3:2006 Quantities and Units - Space and Time〕〔IEEE/ASTM SI 10-2010, American National Standard for Metric Practice〕). : 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 ephemeris seconds and a speed of light of produced a light-year of (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. Other high-precision values are not derived from a coherent IAU system. A value of found in some modern sources is the product of a mean Gregorian year (365.2425 days or ) and the defined speed of light (). Another value, , is the product of the J1900.0 mean tropical year and the defined speed of light. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Light-year」の詳細全文を読む スポンサード リンク
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