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Julian year (astronomy)
In astronomy, a Julian year (symbol: a) is a unit of measurement of time defined as exactly 365.25 days of SI seconds each.〔P. Kenneth Seidelmann, ed., (''The explanatory supplement to the Astronomical Almanac ), (Mill Valley, Cal.: University Science Books, 1992), pp. 8, 696, 698-9, 704, 716, 730.〕〔(【引用サイトリンク】 Recommendations Concerning Units ) Reprinted from the "IAU Style Manual" by G.A. Wilkinson, Comm. 5, in IAU Transactions XXB (1987).〕〔Harold Rabinowitz and Suzanne Vogel, (''The manual of scientific style'' ) (Burlington, MA: Academic Press, 2009) 369.〕 The Julian year is the average length of the year in the Julian calendar used in Western societies in previous centuries, and for which the unit is named. Nevertheless, because an astronomical Julian year measures duration rather than designating a date, this Julian year does not correspond to years in the Julian calendar or any other calendar. Nor does it correspond to the many other ways of defining a year.
== Usage ==
The Julian year is not a unit of measurement in the International System of Units (SI), but it is recognized by the International Astronomical Union (IAU) as a non-SI unit for use in astronomy.〔 Before 1984, both the Julian year and the mean tropical year were used by astronomers. In 1898, Simon Newcomb used both in his ''Tables of the Sun'' in the form of the Julian century (36,525 days) and the "solar century" ( days), a rounded form of 100 mean tropical years of each according to Newcomb.〔Simon Newcomb, (''Tables of the four inner planets'' ), vol. 6 of ''Astronomical papers prepared for the use of the American Ephemeris and Nautical Almanac'' (Washington, DC: 1898) 10–11.〕 However, the mean tropical year is not suitable as a unit of measurement because it varies from year to year by a small amount, days according to Newcomb.〔 In contrast, the Julian year is defined in terms of SI units so is as accurate as those units and is constant. It approximates both the sidereal year and the tropical year to about ±0.008 days.
The Julian year is the basis of the definition of the light-year as a unit of measurement of distance.〔
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