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An astronomical constant is a physical constant used in astronomy. Formal sets of constants, along with recommended values, have been defined by the International Astronomical Union (IAU) several times: in 1964〔Resolution No.4 of the (XIIth General Assembly of the International Astronomical Union ), Hamburg, 1964.〕 and in 1976〔Resolution No. 1 on the recommendations of Commission 4 on ephemerides in the (XVIth General Assembly of the International Astronomical Union ), Grenoble, 1976.〕 (with an update in 1994). In 2009 the IAU adopted a new current set, and recognizing that new observations and techniques continuously provide better values for these constants, they decided 〔 Resolution B2 of the (XXVIIth General Assembly of the International Astronomical Union ), Rio de Janeiro, 2009. 〕 to not fix these values, but have the Working Group on Numerical Standards continuously maintain a set of Current Best Estimates 〔 IAU Division I Working Group on Numerical Standards for Fundamental Astronomy and Astronomical Constants: Current Best Estimates (CBEs) () 〕 . The set of constants is widely reproduced in publications such as the ''Astronomical Almanac'' of the United States Naval Observatory and HM Nautical Almanac Office. Besides the IAU list of units and constants, also the International Earth Rotation and Reference Systems Service defines constants relevant to the orientation and rotation of the Earth, in its technical notes .〔 For complete document see 〕 The IAU system of constants defines a system of astronomical units for length, mass and time (in fact, several such systems), and also includes constants such as the speed of light and the constant of gravitation which allow transformations between astronomical units and SI units. Slightly different values for the constants are obtained depending on the frame of reference used. Values quoted in barycentric dynamical time (TDB) or equivalent time scales such as the ''T''eph of the Jet Propulsion Laboratory ephemerides represent the mean values that would be measured by an observer on the Earth's surface (strictly, on the surface of the geoid) over a long period of time. The IAU also recommends values in SI units, which are the values which would be measured (in proper length and proper time) by an observer at the barycentre of the Solar System: these are obtained by the following transformations:〔 : : : == Astronomical system of units == (詳細はGaussian gravitational constant (''k'') takes the value when the units of measurement are the astronomical units of length, mass and time.〔 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「Astronomical constant」の詳細全文を読む スポンサード リンク
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