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The International Astronomical Union at its XVIth General Assembly in Grenoble in 1976, accepted (Resolution No. 1 〔 〕) a whole new consistent set of astronomical constants 〔IAU(1976) ''ibidem'': Commission 4 (Ephemerides) recommendations 1,2,3,5,6: pp.52..67〕 recommended for reduction of astronomical observations, and for computation of ephemerides. It superseded the IAU's previous recommendations of 1964 (see IAU (1964) System of Astronomical Constants), became in effect in the Astronomical Almanac from 1984 onward, and remained in use until the introduction of the IAU (2009) System of Astronomical Constants. In 1994 〔 〕 the IAU recognized that the parameters became outdated, but retained the 1976 set for sake of continuity, but also recommended to start maintaining a set of "current best estimates" ;〔IAU(1994) ''ibidem'', Resolution No. C 6〕 this "sub group for numerical standards" had published a list, which included new constants (like those for relativistic time scales) 〔 〕 . The system of constants was prepared 〔 〕 by Commission 4 on ephemerides led by P. Kenneth Seidelmann (after whom asteroid 3217 Seidelmann is named). At the time, a new standard epoch (J2000.0) was accepted; followed later 〔 〕 〔 〕 by a new reference system with fundamental catalogue (FK5), and expressions for precession of the equinoxes, and in 1979 by new expressions for the relation between Universal Time and sidereal time 〔IAU(1979) ''ibidem'', recommendation by Commissions 4 (Ephemerides), 8 (Positional Astronomy), 19 (Rotation of the Earth), 31 (Time)〕 〔 〕 〔IAU(1982) ''ibidem'', Resolution No. C 5〕 , and in 1979 and 1980 by a theory of nutation 〔IAU(1979) ''ibidem'', recommendation by Commissions 4 (Ephemerides), 19 (Rotation of the Earth), 31 (Time)〕 〔IAU(1982) ''ibidem'', Resolution No. R 3〕 . There were no reliable rotation elements for most planets,〔〔 but a joint working group on Cartographic Coordinates and Rotational Elements was installed to compile recommended values 〔IAU(1976) ''ibidem'', recommendation by Commissions 4 (Ephemerides) and 16 (Physical Study of Planets and Satellites)〕 〔IAU(1979) ''ibidem'', recommendation by Commissions 4 (Ephemerides) and 16 (Physical Study of Planets and Satellites)〕 . == Units == The IAU(1976) system is based on the astronomical system of units: * The astronomical unit of ''time'' is the day (D) of 86,400 SI seconds, which is close to the mean solar day of civil clock time. * The astronomical unit of ''mass'' is the mass of the Sun (S). * The astronomical unit of ''length'' is known as the astronomical unit (A or au), which in the IAU(1976) system is defined as the length for which the gravitational constant, more specifically the Gaussian gravitational constant k expressed in the astronomical units (''i.e.'' k2 has units A3S−1D−2), takes the value of . This astronomical unit is approximately the mean distance between the Earth and the Sun. The value of k is the angular velocity in radians per day (i.e. the daily mean motion) of an infinitesimally small mass that moves around the Sun in a circular orbit at a distance of 1 AU. 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「IAU (1976) System of Astronomical Constants」の詳細全文を読む スポンサード リンク
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