For the standard data regarding precession and the motion of the earth round the sun see S. Newcomb, "Tables of the Motion of the Earth," vol. vi. (1898), and "Determination of the Pre cessional Constant," vol. viii. (1905) in "Astronomical Papers" of the American Ephemeris. Quoting those relevant to this article, we have Tropical year . . . . . 365.24220 mean solar days Sidereal year . . . . . 365.25636 mean solar days General precession in one tropical year 50.2453" (epoch 1900) The general precession is compounded by taking the luni-solar pre cession, less the regressive motion of the ecliptic itself ( 12.31" per century). The luni-solar precession varies in proportion with the cosine of the obliquity of the ecliptic.
From the length of the tropical year we deduce I mean solar day = 24h. 3m. 56.5554s., sidereal I sidereal day = 23h. 56m. 4.091s. mean solar time.
tion of latitude: S. Newcomb, Roy. Astr. Soc., Monthly Notices, vol. lii. (1892) ; S. S. Hough, Roy. Soc. Phil. Trans., vol. clxxxvii. (1895) ; H. Spencer Jones, General Astronomy (observational results up to 192o) (1922). Secular slowing of the earth's rotation: E. W. Brown, Brit. Assoc. Report (1914) ; J. Larmor, Roy. Astr. Soc., Monthly Notices, vol. lxxv. (1915) ; H. Glauert, ibid.; J. T. Fother ingham, ibid., vol. lxxxi. (1921). Effects of tidal friction: G. I. Taylor, Roy. Soc. Phil. Trans. A. vol. ccxx. (1919) ; H. Jeffreys, ibid. vol. ccxxi. (192o) ; The Earth (5924) ; W. de Sitter, Nature, vol. cxxi. (1928). Anomalies in direct determination of time: R. A. Sampson, Roy. Astr. Soc. Monthly Notices, vol. lxxxii. (1922) ; vol. lxxxv. (1925) ; W. Bowie, U.S. Coast and Geodetic Survey, special publica tions No. 92 (1923). Accepted zone times and date line: World Time Zone Chart (the Admiralty, 1927). (R. A. S.)