Explanatory supplement to the Astronomical Almanac

original Explanatory to the and the and Nautical Almanac was published in 1961 and revised in 1974. In 1981 the series of publications titled The Ephemeris and the American and Nautical Almanac were continued under the single title The Almanac. Thus, the title of the new edition of the Explanatory Supplement. In the 1970s it was recognized that there was a need to revise the planetary and lunar ephemerides, the FK4 star catalog, the time scales, the System of Constants and the Theory of Nutation. In 1984 all these revisions were introduced using the new standard epoch J2000.0, thus making the original Explanatory Supplement out of date. Hence, the new version was prepared based on the FK5 star catalog, the IAU (1976) System of Constants, the new time scales, including those adopted in 199 1, the IAU (1980) Theory, of Nutation, and the DE200/LE200 ephemerides. Improvements since 1984 have also been incorporated. Current values of astrometric and planetary data are included. Explanatory to the Almanac provides a detailed description of the means of performing practical astronomy or spherical astronomy. It shows how computations for the Astronomical Almanac are performed and the underlying information required by someone using that publication or wanting to perform related computations. It is intended to be a reference volume for anybody working with accurate positional astronomical data and anyone interested in investigations of the solar system.