The ExplanatorySupplement to the Astronomical Almanac offers explanatory material,supplemental information, and detailed descriptions of the computational modelsand algorithms used to produce The Astronomical Almanac, which isan annual publication prepared jointly by the US Naval Observatory and HerMajesty's Nautical Almanac Office in the UK. Like The AstronomicalAlmanac, The Explanatory Supplement provides detailed coverage of modernpositional astronomy. Chapters are devoted to the celestial and terrestrialreference frames, orbital ephemerides, precession, nutation, Earth rotation, andcoordinate transformations. These topics have undergone substantial revisionssince the last edition was published in 1992. Astronomical positions areintertwined with timescales and relativity in The Astronomical Almanac,so related chapters are provided in The Explanatory Supplement. TheAstronomical Almanac also includes information on lunar and solar eclipses,physical ephemerides of solar system bodies, and calendars, so TheExplanatory Supplement expounds upon each of these topics as well.The book is written at a technical, but non-expert level. As such, it provides animportant reference for a full range of users including astronomers, engineers,navigators, surveyors, space scientists, and educators.