The Explanatory Supplement provides a detailed description of how to perform practical astronomy or spherical astronomy, how the computations for the Astronomical Almanac are performed, and the underlying information required by someone using the Almanac or wanting to perform related computations. This completely revised and rewritten edition of the Explanatory Supplement is an authoritative source on astronomical phenomena and calendars and includes details for FK5 J2000.0 reference system and transformations; details on celestial and terrestrial coordinate systems; current values of astrometric quantities; explanation of eclipse of Sun and Moon computations; details of time scales and their relationships; and detailed descriptions of ephemerides and characteristics of the planets.