Section L specifies the sources for the theories and data used to construct the ephemerides in The Astronomical Almanac, explains the basic concepts required to use the ephemerides, and where appropriate, states the precise meaning of tabulated quantities. Definitions of individual terms appear in the Glossary (Section M). The Explanatory Supplement to the Astronomical Almanac, 1992, published by University Science Books, contains additional information about the theories and data used.

To the greatest extent possible, The Astronomical Almanac is prepared using standard data sources and models recommended by the International Astronomical Union (IAU), including those adopted in resolutions of the 1997, 2000, and 2006 General Assemblies. The 2000 resolutions involving precession, nutation, and Earth rotation were first implemented in the 2006 edition of The Astronomical Almanac. The 2006 resolutions are adopted beginning with the 2009 edition. Detailed information on several of the recent IAU resolutions may be found in USNO Circular 179.