The Explanatory Supplement to the Astronomical Almanac is a technical reference detailing the algorithms, coordinate systems, and dynamical theories used to generate data in the annual Astronomical Almanac aa.usno.navy.mil/publications/asa. The current third edition (2013) covers modern ICRS standards, while official, full-text access is available through academic publishers like MIT Press. For more information on accessing the publication, visit US Naval Observatory aa.usno.navy.mil/publications/exp_supp. Harvard University
The Explanatory Supplement to the Astronomical Almanac PDF is widely available online, and can be accessed through various sources, including: the explanatory supplement to the astronomical almanac pdf