P. Kenneth Seidelmann is a research professor of astronomy at the University of Virginia and is a former Director of Astrometry at the US Naval Observatory. He has led and been a member of a Division, various Commissions, and Working Groups of the International Astronomical Union, has co-authored two other books: Fundamentals of Astrometry (Cambridge, 2004) and Celestial Mechanics and Astrodynamics (2016), and is co-editor of the Explanatory Supplement to the Astronomical Almanac (2012).