The Earth wobbles. A lot. This chapter provides the analytical series (derived from VLBI observations) that allow you to calculate the changes in obliquity and the equation of the equinoxes to microarcsecond precision.
The 1992 edition, prepared by the USNO and HMNAO, is widely regarded as the "classic" text. It explains the fundamental concepts that governed the latter half of the 20th century. For many years, scanned PDF versions of this edition circulated freely on university servers and astronomy forums. the explanatory supplement to the astronomical almanac pdf
First published in 1961 (as a supplement to the American Ephemeris and Nautical Almanac ) and then radically revised in 1992 and again in 2013, The Explanatory Supplement is not a casual read. It is a 700+ page treatise on the fundamental astronomy required to compute the positions, motions, and phenomena of celestial bodies. The Earth wobbles
There are two major editions, and the right one depends on your work: The 1992 edition, prepared by the USNO and
Explanatory Supplement to the Astronomical Almanac - NASA ADS
Need the Explanatory Supplement to the Astronomical Almanac ? Here’s what you should know (PDF edition)
The 2013 edition is a massive overhaul. It moves away from the dynamical definitions of the past and embraces the relativistic framework used in modern astrometry. It covers: