If you’re doing serious positional astronomy (ephemerides, reference frames, time scales, nutation, etc.), you’ve probably heard that the is the ultimate reference. It’s the "how and why" behind the numbers in the Astronomical Almanac .
Need the Explanatory Supplement to the Astronomical Almanac ? Here’s what you should know (PDF edition) the explanatory supplement to the astronomical almanac pdf
Hope this helps you spend more time on astronomy and less on searching! the explanatory supplement to the astronomical almanac pdf