The Explanatory Supplement To The Astronomical Almanac Pdf ((link)) Page

The 2013 edition is critical because it fully incorporates the IAU 2006/2009 resolutions. These resolutions fundamentally changed how we define "north," "precession," and "time."

If the Astronomical Almanac tells you that the Sun’s right ascension is 18h 45m 12.34s on a specific date, the Explanatory Supplement tells you: the explanatory supplement to the astronomical almanac pdf

There are two major editions, and the right one depends on your work: The 2013 edition is critical because it fully

However, this quest is fraught with challenges regarding currency and copyright. for modern precision work

While historically significant, the 1992 edition relies on older reference systems. It discusses concepts that have since been redefined. For historians of science or those reducing historical data, the 1992 PDF is an essential artifact. However, for modern precision work, it is technically obsolete.