| Resource | Content | Best for | |----------|---------|----------| | The Astronomical Almanac Online (USNO) | Current data tables, some explanatory notes. | Daily or yearly lookups, not deep theory. | | IAU SOFA (Standards of Fundamental Astronomy) | C++/Fortran code and documentation. | Implementing the algorithms yourself. | | Naval Observatory Vector Astrometry Subroutines (NOVAS) | Code and annotated references. | Practical astrometry programming. | | Caltech/JPL Solar System Dynamics | Online ephemerides and descriptions. | Understanding DE ephemeris format. | | Urban & Seidelmann (2012) – partial preview on Google Books | Tables of contents, some chapter previews. | Deciding if you need the full book. |
You can download the Explanatory Supplement to the Astronomical Almanac in PDF format from: | Resource | Content | Best for |
: In-depth explanations of celestial and terrestrial reference systems. | Implementing the algorithms yourself
: You can borrow or stream digital scans of the 1992 edition (and others) for free. This is one of the most reliable sources for full-text access of older versions. USNO Publications Page U.S. Naval Observatory (USNO) | | Caltech/JPL Solar System Dynamics | Online