The SOFA Collection consists of libraries of routines in ANSI C. There is a
suite of vector/matrix routines and various utilities that underpin the
astronomy algorithms, which include routines for the following:

* Astrometry
* Calendars
* Time Scales
* Ecliptic Coordinates
* Earth Rotation and Sidereal Time
* Ephemerides (medium precision)
* Fundamental Arguments
* Galactic Coordinates
* Geocentric/Geodetic Transformations
* Precession, Nutation and Polar Motion
* Star Catalog Conversion

WWW: https://www.iausofa.org/