ERFA

ERFA, short for Essential Routines for Fundamental Astronomy, is an open-source software library that provides a compact, portable collection of routines for fundamental astronomical calculations. It is derived from the International Astronomical Union's SOFA (Standards of Fundamental Astronomy) library and offers a subset of its functions with a stable API suitable for integration into a wide range of software.

Purpose and scope: ERFA covers time scale conversions (TAI, TT, TCG, TDB, UTC, UT1), Earth orientation parameters and Earth rotation, precession and nutation, aberration, light travel time corrections, and coordinate transformations among common celestial and terrestrial reference frames (ICRS, ICRF, GCRS, ITRS, TEME, etc.). It provides routines to compute astrometric quantities, transform coordinates, and to generate ephemeris-independent results necessary for precise astronomy and spaceflight applications.

Relation to SOFA and compliance: ERFA is a smaller, independently maintained port of the SOFA routines designed for portability and easier licensing. It aims to be compatible with the SOFA API where possible, enabling users to substitute ERFA in places where SOFA would be used, or to use its Python or other language bindings.

Usage and ecosystem: ERFA is widely used in astronomical software and educational projects. It forms the core of Python bindings (pyerfa) and is used by packages such as Astropy for coordinate and time-scale computations, among others. The library is openly available via public repositories and is maintained by a community of contributors aligned with IAU standards.

This provides a concise, neutral overview of ERFA and its role in standard, accurate astronomy calculations.