Vendored from https://github.com/cosinekitty/astronomy

- Files: `source/c/astronomy.h`, `source/c/astronomy.c`
- Pinned commit: `865d3da7d8112bbc7911238052c6af4aaf877181` (master, 2025-01-27)
- License: MIT (see `LICENSE` in this directory)
- Unmodified from upstream.
