The Skyfield library for astronomical computations depends on various data
files, which are automatically downloaded at runtime if not already found in
the Loader's path, or if found but the file has expired (in which case a new
version is fetched).

Skyfield-data provides offline versions of common data files:

- de421.bsp, provided by the Jet Propulsion Laboratory, this has fairly
long validity

- finals2000A.all, provided by the International Earth Rotation and
Reference Systems Service, however be awware that skyfield-data does not
update this frequently and it's often outdated - users are advised to use
that from py3-astropy-iers-data instead
