docs: def load_skyfield_data():
      """Load data files used in Skyfield. This will download files from the
      internet if they haven't been downloaded before.
  
      Skyfield downloads files to the current directory by default, which is not
      ideal. Here we abuse astropy and use its cache directory to cache the data
      files per-user. If we start downloading files in other places in pwkit we
      should maybe make this system more generic. And the dep on astropy is not
      at all necessary.
  
      Skyfield will print out a progress bar as it downloads things.
  
      Returns ``(planets, ts)``, the standard Skyfield ephemeris and timescale
      data files.
  
      """
      import os.path
      from astropy.config import paths
      from skyfield.api import Loader
  
      cache_dir = os.path.join(paths.get_cache_dir(), 'pwkit')
      loader = Loader(cache_dir)
      planets = loader('de421.bsp')
      ts = loader.timescale()
      return planets, ts