code: def subpoint(self):
          
          if self.center != 399:  # TODO: should an __init__() check this?
              raise ValueError("you can only ask for the geographic subpoint"
                               " of a position measured from Earth's center")
          t = self.t
          xyz_au = einsum('ij...,j...->i...', t.M, self.position.au)
          lat, lon, elevation_m = reverse_terra(xyz_au, t.gast)
  
          # TODO. Move VectorFunction and Topos into this file, since the
          # three kinds of class work together: Topos is-a VF; VF.at() can
          # return a Geocentric position; and Geocentric.subpoint() should
          # return a Topos. I'm deferring the refactoring for now, to get
          # this new feature to users more quickly.
          from .toposlib import Topos
          return Topos(latitude=Angle(radians=lat),
                       longitude=Angle(radians=lon),
                       elevation_m=elevation_m)
docs: Return the latitude and longitude directly beneath this position.
  
          Returns a :class:`~skyfield.toposlib.Topos` whose ``longitude``
          and ``latitude`` are those of the point on the Earth's surface
          directly beneath this position, and whose ``elevation`` is the
          height of this position above the Earth's surface.