code: def eccentricity(self, **kw):
          r
          ra = self.apocenter(**kw)
          rp = self.pericenter(**kw)
          return (ra - rp) / (ra + rp)
docs: r"""
          Returns the eccentricity computed from the mean apocenter and
          mean pericenter.
  
          .. math::
  
              e = \frac{r_{\rm apo} - r_{\rm per}}{r_{\rm apo} + r_{\rm per}}
  
          Parameters
          ----------
          **kw
              Any keyword arguments passed to ``apocenter()`` and
              ``pericenter()``. For example, ``approximate=True``.
  
          Returns
          -------
          ecc : float
              The orbital eccentricity.