docs: def duration(self):
          """
          The approximate duration of the transit :math:`T_\mathrm{tot}` from
          Equation (14) in Winn (2010).
  
          """
          self._check_ps()
  
          rstar = self.system.central.radius
          k = self.r/rstar
          dur = self.period / np.pi
          arg = rstar/self.a * np.sqrt((1+k)**2 - self.b**2)
          arg /= np.sin(np.radians(self.incl))
          dur *= np.arcsin(arg)
          if self.e > 0.0:
              dur *= np.sqrt(1 - self.e**2) / (1 + self.e * np.sin(self.omega))
          return dur