Compute cartesian CIRS coordinates at a given epoch (x, y, z).

        Calculate coordinates in the Celestial Intermediate Reference System
        (CIRS), a dynamical coordinate system referenced to the Celestial
        Intermediate Origin (CIO). As this is a dynamical system it must be
        calculated at a specific epoch.

def cirs_xyz(self, epoch):
        """Compute cartesian CIRS coordinates at a given epoch (x, y, z).

        Calculate coordinates in the Celestial Intermediate Reference System
        (CIRS), a dynamical coordinate system referenced to the Celestial
        Intermediate Origin (CIO). As this is a dynamical system it must be
        calculated at a specific epoch.
        """
        if isinstance(epoch, Time):
            pass
        elif isinstance(epoch, float):
            epoch = Time(None, tt=epoch)
        elif epoch == 'date':
            epoch = self.t
        else:
            raise ValueError('the epoch= must be a Time object,'
                             ' a floating point Terrestrial Time (TT),'
                             ' or the string "date" for epoch-of-date')

        vector = einsum('ij...,j...->i...', epoch.C, self.position.au)
        return Distance(vector)