def propagate_pos(self, phi, theta, parallax, muphistar, mutheta, vrad, t0, t1):
phi1, theta1, parallax1, muphistar1, mutheta1, vrad1 = self.epoch_prop_astrometry(phi, theta, parallax, muphistar, mutheta, vrad, t0, t1)<EOL>return phi1, theta1<EOL>
Propagate the position of a source from the reference epoch t0 to the new epoch t1.

Parameters
----------

phi : float
    Longitude at reference epoch (radians).
theta : float
    Latitude at reference epoch (radians).
parallax : float
    Parallax at the reference epoch (mas).
muphistar : float
    Proper motion in longitude (including cos(latitude) term) at reference epoch (mas/yr).
mutheta : float
    Proper motion in latitude at reference epoch (mas/yr).
vrad : float
    Radial velocity at reference epoch (km/s).
t0 : float
    Reference epoch (Julian years).
t1 : float
    New epoch (Julian years).

Returns
-------

Coordinates phi and theta at new epoch (in radians)