Compute the phase angle of a body viewed from Earth.

    The ``body`` should be an integer or string that can be looked up in
    the given ``ephemeris``, which will also be asked to provide
    positions for the Earth and Sun.  The return value will be an
    :class:`~skyfield.units.Angle` object.

def phase_angle(ephemeris, body, t):
    """Compute the phase angle of a body viewed from Earth.

    The ``body`` should be an integer or string that can be looked up in
    the given ``ephemeris``, which will also be asked to provide
    positions for the Earth and Sun.  The return value will be an
    :class:`~skyfield.units.Angle` object.

    """
    earth = ephemeris['earth']
    sun = ephemeris['sun']
    body = ephemeris[body]
    pe = earth.at(t).observe(body)
    pe.position.au *= -1     # rotate 180 degrees to point back at Earth
    t2 = t.ts.tt_jd(t.tt - pe.light_time)
    ps = body.at(t2).observe(sun)
    return pe.separation_from(ps)