def azimuth(v):
    """Return the azimuth angle in radians.

    ``phi``, ``theta`` is the opposite of ``zenith``, ``azimuth``.

    This is the 'normal' azimuth definition -- beware of how you
    define your coordinates. KM3NeT defines azimuth
    differently than e.g. SLALIB, astropy, the AAS.org
    """
    v = np.atleast_2d(v)
    azi = phi(v) - np.pi
    azi[azi < 0] += 2 * np.pi
    if len(azi) == 1:
        return azi[0]
    return azi