@classmethod
def dphi_dt0(cls, theta, phi, phi1, phi2, r1=10, r2=10):
    """Fokkema2012, eq 4.19"""
    return -(cls.dphi_dt1(theta, phi, phi1, phi2, r1, r2) + cls.dphi_dt2(
        theta, phi, phi1, phi2, r1, r2))
