'Converts 3D rectangular cartesian coordinates to spherical polar
coordinates.'

@classmethod
def from_cartesian(cls, cart):

    s = np.hypot(cart.x, cart.y)
    lon = np.arctan2(cart.y, cart.x)
    lat = np.arctan2(cart.z, s)
    return cls(lon=lon, lat=lat, copy=False)
