Return the angular separation between two 3 dimensional vectors.

    Parameters
    ----------
    n1, n2: array-like, shape (3,)
        Coordinates of two vectors.
        Their magnitude does not matter.

    Returns
    -------
    d_sigma: float
        Angle between n1 and n2 in radians.

def angular_distance(n1, n2):
    """Return the angular separation between two 3 dimensional vectors.

    Parameters
    ----------
    n1, n2: array-like, shape (3,)
        Coordinates of two vectors.
        Their magnitude does not matter.

    Returns
    -------
    d_sigma: float
        Angle between n1 and n2 in radians.
    """
    return np.arctan2(vector.vector_mag(np.cross(n1, n2)), np.dot(n1, n2))