Angular separation (deg) between two sky coordinates.

def angsep2(lon_1, lat_1, lon_2, lat_2):
    """
    Angular separation (deg) between two sky coordinates.
    """
    import healpy

    v10, v11, v12 = healpy.ang2vec(np.radians(90. - lat_1), np.radians(lon_1)).transpose()
    v20, v21, v22 = healpy.ang2vec(np.radians(90. - lat_2), np.radians(lon_2)).transpose()
    val = (v10 * v20) + (v11 * v21) + (v12 * v22)
    val = np.clip(val, -1., 1.)
    return np.degrees(np.arccos(val))