def _update_sky_delta(skyline, skyval_brightness):
    """
    Updates `skyuser_delta` of each member of the
    input skyline. It *does not* modify image data or headers.

    .. note::
        This function does not update members of mosaiced skylines.

    Parameters
    ----------
    skyline: `SkyLine` object
        The `Skyline` of the image to update.

    skyval_brightness: float
        Value of sky (in units of brightness) to be added to
        `skyuser_delta` of each member of the input skyline.

    """
    if skyline.is_mf_mosaic or len(skyline.members) < 1:
        return
    for m in skyline.members:
        skyuser_delta = m.brightness2data(skyval_brightness)
        m.update_skydelta(skyuser_delta)
    return
