def lens_rotation(alpha0, s0, dalpha, ds, t, tb):
    """Compute the angle alpha and projected separation s for each
    time step due to the lens orbital motion.

    :param alpha0: angle alpha at date tb.
    :param s0: projected separation at date tb.
    :param dalpha: float, angular velocity at date tb
        (radians.year^-1).
    :param ds: change rate of separation (year^-1).
    :param t: list of dates.
    :param tb: time reference for linear development.
    :type alpha0: float
    :type s0: float
    :type dalpha: float
    :type ds: float
    :type t: numpy array
    :type tb: float
    :return: unpacked list of actual alpha and s values at each date.
    :rtype: numpy array, numpy array
    """
    Cte_yr_d = 365.25  # Julian year in days
    alpha = alpha0 - (t - tb) * dalpha / Cte_yr_d
    s = s0 + (t-tb) * ds / Cte_yr_d
    return alpha, s