def D_to_M(D):
    r"""Mean anomaly from parabolic anomaly.

    Parameters
    ----------
    D : float
        Parabolic anomaly.

    Returns
    -------
    M : float
        Mean anomaly.

    Notes
    -----
    We use this definition:

    .. math::

        M = B + \frac{B^3}{3}

    Notice that M < ν until ν ~ 100 degrees,
    then it reaches π when ν ~ 120 degrees,
    and grows without bounds after that.
    Therefore, it can hardly be called an "anomaly"
    since it is by no means an angle.

    """
    M = D + D**3 / 3
    return M