def M_to_D(M):
    """Parabolic eccentric anomaly from mean anomaly.

    Parameters
    ----------
    M : ~astropy.units.Quantity
        Mean anomaly.

    Returns
    -------
    D : ~astropy.units.Quantity
        Parabolic eccentric anomaly.

    """
    return (M_to_D_fast(M.to_value(u.rad)) * u.rad).to(M.unit)