def declination_cooper69(dayofyear):
    """
    Solar declination from Duffie & Beckman [1] and attributed to Cooper (1969)

    .. warning::
        Return units are radians, not degrees.

    Declination can be expressed using either sine or cosine:

    .. math::

       \\delta = 23.45 \\sin \\left( \\frac{2 \\pi}{365} \\left(n_{day} + 284
       \\right) \\right) = -23.45 \\cos \\left( \\frac{2 \\pi}{365}
       \\left(n_{day} + 10 \\right) \\right)

    Parameters
    ----------
    dayofyear : numeric

    Returns
    -------
    declination (radians) : numeric
        Angular position of the sun at solar noon relative to the plane of the
        equator, approximately between +/-23.45 (degrees).

    References
    ----------
    [1] J. A. Duffie and W. A. Beckman,  "Solar Engineering of Thermal
    Processes, 3rd Edition" pp. 13-14, J. Wiley and Sons, New York (2006)

    [2] J. H. Seinfeld and S. N. Pandis, "Atmospheric Chemistry and Physics"
    p. 129, J. Wiley (1998)

    [3] Daryl R. Myers, "Solar Radiation: Practical Modeling for Renewable
    Energy Applications", p. 4 CRC Press (2013)

    See Also
    --------
    declination_spencer71
    """
    day_angle = _calculate_simple_day_angle(dayofyear)
    dec = np.deg2rad(23.45 * np.sin(day_angle + (2.0 * np.pi / 365.0) * 285.0))
    return dec