def declination_cooper69(dayofyear):
day_angle = _calculate_simple_day_angle(dayofyear)<EOL>dec = np.deg2rad(<NUM_LIT> * np.sin(day_angle + (<NUM_LIT> * np.pi / <NUM_LIT>) * <NUM_LIT>))<EOL>return dec<EOL>
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