def declination_spencer71(dayofyear):
day_angle = _calculate_simple_day_angle(dayofyear)<EOL>return (<EOL><NUM_LIT> -<EOL><NUM_LIT> * np.cos(day_angle) + <NUM_LIT> * np.sin(day_angle) -<EOL><NUM_LIT> * np.cos(<NUM_LIT> * day_angle) + <NUM_LIT> * np.sin(<NUM_LIT> * day_angle) -<EOL><NUM_LIT> * np.cos(<NUM_LIT> * day_angle) + <NUM_LIT> * np.sin(<NUM_LIT> * day_angle)<EOL>)<EOL>
Solar declination from Duffie & Beckman [1] and attributed to
Spencer (1971) and Iqbal (1983).

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

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. W. Spencer, "Fourier series representation of the position of the
sun" in Search 2 (5), p. 172 (1971)

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

See Also
--------
declination_cooper69