Solar declination from Duffie & Beckman  and attributed to Spencer (1971) and Iqbal (1983).
Return units are radians, not degrees.
dayofyear (numeric) –
declination (radians) (numeric) – Angular position of the sun at solar noon relative to the plane of the equator, approximately between +/-23.45 (degrees).
 J. A. Duffie and W. A. Beckman, “Solar Engineering of Thermal Processes, 3rd Edition” pp. 13-14, J. Wiley and Sons, New York (2006)
 J. W. Spencer, “Fourier series representation of the position of the sun” in Search 2 (5), p. 172 (1971)
 Daryl R. Myers, “Solar Radiation: Practical Modeling for Renewable Energy Applications”, p. 4 CRC Press (2013)