code: def declination_cooper69(dayofyear):
      
      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
docs: 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