solar declination angle derivations

1. solar declination angle from datetime

   symbol	description	unit	variable name
   \(t\)	datetime (UTC)	\(s\) since 2000-01-01	datetime {time}
   \(\delta\)	solar declination angle	\(deg\)	solar_declination_angle {time}

   \begin{eqnarray} A & = & 2\pi \left( \frac{t + 10 \cdot 86400}{365.2422 \cdot 86400} - \lfloor \frac{t + 10 \cdot 86400}{365.2422 \cdot 86400} \rfloor \right) \\ B & = & A + 2 \cdot 0.0167 \sin( 2\pi \left( \frac{t - 2 \cdot 86400}{365.2422 \cdot 86400} - \lfloor \frac{t - 2 \cdot 86400}{365.2422 \cdot 86400} \rfloor \right) ) \\ \delta & = & -\frac{180}{\pi} \arcsin( \sin(\frac{\pi}{180} 23.44) \cos(B) ) \end{eqnarray}