====== Solar declination ======

The solar declination is given by:

$$ \delta = 0.4093 * sin \left ( \frac {2*\pi }{365}*J-1.405 \right ) $$

where $J$ is the Julian day number. The solar declination is needed to calculate the [[en:hydro:sunset-hour-angle|sunset hour angle]]. The sunset hour angle is needed for the calculation of [[en:hydro:daylight-hours|possible daylight hours]].

The solar declination can be calculated with the following Python code.

<file py solar-declination.py>
from pylab import *
def declination(J):
    ds=0.4093*sin(2*pi/365*J-1.405)
    return ds
J = arange(1,365,1)
plot(J,declination(J))
ytext = ylabel('solar declination in radians')
xtext = xlabel('Julian day')
show()
</file>