Solar Declination

The solar declination is the earth's latitude at which incoming solar rays are normal to the earth's surface. The solar declination is zero at the spring and fall equinoxes, approximately +23½° at the summer solstice, and approximately -23½° at the winter solstice. A simple formula to calculate solar declination from Perrin de Brichambaut (1975) is:

$\delta = sin^{-1} \{0.4sin [ 2 \pi /365] (d_n - 82)\}$ 1:1.1.2

where $\delta$ is the solar declination reported in radians and $d_n$ is the day number of the year.