Garden with Insight v1.0 Help: Weather - Day length

Day length is a function of the time of year and latitude as expressed in the equation [Equation 317] where LAT is the latitude of the watershed in degrees and SD is the sun's declination angle.

Equation 317

HRLT = 7.64 * acos((-sin((2pi/360) * LAT) * sin(SD) - 0.044)
/ (cos((2pi/360) * LAT) * cos(SD))
Code:
HRLT = 7.72 * acos(-tan((2pi/360) * LAT) * tan(SD))
since tan is sin/cos, that is the same, the only two differences are
the 0.044 lost in the code and the change of constant from 7.64 to 7.72
Variables:
HRLT = DayLength_hr
LAT = siteLatitude_rad
SD = sunDeclinationAngle_rad

The sun's declination angle, SD, is defined by the equation [Equation 318].

Equation 318

SD = 0.4102 * sin((2pi/365) * (i - 80.25))
Code:
same except in code they calculate this once per month
we will calculate it every day
Variables:
SD = SunDeclinationAngle_rad
i = julianDay