270
0 degrees latitude
Radiation Fluxes in Natural Environments
270
25 degrees latitude
270
50 degrees latitude
270
75 degrees latitude
FIGURE 11-1. Sun tracks at declination angles of -23.5, -10, 0, 10, and 23.5"
for four different latitudes. Zenith angle grids are the concentric circles. Azimuth
angles are shown around the outer circle. North is 0°, east is 90". Large dots are
at one hour time increments.
labeling conventions exist, such as mathematical, astronomical or geographical, so that the reader should be prepared to convert between
various labeling conventions.
Equation (1 1.1) can be rearranged to solve for daylength. It works best
to write the equation in terms of the half daylength, h,, which is the time
(in degrees) from sunrise to solar noon. The half daylength is
cos @ - sin4 sin6
h, = cos-I
cos 4 cos 6
where cos @ = 0 for a geometric sunset (no atmospheric refraction) and
a small negative value when twilight is considered. The time of sunrise