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96 INTRODUCTION TO SOLAR POWER SYSTEM DESIGN

Figure 4.1 Solar declination angle.

axis at these two seasonal changes, referred to as the summer and winter equinoxes,
is 0 degrees.
The solar declinations described in below result from seasonal cyclic variations and
solar variations in insolation. For the sake of discussion if we consider Earth as being
a sphere of 360 degrees, within a 24-hour period Earth rotates 15 degrees around its
axis every hour, commonly referred to as the hour angle. It is the daily rotation of
Earth around its axis that gives the notion of sunrise and sunset.
The hour angle (H) (shown in Figure 4.2) is the angle through which Earth has rotated
since midday or the solar noon. At the noon hour when the sun is exactly above our
heads and does not cast any shadow of vertical objects, the hour angle equals 0 degrees.
By knowing the solar declination angle and the hour angle we can apply geometry
and find the angle from the observer’s zenith point looking at the sun, which is referred
to as the zenith angle (Z) (shown in Figure 4.3).
The amount of average solar energy striking the surface of Earth is established by
measuring the sun’s energy rays that impact perpendicular to a square meter area, which
is referred to as the solar constant (S). The amount of energy on top of Earth’s atmos-
2
phere measured by satellite instrumentation is 1366 watts per square meter (W/m ).
Because of the scattering and reflection of solar rays when they enter the atmosphere,

Figure 4.2 Solar hour angle.