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In order to find the beam energy falling on a surface having any orientation, it is necessary to convert the value of the beam flux coming from the direction of the sun to an equivalent value corresponding to the normal direction to the surface.

**SOLAR RADIAION
GEOMETRY**

In
order to find the beam energy falling on a surface having any orientation, it
is necessary to convert the value of the beam flux coming from the direction of
the sun to an equivalent value corresponding to the normal direction to the
surface.

If
‘q’
is the angle between an incident beam of flux ‘I_{bn}’ and the normal
to a plane surface, then the equivalent flux falling normal to the surface is
given by I_{bn}Cosq. The angle ‘q’
can be related by a general equation to ‘ϕ’ latitude , ‘β’ the slope ‘γ’ the
surface azimuth angle, ‘δ’ the declination and ‘ω’ the hour angle.

The
Latitude ‘ϕ’ of a location is the angle made by the radial line joining the
location to the centre of the earth with the projection of the line on the
equatorial plane. It varies from – 90^{o} to +90^{o}. By
convention, the latitude is measured as positive for the northern hemisphere.

The
slope ‘β’ is the angle made by the plane surface with the horizontal. It varies
from 0 to 180^{o}.

The
surface Azimuth Angle ‘γ’ is the angle made in the horizontal plane between the
horizontal line due south and the projection of the normal to the surface on
the horizontal plane. It varies from -180^{o} to + 180^{o}.

We
adopt the convention that the angle is positive, if the normal is East of South
and Negative if West of South.

The
Declination ‘δ’ is the angle made by the line joining the centres of the Sun
and the Earth with the projection of this line on the equatorial plane.

Where,
*n* is the day of the year(or Day
Number).

The
hour angle ‘ω’ is an angular measure of time. It varies from -90^{o }to
+90^{o}.

It
is the angle through which the earth must be rotated to bring the meridian of
plane directly under the sun. In order words, it is the angular displacement of
sun east and west of the local meridian, due to the rotation of the earth on
its axis 15^{o} per hour. The hour angle is zero at solar noon,
negative in the morning and positive in the afternoon(Table 4.2) for northern
hemisphere and vice-versa for southern hemisphere. Expression for hour angle by
ω= (ST-12) 15^{o}.

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