**
****2 **__Solar Angles__

**
****2.1 Solar Altitude**

Solar altitude (b )
is the angle ROQ (Figure 3) on a vertical plane between the sun's rays and the horizontal
plane on the earth's surface.

**
****2.2 Solar Azimuth**

Solar azimuth (f ) is the angle SOQ
(Figure 3) on a horizontal plane between the due-south direction line and the horizontal
projection of the sun's rays.

**
****2.3 Solar Declination**

Solar declination (d) is the angle
between the earth-sun line and the equatorial plane (Figure 2). Solar declination varies
throughout the year.

**
****2.4 Surface-solar Azimuth**

Surface-solar azimuth (g) is the angle
POQ (Figure 3) on a horizontal plane between the normal to a vertical surface and the
horizontal projection of the sun's rays.

*
*Figure 3 Solar Angle with Respect to the
Figure
4 Solar Intensity and

*
Horizontal and Vertical
Surfaces
Angle
of Incidence*

**
****2.5 Surface Azimuth**

Surface azimuth (y) is the angle
POS (Figure 3) on a horizontal plane between the normal to a vertical surface and the
north-south direction line.

**
****2.6 Hour Angle**

Hour angle (H) is the angle SPD (Figure 3)
on a horizontal plane between the local solar noon (meridian which contains the
south-north line) PS and the horizontal projection of the sun's rays PD. The hour angle is
given by:

degree (1)

where T = solar time

In the morning, the hour angle is positive.
At noon the hour angle is zero. In the afternoon, the hour angle is negative.

**
****2.7 Latitude Angle**

Latitude angle (L) is the angle S'OP
(Figure 2) on the longitudinal plane between the equatorial plane and the line PO which
joins the point P on the earth surface and the centre of the earth O.

**
****2.8 Angle of Incidence**

The angle of incidence (q) is the angle
between the sun's rays irradiated on a surface and the line normal to this surface. In
Figure 3, the angle of incidence q H for a horizontal surface is ROV; for vertical
surface, the angle of incidence q V is ROP. In Figure 4, the angle of incidence between
the sun's rays and a tilted surface is q S , where q S is given by:

(2)

**
****2.9 Solar Intensity**

The solar intensity at a direction normal
to the title surface (I_{S} ) is the vector sum of the components along the line normal of the
titled surface.

(3)

where I_{DN} = the solar intensity
irradiated on a surface normal to the sun's rays

For a horizontal surface S = 0^{o},
the solar intensity normal to the horizontal surface is:

(4)

For a vertical surface S = 90^{o},
the solar intensity normal to the vertical surface is:

(5)

*
**Figure 5 Solar Spectra Compared
with The 600K Black Body Emission Spectrum*