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 (IS ) is the vector sum of the components along the line normal of the titled surface. (3) where IDN = the solar intensity irradiated on a surface normal to the sun's rays For a horizontal surface S = 0o, the solar intensity normal to the horizontal surface is: (4) For a vertical surface S = 90o, the solar intensity normal to the vertical surface is: (5) Figure 5 Solar Spectra Compared with The 600K Black Body Emission Spectrum