The angular position of the sun as seen from a particular place on the surface of the earth varies from hour to hour and from season to season. The basic position of the sun at any instant can be described by two angles: the solar altitude (angle gamma) and azimuth (angle alpha). The reference plane for the solar altitude is the horizontal plane. The altitude is the angle between the line to the centre of the sun and the horizontal plane. When the sun is on the horizon, the solar altitude is 0°; when directly overhead 90°.solar azimuth and altitude The reference plane for the solar azimuth is the vertical plane running north-south through the poles. The azimuth is the angle between true south and the point on the horizon directly below the sun. By convention, it is negative before noon and positive after noon. The azimuth is referenced to due south in the Northern hemisphere. The example to the right illustrates the sun angles for 56 degrees North latitude.
At the northern hemisphere's summer solstice (21June), the sun's rays make an angle of 23° 27' to the equatorial plane. The beam is approximately perpendicular to the Tropic of Cancer (22° 30'N). In the northern hemisphere, the day length reaches its maximum value on this date. North of latitude 23° 27'N, the solar altitude at noon is at its greatest value for the year. At winter solstice (21 December), the sun's rays make an angle of -23° 27' to the equatorial plane. The beam radiation is approximately perpendicular to the Tropic of Capricorn (20°30'S). In the northern hemisphere, the days are at there shortest and the solar altitude at noon has its lowest values.
At the northern hemisphere spring and autuum equinoxes (21 March and 22 September), the sun's rays are perpendicular to the equator. The day and night lengths are almost equal everywhere in the world.
The solar altitude and azimuth over the year can be plotted on a solar chart. The altitude scale is shown on a series of concentric circles. The azimuth scale is set around the perimeter of the chart. The azimuth angle is read by setting a straight edge from the centre of the chart to the intersection of the required hour and date path lines and noting where it cuts the chart perimeter. Different charts are required for different latitudes.
In Southern Europe, at a latitude of 36°N, the sun's path is 240° wide at the summer solstice and the maximum solar altitude is 77°. At the winter solstice, the sun's path is 120° wide and the maximum solar altitude is 30°. In Northern Europe, at a latitude of 56°N, the sun's path is 270° wide at the summer solstice and the maximum solar altitude is 58°. At the winter solstice, the sun's path is 90° wide and the maximum solar altitude is 11°.