The apparent motion of the sun, caused by the rotation of the Earth about its axis, changes the angle at which the direct component of light will strike the Earth. From a fixed location on Earth, the sun appears to move throughout the sky. The position of the sun depends on the location of a point on Earth, the time of day and the time of year. This apparent motion of the sun is shown in the figure below..
This apparent motion of the sun has a major impact on the amount of power received by a solar collector. When the sun's rays are perpendicular to the absorbing surface, the power density on the surface is equal to the incident power density. However, as the angle between the sun and the absorbing surface changes, the intensity on the surface is reduced. When the module is parallel to the sun's rays (and the angle to the module normal = 90°) the intensity of light essentially falls to zero. For intermediate angles, the relative power density is cos(θ) where θ is the angle between the sun's rays and the module normal.
The angle between the sun and a fixed location on Earth depends on the particular location (the longitude of the location), the time of year and the time of day. In addition, the time at which the sun rises and sets depends on the longitude of the location. Therefore, complete modeling of the sun's angle to a fixed position on Earth requires the latitude, longitude, day of the year, and time of day. This is discussed in the following pages.