The point of interest in sun tracking is to follow the path of the sun on local visible sky. This is because in this area, the receiver part of the conversion system has to intercept the sunshine during the day.

The orientation principle is based on the input data referring to the position of the sun on the sky dome. For the highest conversion efficiency, the sun rays have to fall normal on the receiver so the system must periodically modify its position in order to maintain this relation between the sun rays and the panel. (Fig .2)

The positions of the Sun on its path along the year represent an input data in designing the tracking system, so the geometrical relationship between the Earth and the Sun has to be considered.

The Earth describes along the year a rotational motion following an elliptical path around the sun. During one day (24 hours) the Earth also spins around its own axis describing a complete rotation (360°).

Comparative with the first mentioned motion which doesn’t affect the behaviour of the sun on the sky dome, the second one generates the sunrises and the sunsets.

The variation of the altitude of the sun on the celestial sphere during one year is determined by another rotational motion called precession responsible for a declination of the Earth axis in consideration with the plane of the elliptic yearly path. This motion generates the seasons because of the alternative exposure of the northern and southern hemisphere to the sun rays trajectory. So the combined motions of the Earth are similar with behaviour of a spindle.

Considering as a reference point the ground plain of the loco, the relative movement of the Sun along the year covers a sector in the celestial sphere.

This "belt” of trajectories results from the combination between the daily rotation of the Earth around its axis and the precession motion of the Earth axis (Fig.3) [1, 4, 6].

The fact that the orientation can be obtained by two independent rotational motions has to be considered in the mechanism design process.

There are particular conditions imposed by the principle of orientation:

— the motions have to be independent in respect to the relative motions of the Sun;

— the motion that tracks the daily trajectory of the Sun is recommended to be a rotation around an axis parallel with the polar one.