Abstract: A sensor for detecting the position of the sun in the sky and for use in systems adapted to track the movement of the sun in the sky, said sensor comprising a housing provided with a predetermined opening in the vertical and in the horizontal directions, a member having a reflecting surface at least part of which is of spherical shape, a photosensor adapted to provide an electrical signal when illuminated by rays of the sun reflected from said member with spherical surface, said photosensor being located in said housing or in a separate housing, said element with spherical surface being positioned so that rays of the sun entering said opening are reflected onto said photosensor, means being provided to utilize the resulting signal for locking onto the sun and for tracking its movement; systems for such tracking comprising a plurality of such sensors and means for tracking the sun by equalization of the signals obtained from at least two sensors, each covering a predetermined part of the sky.