Abstract: An instrument includes three surfaces arranged in contact with one another and rotating around a common coaxial center. By appropriate reciprocal rotations of the three surfaces, it is possible to determine, directly and simultaneously, at any latitude and longitude and during the entire course of the year, at the various solar times, the time of sunrise and sunset, the altitude and azimuth angles of the sun, the points to trace daily trajectories of the sun, the optimum inclination of a plane placed in a North-South direction to enjoy maximum insolation within a predetermined time period, as well as the correspondence between the actual solar time and the conventional time of the selected site. The instrument makes possible a rapid and direct determination of these values without having to resort to various tables or to analytical calculus.