Abstract: A mechanical device adapted for determining the position of an observer by star-sight observations comprising a hollow portable sphere, having a number of sets of pairs of holes, the two holes in each set lying at diametrical extremities by which the sphere can be releasably engaged in a rigid ring, such that the sphere is rotatably mounted in the ring for rotation about an axis passing through the diametrically-opposed holes. A slidable carriage is mounted on the ring for travel in the plane thereof and markings are formed on the ring for determining the position of the carriage thereon. The position of an observer is determined from three simultaneous observations of suitably selected stars by means of the corresponding true circles of altitudes which are inscribed on the surface of the sphere for observation. star of obsevation.