A transit to which is fastened a micrometer for following the motion of a star through the field of view and determining the instant at which a star passes through a given vertical circle. The micrometer has one or more movable threads which lie in the focal plane of the telescope. The threads are moved in the direction of azimuth by a single screw with a graduated head. The star is followed by rotating the screw and recording an observation at each of the main graduations. Local sidereal time can be determined if the telescope is set up to move in the meridional plane or to determine latitude if set up to rotate in the plane of the prime vertical.