Abstract: The combination with a sextant embodying an index mirror rotatable about a predetermined axis to bring a sighted object into juxtaposition with the line of sight of the optical system at the center of the horizontal mirror of an arcuate informational grating concentric with the axis of rotation of the index mirror and an index arm rotatable in consonance with the index mirror so that its distal end travels along the informational grating, a reference grating at the distal end of the index arm concentric with the information grating, a light-emitting diode at the proximal end of the index arm arranged to project a beam through the reference grating at the distal end of the arm onto the informational grating and a phase detector arranged at the proximal end of the index arm to receive the beam reflected from the conjuncted gratings.

Abstract: An improved sextant which provides a very accurate angular measurement of the position of a celestial body relative to the horizon. The sextant is a typical prior art sextant having a horizon glass and an index mirror, and wherein the sextant is sighted by adjusting the angular position of the index mirror to align an image of the celestial body with a sighting of the horizon. An angular position encoder is coupled to the index mirror to provide a digital electrical output signal indicative of the exact angular position of the index mirror. The sextant includes an electrical clock for producing an electrical output signal which indicates the exact time the sighting is taken. The sextant includes electronics for automatically synchronizing the electrical output signal from the encoder with the electrical output signal from the clock.