Abstract: A sextant having a telescope, a fixed split horizon mirror, a pivotal index arm supporting an index mirror and a stepping motor coupled to the index arm to cause rotation thereof on receipt of control pulses, the number of pulses applied to the stepping motor being proportional to the angle of the resultant pivotal movement of the index arm and thus the altitude angle of a heavenly body being observed through the sextant telescope. A microprocessor is programmed with information relating to the position data of heavenly bodies and monitors the pulses applied to the stepping motor and is arranged to calculate in accordance with known navigational equations, lattitude and longitude from time data and pulses applied to the stepping motor indicative of the altitude angle of the heavenly body being observed.