SEXTANT, a sixth part of a circle. The name is applied especially to an optical instrument for measuring angular dis tances, invented by John Hadley in 1731. Hadley's original in strument was, strictly, an octant, employing a graduated arc of one-eighth of a circle. The arc was enlarged to one-sixth, to meet the needs of navigation, by Capt. Campbell in 1757.
The instrument is mainly used at sea, and the angle that is measured is the altitude of the sun (or a star) above the horizon (see NAVIGATION). A familiar sight on an ocean steamer is the officer on the bridge "shooting the sun" at noon, in order to de termine his latitude. The officer is looking through a small tele scope straight at the sea horizon; but he sees also an image of the sun (dimmed by an interposed dark glass) which has been re flected into his field of view by an arrangement of mirrors de scribed below. He is slowly mov ing an arm which turns one of the mirrors until the solar image appears just to touch the sea horizon.
The figure shows the construc tion of the sextant. ABC is a light framework of brass in the shape of a sector of 6o°, the limb AB having a graduated arc of silver inlaid. It is held in the hand by a small handle at the back, either vertically in a position in front of the eye to measure the alti tude of an object, or in the plane passing through two objects the angular distance of which is to be found. It may also be mounted on a stand. CD is a radius movable round C, where a small plane mirror of silvered plate-glass (called the "index glass") is fixed perpendicular to the plane of the sextant and in the line CD. At D is a vernier read through a microscope, also a clamp and a tangent screw for giving the arm CD a slow motion. At E is another mirror "the horizon glass," also perpendicular to the plane of the sextant and parallel to CB. F is a small telescope fixed
across CB, and pointed to the mirror E. As only the lower half of E is silvered, the observer can see the horizon in the telescope through the unsilvered halt, while the light trom tne sun or a star S may be reflected from the index glass C to the silvered half of E and thence through F to the observer's eye. If CD has been moved so as to make the image of a star or of the limb of the sun coincide with that of the horizon, it is seen that the angle SCH (the altitude of the star or solar limb) equals twice the angle BCD. The limb AB is graduated so as to avoid the necessity of doubling the measured angle, a space marked as a degree on AB being in reality only 3o'.
If the sextant is used on land an "artificial horizon" is required instead of the sea-horizon. This consists of a trough containing a shallow layer of mercury, which gives a truly horizontal re flecting surface. The telescope F is now pointed downwards so as to view the sun's image reflected in the mercury trough; an image of the sun reflected by the sextant mirrors appears as be fore, and the two images are made to touch. The reading now gives the angle between the sun and its image in the mercury trough, which is double the angle between the sun and the horizon.
In the air, however, the visible horizon is of no use, since its "dip" (below the truly horizontal direction) is large and unknown.
The mercury trough is obviously unsuitable for use in an aero plane. Hence some form of "bubble sextant" (see AERIAL NAVI GATION) is used, in which a spirit level is reflected into the field of view in such a way that the centre of the bubble indicates the true horizon.