sextant, instrument for measuring the altitude of the sun or another celestial body; such measurements can then be used to determine the observer's geographical position or for other navigational, surveying, or astronomical applications. The term sextant is used generally to include related devices such as the quadrant, quintant, and octant. The sextant was invented independently in England and America in 1731. Its construction is based on the principle that a reflected ray of light leaves a plane surface at the same angle at which the direct ray strikes the surface. The sextant consists basically of a triangular frame, the bottom of which is a graduated arc of 60°; a telescope is attached horizontally to the plane of the frame. A small index mirror is mounted perpendicular to the frame at the top of a movable index arm or bar, which swings along the arc. In front of the telescope is the horizon glass, half transparent and half mirror. The image of the sun or other body is reflected from the index mirror to the mirror half of the horizon glass and then into the telescope. If the index (or image) arm is then adjusted so that the horizon is seen through the transparent half of the horizon glass, with the reflected image of the sun lined up with it, the sun's altitude can be read from the position of the index arm on the arc. By reference to navigational tables, the geographical position can then be determined. A sextant may be used on land with an "artificial horizon"—a small, shallow receptacle containing mercury, which gives a truly horizontal surface. In aerial navigation a bubble octant—sometimes called a bubble sextant—is used, in which a spirit level is reflected into the field of view in such a way that the center of the bubble indicates the true horizon.
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