sextant

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A navigational instrument containing a graduated 60-degree arc, used for measuring the altitudes of celestial bodies to determine latitude and longitude.
2. n. See Sextans.

Century Dictionary and Cyclopedia

1. n. In mathematics, the sixth part of a circle. Hence An important instrument of navigation and surveying, for measuring the angular distance of two stars or other objects, or the altitude of a star above the horizon, the two images being brought into coincidence by reflection from the transmitting horizon-glass, lettered b in the figure. The frame of a sextant is generally made of brass, the arc h being, graduated upon a slip of silver. The handle a is of wood. The mirrors b and c are of plate-glass, silvered. The horizon-glass b is, however, only half silvered, so that rays from the horizon or other direct object may enter the telescope e. This telescope is carried in the ring d, and is capable of being adjusted, once for all, by a linear motion perpendicular to the plane of the sextant, so as to receive proper proportions of light from the silvered and unsilvered parts of the horizon-glass. The figure does not show the colored glass shades which may be interposed behind the horizon-glass and between this and the Index-glass c, upon which the light from one of the objects is first received, in order to make the contact of the images more distinct. This index-glass is attached to the movable arm feminine The movable arm is clamped by the screw i, and is furnished with a tangent screw j. The arc is read by means of a venier carried by the arm, with the reading-lens g. In the hands of a competent ob-server, the accuracy of work with a sextant is surprising.
2. n. [capitalized] Same as Sextans, 2.

Wiktionary

1. n. nautical A navigational device for deriving angular distances between objects so as to determine latitude and longitude.
2. n. geometry One sixth of a circle or disc; a sector with an angle of 60°.

GNU Webster's 1913

1. n. (Math.) The sixth part of a circle.
2. n. An instrument for measuring angular distances between objects, -- used esp. at sea, for ascertaining the latitude and longitude. It is constructed on the same optical principle as Hadley's quadrant, but usually of metal, with a nicer graduation, telescopic sight, and its arc the sixth, and sometimes the third, part of a circle. See Quadrant.
3. n. (Astron.) The constellation Sextans.

WordNet 3.0

1. n. a measuring instrument for measuring the angular distance between celestial objects; resembles an octant
2. n. a unit of angular distance equal to 60 degrees