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sextant

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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sextant

sextant (astron.) instrument resembling a quadrant having a graduated are equal to ⅙ of a circle. XVII. — modL. use of L. sextāns, -ant- sixth part (of an as, etc.), f. sextus SIXTH. sextet(te) XIX. alt. of SESTET after L. sex SIX. sextile (astrol.) pert. to the aspect of two heavenly bodies which are 60 or ⅙ of the zodiac distant. XVI. — L. sextīlis, f. sextus SIXTH. sextillion see BILLION. †sextodecimo size of a book in which the leaf is 1/16 of the sheet, †decimosexto; sixteenmo. XVII. sextuple sixfold. XVII.

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sextant

sextant Optical instrument for finding latitude (angular distance n or s of the Equator). A sextant consists of a frame with a curved scale marked in degrees, a movable arm with a mirror at the pivot, a half-silvered glass and a telescope. The instrument measures the angle of a heavenly body above the horizon, which depends on the observer's latitude. A set of tables gives the corresponding latitude for various angles measured.

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sextant

sex·tant / ˈsekstənt/ • n. an instrument with a graduated arc of 60° and a sighting mechanism, used for measuring the angular distances between objects and esp. for taking altitudes in navigation. ORIGIN: late 16th cent. (denoting the sixth part of a circle): from Latin sextans, sextant- ‘sixth part,’ from sextus ‘sixth.’

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sextant

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