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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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Sextant

Sextant

A sextant is a navigational instrument that uses a telescope and an angular scale to calculate latitude and longitude. Making a measurement with a sextant is called sighting the object or taking a sight. The optical instruments called sextants have been used as navigation aids for centuries, especially by seafarers. The sextant replaced the astrolabe, which was used by ancient astronomers for navigation. In its simplest form, a sextant consists of an eyepiece and an angular scale called the arc, fitted with an arm to mark degrees. By manipulating the parts, a user can measure the angular distance between two celestial bodies, usually Earth and either the sun or the Moon. The observer can thereby calculate his or her position of latitude by using a trigonometric operation known as triangulation. The word sextant derives from a Latin term for one sixth of a circle, or 60 degrees. This term is applied generally to a variety of instruments today regardless of the spans of their arcs.

One of the earliest precursors to the sextant was referred to as a latitude hook. This invention of the Polynesians could only be used to travel from one place at one latitude to another at the same latitude. The hook end of the device served as a frame for the North Star, a fixed celestial body also known as Polaris. By sighting the star through the hook at one tip of the wire, one could discover if the ship was off course if the horizon line did not exactly intersect the straight tip at the opposite end.

Navigator and discoverer Christopher Columbus (c. 14511506), probably either originally from Italy or Spain, used a quadrant during his maiden voyage. The measuring was done by a plumb bob, a little weight hung by a string that was easily disturbed by the pitching or acceleration of a ship. The biggest drawback to such intermediate versions of the sextant was the persistent requirement to look at both the horizon and the chosen celestial body at once. This always introduced a reading error, caused by ocular parallax, which could set a navigator up to 90 mi (145 m) off-course. Inventions such as the cross-staff, back-staff, sea-ring and nocturnal could not ease the tendency towards such errors.

Although English physicist and mathematician Sir Isaac Newton (16421727) discovered the principle that guides modern sextants, and even designed a prototype in 1700, English inventor John Hadley (16821744) in England and American optician and inventor Thomas Godfrey (17041749) in the United

States simultaneously constructed working models of the double-reflecting sextant 30 years later. These machines depended upon two mirrors placed parallel to each other, as in a periscope. Just the way a trans-versing line cuts two parallel lines at matching angles, a ray of light bounces on and off first one, then the other mirror. One displaces the mirrors by adjusting the measuring arm along the arc, in order to bring a celestial object into view. The number of degrees of this displacement is always half the angular altitude of the body, in relation to the horizon.

Although it has been largely replaced by radar and laser surveillance technology, the sextant is still used by navigators of small craft, and applied to simple physics experiments. Marine sextants depend upon the visible horizon of the seas surface as a base line. Air sextants were equipped with a liquid, a flat pane of glass, and a pendulum or gyroscope to provide an artificial horizon.

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Sextant

Sextant

The optical instruments called sextants have been used as navigation aids for centuries, especially by seafarers. In its simplest form, a sextant consists of an eyepiece and an angular scale called the "arc," fitted with an arm to mark degrees. By manipulating the parts, a user can measure the angular distance between two celestial bodies, usually Earth and either the Sun or Moon . The observer can thereby calculate his or her position of latitude by using a trigonometric operation known as triangulation. The word sextant derives from a Latin term for one sixth of a circle , or 60 degrees. This term is applied generally to a variety of instruments today regardless of the spans of their arcs.

One of the earliest precursors to the sextant was referred to as a latitude hook. This invention of the Polynesians could only be used to travel from one place at a particular latitude to another at the same latitude. The hook end of the device served as a frame for the North Star, a fixed celestial body also known as Polaris. By sighting the star through the hook at one tip of the wire, you could discover you were off-course if the horizon line did not exactly intersect the straight tip at the opposite end.


Christopher Columbus used a quadrant during his maiden voyage. The measuring was done by a plumb bob, a little weight hung by a string that was easily disturbed by the pitching or acceleration of a ship. The biggest drawback to such intermediate versions of the sextant was the persistent requirement to look at both the horizon and the chosen celestial body at once. This always introduced a reading error , caused by ocular parallax , which could set a navigator up to 90 mi (145 m) off-course. Inventions such as the cross-staff, backstaff, sea-ring and nocturnal could not ease the tendency towards such errors.

Although Isaac Newton discovered the principle which guides modern sextants, and even designed a prototype in 1700, John Hadley in England and Thomas Godfrey in America simultaneously constructed working models of the double-reflecting sextant 30 years later. These machines depended upon two mirrors placed parallel to each other, as in a periscope. Just the way a transversing line cuts two parallel lines at matching angles, a ray of light bounces on and off first one, then the other mirror. You displace the mirrors by adjusting the measuring arm along the arc , in order to bring a celestial object into view. The number of degrees of this displacement is always half the angular altitude of the body, in relation to the horizon.

Although it has been largely replaced by radar and laser surveillance technology, the sextant is still used by navigators of small craft, and applied to simple physics experiments. Marine sextants depend upon the visible horizon of the sea's surface as a base line. Air sextants were equipped with a liquid, a flat pane of glass , and a pendulum or gyroscope to provide an artificial horizon.

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