Sextant

views updated Jun 11 2018

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.

Sextant

views updated May 11 2018

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.

sextant

views updated May 21 2018

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.

sextant

views updated May 29 2018

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.’

sextant

views updated May 17 2018

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.