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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.

A sextant. Photograph by Gabe/Bikderberg Palmer. Stock Market. Reproduced by permission.

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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