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Nautical sextants are designed to measure the precise angle between two objects that, through the use of mirrors, can be viewed simultaneously for highly accurate readings. Depending upon the objects sighted, a navigator is then able to consult charts or use mathematical calculations to determine their exact position on a map or the distance from an object sighted.

The most common use of a sextant is to sight the Sun at solar noon (i.e. “local noon” relative to a person’s actual position on the globe, rather than according to an arbitrary Time Zone) in order to determine the viewer’s latitude. A similar sighting at night of the North Star, Polaris, enables one to determine latitude as well.

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Sextants & Leveling Instruments

Longitude may determined by measuring the angle between the Moon and another celestial body such as a star or planet. This reading of “lunar distance” is then compared to an astronomical almanac of sightings calculated for the prime meridian in Greenwich, England. By calculating the difference between Greenwich time and local time, the navigator is then able to determine their longitude.

Another use of the sextant is to determine the position from a single landmark of known height, such as a lighthouse, or to triangulate and plot position relative to a pair of fixed landmarks of known location and distance from one another. By holding the sextant vertically and measuring the angle between the base and the lamp of a lighthouse, a navigator could use simple trigonometry to determine the distance of the ship from the lighthouse simply by knowing its height. Holding the sextant horizontally allowed for reading the angle between two landmarks, and then determining one’s position upon a map or chart.

Navigation at sea was extremely dangerous prior to the development of the sextant and the accurate measurements the instrument allowed. Ancient mariners often kept within sight of known land and sailed only by day, forcing them to take indirect and lengthy courses to their destinations while placing them at the mercy of rocky shores, storms that might drive them aground and coastal pirates.

The Minoans are among the first sailors known to use celestial navigation. By carefully observing the location and movements of certain stars and constellations, they could maintain a basic orientation of their ship throughout the night, allowing them to sail voyages across open water that took longer than a single daylight period to complete.

During the Middle Ages, the development of the magnetic compass was complimented by the invention of several crude measuring devices for determining angles. One of the earliest was the cross-staff, which consisted of a central beam with graduated markings, and a crosspiece that could slide along it. By placing the base of the staff before one’s eye and then sliding the crosspiece so that one object would appear to be at the end of one arm while a second object appeared positioned off the opposite arm, the observer could determine the angle between the two objects.

The cross-staff had two great shortcomings. The first was that taking measurements of solar position required looking directly into the sun, resulting in eventual blindness for numerous mariners. The second problem was that because the navigator could not view the objects at both arms of the staff simultaneously but instead had to constantly switch his focus between them, the accuracy of the measurements was severely compromised.

Sun-blindness was temporarily solved by a device called the back-staff, which was later refined as the Davis quadrant. Using this device, the navigator turned his back to the sun, and instead measured using a shadow cast upon the device. However, the Davis quadrant still required the user to sight two objects with consecutive rather than simultaneous focus.

The issue of simultaneous focus led to the suggestion by Sir Isaac Newton that mirrors be utilized to bring separate objects into view together. This led to the development of several types of reflecting instruments for navigation, wherein a celestial body could be viewed at the same time as the horizon or another object, and the angle between them measured with much greater accuracy. The shortcoming of these initial devices was their limited precision as well as a general return of the sun-blindness problem.

Both the Davis quadrant and the various reflecting instruments remained in use until the 1730s, when over a period of three years at least four different inventors independently created very similar devices known as “octants”. Taking their name from the fact that their arc was one-eighth of a complete circle, the octants used a pair of mirrors (or in one case, prisms) to reflect an object into view beside another that was directly sighted. The use of dark filters solved the sun-blindness hazard, while increasingly precise manufacturing methods of the period allowed for considerable accuracy in measurements.

Octants quickly eclipsed all previous devices, relegating them to obsolescence within 50 years. However, they primacy of the octant was also short lived. In 1767 British Admiral John Campbell was conducting sea-trials of the lunar-distance method of calculating time and position, but discovered that sometimes the angle he was required to measure was greater than the 90-degrees of which the octant was capable. He suggested increasing the arc of the instrument to one-sixth of a circle, and thus the “sextant” quickly ascended to become the reigning instrument of nautical navigation.

Despite late-20th century advances in technology, including LORAN and GPS navigation, skillful and knowledgeable use of a sextant is an important skill even for modern sailors and navigators. In the event of mechanical or electronic failure, poor signal reception or any other issues which compromise these technological systems, the sextant remains an accurate and reliable method for determining one’s location at sea.