Images(2)

Double framed sextant

Manufacturer Troughton and Simms

Date1810 - 1860

Object number00036876

NameSextant

MediumCopper alloy, wood, glass

DimensionsOverall: 109 x 260 x 300 mm, 1.46 kg

ClassificationsTools and equipment

Credit LineANMM Collection

Terms

DescriptionSextants were used to obtain the angles or height from the horizon of a celestial body such as stars, planets, the moon or the sun. This information could then be used to calculate a vessel's latitude and, aided by a chronometer and mathematical tables, its longitude. The construction technique used for this sextant produced a very rigid, robust and accurate instrument, and was first developed by Troughton and Simms in the early 1800s and remained popular until the development of lighter sextants in the 1850s. HistoryThe sextant is a navigational instrument designed to calculate the measurement of horizontal and vertical angles at sea. Using a series of mirrors, a sextant is capable of measuring angles up to 120 degrees. The sextant was developed in 1757 from its antecedent the quadrant, which could measure angles only up to 90 degrees, following a suggestion from Captain John Campbell of the Royal Navy. Although the quadrant was a capable instrument and remained in use until the mid-1800's, it was not capable of taking lunar observations which were necessary for the calculation of longitude in celestial navigation (out of sight of land at sea). The sextant employs the same optical principles as those developed by John Hadley and demonstrated in Hadley's quadrant. A fixed mirror is half silvered, so that the horizon can be seen through non-silvered half and the reflected object can be seen in the silvered half. A second moveable mirror is mounted on the index bar, which is pivoted at the centre of the sextant. When a navigator takes an altitude of a celestial body, such as the sun or a moon, the sextant is held perpendicular to the navigator who then observes the horizon through the fixed mirror. At the same time the navigator moves the index bar until they can also see the celestial body which has been reflected from the moving to the fixed mirror. Once the horizon line and the celestial body are observed, the altitude of the body can be read off the arc scale at the base of the instrument. In order to calculate a position at sea out of sight of land, the navigator uses the sextant to calculate the altitudes of two or more celestial bodies, and records the time on board ship when they took the observation, and the time at Greenwich (0 degrees longitude) using a coded time signal or chronometer. Using Greenwich Mean Time (GMT) the navigator looks up the altitude and position of the observed celestial bodies in the Nautical Almanac. The Almanac is an astronomical table which records the position of the majority of observed celestial body's for every second of every day. These positions are given in declination and Greenwich Hour Angle (GHA) from which the latitude and longitude of the celestial body's geographical position (GP) can be found. Knowing the altitude of the celestial bodies and the geographical position allows the navigator to calculate circles of equal altitude on a chart which can be used - aided by 'dead reckoning' - to determine the vessel's latitude and longitude. SignificanceThis is a fine and rare example of a sextant which represents the high level of technological development achieved in the design and manufacture of navigational instruments in the early 19th century. Sextants were essential for the safe navigation of a vessel out of sight of land and were used for ocean voyaging.