Sextant designed by Peter Dollond

Designer Peter Dollond (1731 - 1821)

Datec 1772

Object number00006711

NameSextant

MediumBrass, wood

DimensionsOverall: 109 x 520 x 531 mm, 2.3 kg
Display Dimensions: 500 x 532 mm

ClassificationsTools and equipment

Credit LineANMM Collection

Terms

DescriptionNamed because its arc is equal to one sixth of a circle, a sextant is actually capable of measuring angles up to 120 degrees. It was this feature tat made them particularly useful when attempting to determine longitude using the lunar distance method.HistoryThe sextant was developed in 1757. It is an instrument of double reflection by means of two mirrors, and thus although its actual arc subtends an angle of 60 degrees (1/6th of a circle - hence the name sextant), it is capable of measuring angles up to 120 degrees. The sextant was an improvement on the earlier quadrant, an instrument capable for measuring angles up to 90 degrees (1/4th of a circle hence the name quadrant). The capacity of sextants to read angles greater than 90 degrees was an advantage when using the lunar distance method to determine longitude. This was also useful for taking horizontal angles. This sextant is made to a special design by Peter Dollond which incorporates a pivoting arm on which the horizon glass is mounted. This feature was intended to facilitate adjustments but due to its placement, required a 'kink' in the index arm. Born in England in 1731, Peter Dollond was an English optician who along with his father, younger brother and nephew, designed and manufactured a range of optical instruments. Dollond was also well known for his improvements to microscopes and telescopes. Dollond telescopes voyaged with Captain Cook during the ENDEAVOUR expedition to view the transit of Venus. Admiral Lord Nelson owned several Dollond telescopes. The sextant bears the trade mark of Dutch instrument maker and retailer Jacob Hendrik Onderwijngaart Canzius (1771-1838) who made and sold instruments in Delft from 1797 to 1810. It is unclear if the sextant was made or just retailed by Canzius. It is missing the telescope and both upper and lower shades. (Further information on Canzius can be found in Peter de Clercq's article -"JH Onderdewijngaart Canzius, Instrument manufacturer and Museum Director" in the Bulletin of the Scientific Instrument Society, No. 49 (1996): pp 22-24.)SignificanceThis sextant is representative of the high level of technological development achieved in the design and manufacture of navigational instruments in the second half of the 18th century.