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Developed from the more limited octant in the 1750s, the sextant could be used either to find the height of the Sun above the horizon to determine a ship’s latitude, or measure angles between the Moon and fixed stars. The latter was a more difficult observation to do accurately on a moving vessel but, using published tables, allowed a skilled navigator to calculate his longitude as well. This particular instrument is reputed to have been used by Captain Cook on his third Pacific voyage, 1776–80 and is one of four known to survive from his three expeditions. Cook used lunar-distance sextant observations made on land to check the accuracy of the experimental chronometers he took on his last two Pacific voyages. Jesse Ramsden, was one of the best London instrument makers of the late 18th century.
The instrument has a polished brass scale from -2° to 134° by 20 arcminutes, measuring to 120°. The sextant has a brass vernier measuring to 20 arcseconds, with zero at the right.
The sextant was reputed to have been used on Cook’s third voyage to the Pacific (1776-80).
Peter Dollond patented the horizon glass adjustment (no. 1017 of 1772). D.33 was the Hydrographic Office serial numbering adopted in February 1828, whereby D was the letter given to sextants.
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