Accession No


Brief Description

sextant, 12-inch, by Jesse Ramsden, English, 1780 (c) - 1790 (c)


England; London


Ramsden, Jesse



Earliest Date

Jan. 1, 1780

Latest Date

Dec. 31, 1790

Inscription Date


metal (brass); glass; wood (mahogany, other)


box length 385mm; breadth 435mm; height 120mm; sextant length 360mm; greatest width 385mm

Special Collection


Transferred from Institute of Astronomy, University of Cambridge, Cambridge, England, 10/1975.


‘Ramsden LONDON’ (above scale)
‘Camb: Obs:’ (above scale)
‘Camb. Obs. Sextant’ (on telescope)
‘857’ (reverse, behind index mirror)
‘Vince’ (ink, inside box)
‘C.O. Sextt’ (telescope)
‘C.O. Sextt’ (telescope)
‘C.O. Sextt’ (dark index mirror)

Description Notes

Sextant, 12-inch, by Jesse Ramsden, English, 1780 (c) - 1790 (c).

Brass frame sextant. Lattice structure frame. Index mirror, with 3 shades. Single brass tube across frame connects telescope sight and horizon glass, with 3 shades. Reinforced T-bar brass index arm with vernier on brass, clamp and tangent screws. Arc engraved on brass - 2 - 137 by 1˚ subdivided to 20’. Wooden handle fixed in brass mount. Three brass feet. Additional dark index mirror, eyepiece shades, 2 telescopes. Fitted mahogany box, key broken.

Box good condition. Sextant is dirty, tarnished.


Joshua Nall; ‘Astronomy at Sea’; Explore Whipple Collections online article; Whipple Museum of the History of Science; University of Cambridge; 2020:


Today navigation instruments such as radar, radio and satellites update a ship’s position continuously. During the 17th and 18th centuries manual calculations had to be made using instruments such as the backstaff, octant or sextant.

The term “sextant” refers to an arc of 60°. The sextant is a portable instrument that measures angles between distant objects. It is an instrument that has been used in the fields of astronomy, surveying and navigation. The sextant was born as a compromise between the circle and the octant: it was an attempt to combine a larger arc with lightness and ease of handling.

When navigating, the sextant is used to measure latitude to an accuracy of 0.01 of a degree. To use the sextant the navigator moves the index arm until the index mirror appears to line up the sun within the horizon mirror. By reading the angle off the index arm, the angle of the sun (and therefore the ships latitude) can be calculated.

Much thought was put into the design for the sextant in an attempt to make them as accurate as possible. The first examples of sextants were made of brass and were mostly large and heavy. Over time the frame was designed to be rigid and light. A successful and popular design in the 18th Century was the “double-frame” or “pillar frame” sextant which was patented by Troughton in 1788. An example of this sextant can be seen in the navigation case.

The Search for Longitude
The sextant was also used in an attempt to determine longitude as well as latitude. In the 1750’s Tobias Mayer’s design of a reflecting circle was given to the British Board of Longitude who gave the instrument to Captain John Campbell to test fully at sea. Campbell liked the idea but found the circle too awkward to use. John Bird was ordered by Campbell to design a 60° arc (the sextant), which he thought to be adequate for the longitude measurements required. (To discover more about the search for longitude have a look at some of the books).


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