Accession No
2683
Brief Description
octant, by Jones, Gray and Keen, English, 1845 (c)
Origin
England; Liverpool
Maker
Jones, Gray and Keen
Class
navigation
Earliest Date
1845
Latest Date
1845
Inscription Date
Material
wood (ebony and one other); metal (brass); plastic (ivorine); glass
Dimensions
box length 350mm; breadth 316mm; depth 90mm
Special Collection
Provenance
Transferred from Institute of Astronomy, University of Cambridge, Cambridge, England, 10/1975.
Inscription
‘JONES, GRAY & KEEN. STRAND. LIVERPOOL’ (ivory plate on brace)
Description Notes
Ebony frame, ivory scale. Adjustable index mirror, reinforced brass index arm, clamp and tangent screws, ivorine type-B vernier. Brass double pinhole sight. Ivorine maker’s plate and scale divided -2˚ - 100˚ by 1˚, subdivided to 20´. Pencil hole in frame. Note plate on reverse. Knurled screw for adjusting horizon glass. Two brass feet (third missing). Detached: 3 index mirror shades, horizon glass.
Step wooden box (broken) with trade label for Thos. Parnell, London.
References
Events
Description
John Hadley, an English seafarer, invented the octant (also called a Hadley quadrant) during the 1730’s. It was used to find latitude by measuring the altitude of the sun, moon or stars.
An octant is an angle of 45°, or an eighth of a circle. The octant was first made with mahogany frames and scales on inlaid boxwood. By the early 19th century ebony (a much more hard wearing wood) had replaced the mahogany to craft the frames which had brass index arms.
The octant is used in a similar way to a sextant. The observer raises the octant until the horizon can be seen through the horizon glass. The index arm and attached index glass are moved until the twice-reflected image of the sun or star can be seen superimposed on the horizon, enabling an angle to be read off.
The octant was inexpensive and became the everyday instrument for measuring latitude, whilst the more expensive sextant was used to find longitude by measuring lunar distances.
Created by: Corrina Bower
FM:42762
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