Accession No
1486
Brief Description
octant, by Dombey and Son and W. and T. Gilbert, English, c. 1850
Origin
England; London
Maker
Dombey and Son W. and T. Gilbert
Class
navigation
Earliest Date
1850
Latest Date
1850
Inscription Date
Material
wood (ebony); metal (brass); ivory
Dimensions
length 288mm; breadth 229mm; thickness 79mm
Special Collection
Provenance
Transferred from the Department of Mineralogy and Petrology, University of Cambridge.
Inscription
‘DOMBEY & SON. LONDON’ (maker’s plate on brace)
‘W. & T. Gilbert, London’ (index arm)
Description Notes
Ebony frame, ivory scale. Reinforced brass index arm with ivory, type-B vernier, brass clamp and tangent screw. No optics or evidence of optics. Curious wood and brass fitting replacing index mirror. Inset ivory note plate. Pencil hole in brace. No feet. Ivory plate for maker’s inscription.
Incomplete (optics, pencil and feet missing)
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:43358
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