Accession No
0045
Brief Description
octant, 1800 (c)
Origin
Maker
Class
navigation
Earliest Date
1800
Latest Date
1800
Inscription Date
Material
wood (ebony and one other); ivory; metal (brass, oxidised brass); glass
Dimensions
box length 360mm; breadth 315mm; height 100mm
Special Collection
Robert Whipple collection
Provenance
Purchased by Robert Stewart Whipple from R. Castle, Bath, in 05/1922.
Inscription
Description Notes
Ebony frame, ivory scale. Reinforced brass index arm with ivory, B-type vernier. Brass lamp (later addition) and tangent screw. Oxidised brass mounts for mirrors and shades. Adjustable index mirror with three shades. Horizon glass with lever adjustment moved by knurled screw. Brass double pinhole sight. Backsight and back horizon glass with lever adjustment. Ivory inset nameplate, unmarked. Ivory note plate on reverse. Three brass feet. Step wooden box.
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:42763
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