Accession No

0355

Brief Description

octant, by John Goater, English, 1750 (c)

Origin

England; London; Wapping

Maker

Goater, John

Class

navigation

Earliest Date

1750

Latest Date

1750

Inscription Date

Material

wood (mahogany, boxwood); metal (brass); ivory; glass

Dimensions

Special Collection

Robert Whipple collection

Provenance

Purchased from Antique Art Galleries, Grafton Street, on 03/08/1927.

Inscription

‘I. Goater Maker No 141 Wapping LONDON’

Description Notes

Mahogany frame, boxwood diagonal sclae, mahogany index arm, brass mounts for mirrors, shades and pinhole sights. Adjustable index mirror with two shades (red and green), adjustable horizon glass, two alternative pinhole sights. Backsight and adjustable back-horizon glass. Inset ivory plate with maker’s inscription. Clamp for index arm. Three feet.

Condition good; complete.

References

Joshua Nall; ‘Astronomy at Sea’; Explore Whipple Collections online article; Whipple Museum of the History of Science; University of Cambridge; 2020: https://www.whipplemuseum.cam.ac.uk/explore-whipple-collections/astronomy-and-empire/astronomy-sea

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:39658