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Ebony, brass and ivory octant

Date: c 1790
Overall: 353 x 287 x 80 mm, 1.15 kg
Display Dimensions: 360 x 85 mm, 290 kg
Medium: Ebony, ivory, glass, brass
Credit Line: ANMM Collection Gift from The Australian Institute of Navigation
Classification:Tools and equipment
Object Name: Octant
Object No: 00027259
Place Manufactured:London

User Terms

    This Spencer, Browning and Rust octant has an ebony frame, a brass index bar with an ivory index. The ebony limb (or arc) has an inlaid ivory scale with the initials 'SBR' inscribed at the centre.

    Octants were used for navigation. They could be used for both Sun and Star observations provided the horizon was visible. It was more accurate at measuring lunar distances than the fore-staff which was available prior to its introduction.
    SignificanceThe development of the octant represented a vast improvement in the accuracy of navigational instruments used at sea during the 18th century.
    HistoryAn octant is a portable instrument with an arc of 45 degrees (1/8th of a circle - hence octant). However, through the use of mirrors, it is capable of measuring angles of 90 degrees (1/4th of a circle - hence quadrant).

    John Hadley described an instrument of this sort to the Royal Society of London in 1731 and obtained a British patent in 1734, and so octants are sometimes known as Hadley quadrants. They were still in use in the early twentieth century.

    Early octants have mahogany frames and boxwood scales read by diagonals. Those made after around 1800 often have ebony frames, brass index arms, and ivory scales read by verniers.
    Additional Titles

    Primary title: Octant

    Web title: Ebony, brass and ivory octant

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