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Octant and case

Date: c 1790
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: V00027259
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 meant a vast improvement in the use of navigational instruments at sea.
    HistoryAn octant is a portable instrument that uses a small mirror to bring two images together, those of the sun and the horizon, for instance, to determine latitude at sea by observing the altitude of celestial bodies. It has an arc of 45 degrees or more that measures angles of 90 degrees or more.

    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 have ebony frames, brass index arms, and ivory scales read by verniers. Although early examples were large, heavy, and costly, Ramsden's invention of the dividing engine in 1777 led to the production of smaller and less expensive instruments.
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