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1699638153Code 7691 Antique OctantOctant in ebony wood, brass and marine ivory, early 19th century. signed J. Blair Bristol, scale from 0° to 105° complete with two optics and original mahogany box shaped like the instrument with brass hooks and hinges. In good condition, signs of ancient repairs in the wood of the frame. Box measures 33x30x12.5 cm – 13x.11.5x 4.9 inches. Complete with display base made to measure in wood and brass.
The octant, the eighth part of circle, is a reflection instrument, conceived about in 1731 by John Hadley (1682-1744) and used for measure on the sea the height of the Sun or of a star in comparison with horizon. A mobile arm with a mirror and fixed on a graduated arc makes possible to obtain by reflection the image of the star laid upon the image of horizon directly observed. First instruments was made using valuable wood, ebony or mahogany, and they had ivory gradation; then they were replaced by brass or bronze instruments, and the gradations were engraved on a silver strip, fixed in the metallic arch of border.
The need to make the measurements of angles more accurate required increasingly precise instruments and at the end of the seventeenth century the first document appeared that mentioned the principle of the reflection of flat mirrors, applied to the measurement of angles. In this document, an instrument designed around the 1665 by Robert Hooke (1635-1703) and presented to the scientific community. This instrument, being equipped with only one mirror, allowed simple reflection and was therefore inadequate for the purpose. Hooke realized the inconvenience, but failed to perfect his instrument; However, the idea of making use of reflection was taken into consideration by other researchers who studied how to correct the drawbacks of the initial instrument, until in 1731, John Hadley presented his double reflection octant, made up of a 45° arc, divided into 90 half degrees. In 1732 the British Admiralty made the yacht Chatman available to test Hadley's instrument in navigation, which won the praise of the most illustrious astronomers of the time. The first instruments were made of fine wood, generally ebony and had ivory graduations. They were subsequently replaced by brass or bronze instruments, while the graduation was engraved on a silver strip stuck in the metal arch of the flap.
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Code 7691 Antique Octant
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