A Fine Mahogany and brass octant with pinule, divided blade and vernier on bone, signed by Hughes London.
The Octant also called a reflecting quadrant is a reflecting instrument used in navigation.
The name octant derives from the Latin octans meaning eighth part of a circle, because the instrument’s arc is one eighth of a circle.
Reflecting quadrant derives from the instrument using mirrors to reflect the path of light to the observer and, in doing so, doubles the angle measured.
Isaac Newton’s reflecting quadrant was invented around 1699 and a detailed description of the instrument was given to Edmond Halley, but the description was not published until after Halley’s death in 1742.
It is not known why Halley did not publish the information during his life, as this prevented Newton from getting the credit for the invention that is generally given to John Hadley and Thomas Godfrey.
Along with the sextant (a little later), the Octant was the popular choice for navigators in the latter half of the eighteenth-century and by 1780 had entirely superseded previous instruments.
This was a time when England was a great center of instrument making supported by the Royal Society (a scientific society) and by new developments in manufacturing techniques. Instrument makers such as Thomas Howard established workshops in major cities with trading ports such as London and Liverpool.
Instruments like the octant were the eighteenth-century equivalent to GPS; however, unlike today’s GPS systems, they required a good understanding of mathematics to operate them!
Like earlier instruments based on the quadrant, the octant was used to measure the altitude (height above the horizon) of the sun, usually at noon when the sun had reached its highest point, or other celestial body such as a prominent star at night. In combination with suitable relating the height of the sun to the date (contained in the mariner’s almanac), the latitude of the ship could be worked out from this measurement. Basically, it depended on the fact that the further you were from the equator, the lower the sun would appear in the sky.
These measurements were essential to navigation, particularly when ships were out at sea beyond the sighting of landmarks. When used properly under stable conditions, these instruments were accurate to within a few nautical miles.
This elegant piece dates to the 19th Century.