1731 - Appearance of the Octant

John Hadley, a mathematician aged 49, presents a new instrument of navigation, the octant, also known as the reflecting quadrant, at the Royal Society. A member of the Royal Society, he is already and rightly famous as the inventor of the parabolic reflecting telescope which he presented ten years ago.

Three other person at least are working on an instrument of the same sort these days : (1) Thomas Godfrey, 27, a glazier, likely ahead of the group by one or two years maybe, but he lives in America and, worse for his recognition, he is no more than a humble glazier ; (2) Jean-Paul Grandjean de Fouchy, 24, an astronomer, but a French specimen of this noble corporation ; and (3) Caleb Davis, an insurance broker and a connoisseur in astronomy.

The idea has been in the air for long. Robert Hooke (†1703), who invented almost everything in all domains, made a primitive version in 1666, of which he showed a model to the Royal Society in 1670. As usual, he passed to another idea at once without spending another hour to this one which remained uncompleted.

Hadley’s instrument allows to place on the horizon an image of the aimed celestial object – sun, moon, planet or star –, reflected by a mobile mirror. The viewing angle, read on a graduated arc, is the altitude of said celestial object. This instrument is promised to replace the old Davis’ backstaff as it provides the user with a huge improvement in accuracy by a factor of ten or so.

¤ The rotation of the mirror by a given angle displaces the image by the double angle. An eighth of the circle, hence an octant, therefore permits to aim at a celestial object up to the zenith for a measure of altitude.

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IN RETROSPECT FROM TODAY

NOTE A - On Isaac Newton's invention of a reflecting quadrant.

It is still unknown in 1731 that Isaac Newton (†1727) developed a prototype of his invention in 1699. He showed it to Edmund Halley who trialed and validated the instrument - perhaps aboard the Paramour when he metered magnetic declination throughout the Atlantic ocean in 1699-1701 ? Halley described it and kept Newton’s explanations within his papers which will be made public at his death in 1742. Halley did not dare to unveil the invention without Newton’s formal consent. Newton had constantly been strange enough about what could be unveiled of his works or not, and, in the same time, exceedingly picky about his precedence however. It was impossible to make a guess on what to do to meet his agreement and on what not to do to avoid raising his anger. Halley, an old friend though, kept cautious on this. Anyway, Isaac Newton's report to the Royal Society in 1742 was twofold posthumous : it was made fifteen years after Newton's death and just after Halley's death. We may wonder why Halley did not report to the Royal Society in 1727.

It is worth noticing that Newton insisted on the measurement of the distance of the moon to stars. He had clearly in mind the crucial question of longitude, fifteen years before the creation of the Board of Longitude.

NOTE B - Octant Trials.

The Admiralty ordered trials. Performed in 1732, they were convincing. Accuracy was assessed to be better than two minutes of arc.

Amongst the first users, two noticeable gentlemen are to be mentioned : (1) Pierre Bouguer, a mathematician, who brings an octant with him for his journey to Peru in 1735 for measuring an arc of meridian at the latitude of the equator ; (2) Jean-Baptiste d’Après de Mannevillette, a naval officer and a hydrographer who trials the octant as soon as 1733-34 and publishes in 1739 Le Nouveau Quartier Anglois ou Description et Usage d’un Nouvel Instrument pour Observer la Latitude sur Mer.

Success was immediate in France as in Britain.

NOTE C - On octant evolution : the sextant.

Very large in its beginning – 20 inches – and mainly composed of wood, the octant will quickly decrease in size by a half and will be moulded in a single block of brass. In a second step, after captain Campbell’s trials of the lunar distance method in 1757, the arc will develop from an eighth to a sixth of a circle to meet this key usage. The octant then becomes a sextant.

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BIBLIOGRAPHY

Isaac Newton – Description of an Instrument for observing the Moon’s distance from the Fixt stars at Sea in Philosophical Transactions of the Royal Society 42 – London, 1742 - available on the Internet

Edward Neville da Costa Andrade – Robert Hooke in Wilkins Lecture, 1949 in Royal Society Vol 201 – London, 1950

Eva Germaine Taylor – The Haven-Finding Art : A History of Navigation from Odysseus to Captain Cook – London, 1956

Danielle Fauque – Un Nouvel Instrument pour Observer les Hauteurs, Inventé par M. Grandjean de Fouchy in Revue d’Histoire des Sciences 2008 Vol 61 – Paris, 2008

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CREDIT

Bartholomew Dandridge - Portrait of John Hadley - oil on canvas, circa 1731 - © National Maritime Museum, Greenwich - Hadley looks amazingly juvenile for a 49-year-old man but he is ascertained to be the portrayed character