The sextant The first in a series of articles pertaining to the development and history of surveying instruments Leonard Honey “Captain Nemo, provided with his sextant, took the sun height to find his latitude...” Jules Verne, in his book Twenty Thousand Leagues under the Sea, describes perhaps the traveller and surveyor par excellence. Who invented the sextant? • The mariner's astrolabe It’s between English and Americans. An account of one of these exists Some said the English mathematician, describing how it was used to observe John Hadley (after an idea of Robert the solar eclipse of 3 March 1337. Hooke) developed this instrument in 1731 Amongst early references, the most simultaneously with the American (from famous, especially as it is illustrated, is Pennsylvania) Thomas Godfrey. Others that of Pedro Medina in 1552 in Seville. In said Godfrey invented it in 1730 and John appearance, the mariner's astrolabe Hadley in 1731. resembles a four-spoked, cast bronze wheel, with anointed ring at the top for How did we get to the sextant the navigator's thumb and a pivoted as we now know it? Geodetic measurements. alidade with slits through which he peered • Sea quadrant at the celestial body. Working independently, and in complete One of the first elevation-finding Calibrated 90-0-90 across the two ignorance of each other, John Hadley instruments was the sea quadrant. top quadrants for zenith distances, VPRS (1682-1744) in London and Thomas Originally the tool of the astronomer and usually associated with Portuguese Godfrey, glazier and natural surveyor, this instrument was first used by m a n u f a c t u re or marked for altitude mathematician in Philadelphia, mariners in the 15th century. It was a heights suggesting another school of simultaneously devised an improved form simple arc of a circle made of boxwood makers, they vary considerably in of altitude measuring instrument which (or other close-grained wood) or brass diameter and weight, but an average worked on the same principles. with two sighting pinnules along one size would appear to be 13-18cm (5-7") The Royal Society recognized the straight edge. A plumb bob attached to in diameter and weighing 2-3kg. equality of the two and awarded each a the apex swung across a scale graduated prize of £200. Godfrey received his in 0-90o to show an altitude reading. • The back staff household furniture. This was an ingenious improvement on The Hadley quadrant, as it came to be • The cross staff the quadrant, cross star and mariner’s called, although it was later known as an First described in 1342, it was an astrolabe for taking an elevation. Usually octant, was a brilliantly simple instrument instrument for measuring distances made of wood section 5/8" by 5/8" based on the application of optics. between two stars or the angular (1.6x2cm) in lignum vitae, the instrument Returning to the conception of the sea elevation of a star or the sun above the was formed with a main limb about 24" quadrant with a single arc of 90o, the horizon. About 30" long, it was made of (61cm) long, with a right angled instrument was made of a triangular approximately half-inch section close- accessory, termed a horizon slit, on the frame of wood, lignum vitae or mahogany grained wood on which scales were end. The quadrant was divided into two for strength, with a movable index arm calibrated on all four sides. Three or four arcs made of boxwood. pivoted from the apex. A mirror was fixed alternative cross pieces or 'transoms' The success of the back staff was at this point that would move with the could be moved up and down the shaft, phenomenal. Made almost exclusively in index arm. An observer would peer one at a time. England, although Irish and American through the sighting pinnule, placed on To use the cross star, a navigator ones are known, it was used all over the the limb, and tilt the instrument until he would fit one of the transoms, point the world for nearly 200 years. Termed the could see the horizon in the clear half of staff at the sun and rest the opposite end Davis quadrant, it was the seaman's the second glass fixed on the opposite of it on the bone beside the eye. The trusted servant. However, a notable limb. He then adjusted the index arm until bottom end of the transom should touch exception to its widespread popularity the celestial body appeared to be the horizon, and the top edge of the lower was in Holland, where the conservative reflected onto the horizon. Finally he limb the sun by sliding the transom up Dutch clung to the use of the cross staff checked the vernier, which is fitted at the and down the staff as necessary. The well into the 19th century. end of the index arm over the accurate altitude of the sun could then be read off scale, for fine adjustment and took the the appropriate scale. • The Hadley quadrant reading on the scale. First used by astronomers, it was soon The pressing commercial need for more There are two main principles involved. taken up by navigators and its use accurate navigational methods, which is First, the angle of incidence equals the became widespread. Vasco da Gama was reflected in the offer of a £20,000 prize for angle of reflection in a plane which shown a cross staff by his Arab pilot from a practical solution to the problem of how contains the normal to the reflecting the African coast (now Kenya) when he to determine longitude in Britain and surface at the point of reflection. Second, was taken aboard to guide the explorer to similar prizes in other countries, produced India in 1497-9. if a ray of light suffers two successive a ferment of activity in all aspects of reflections in the same plane, by two Cross staves were still being used in Holland until the 19th century. navigational science. plane mirrors, the angle between the first xx CES Month YEAR www.civilengineeringsurveyor.com xxxx Octants were first made in England, America and Ireland, and then in France — the first by Pierre Lemaire (c1739-60). Ramsden's dividing machine was made by other manufacturers, for he had received an award for its invention and held no patents. Spencer, Browning and Rust (1787-1842) used one of these to great effect for they must have made scales for nearly everyone. The initials SBR will be found in the centre of the ivory scale of many octants bearing either another maker's name, a Apian’s ‘Instrument Buch’. chandler's name or no name at all. and last direction of the ray is twice the angle between the • The sextant mirrors. Because the angle between the two mirrors is half the The familiar brass sextant was based on the same principles as altitude of the object observed, when the mirror on the index the Hadley quadrant, but was intended as an improvement in arm moves from the parallel through to the angle, double the that the wooden frames of the octant were inclined to distort in angle will be read on the arc. Thus the arc will read up to 90o humid conditions and caused errors. although in itself is only an eighth of a circle (45o), hence the Also, the larger arc of the sextant was more useful. It is not term octant. The vernier was added for finite adjustment. clear who first made a brass instrument, but Edward A telescope replaced the sighting pinnule or was offered as Troughton (1753–1836), a founder member of the Royal an alternative. The index arm was either handsomely engraved, Astronomical Society, patented a form of brass sextant in bore a fin for extra rigidity or was simply plain. The wooden 1788. The limb was formed of strips of plate in duplicate; the limbs were blackened or 'ebonized' to reduce glare and, for two joined together with turned brass pillars. This type of greater clarity, the scales were engraved on ivory. A nameplate sextant, termed the Troughton type or double frame, was was provided for the owner as well as a pencil secreted in the being made as late as 1830. cross member to be used to record data on a small ivory plaque In the meantime, Jesse Ramsden and others were on the back. The instruments were fitted with two sets of experimenting with thicker gauge metals and other forms of coloured glass shades for use with the sun and the earlier type manufacture to ensure rigidity. Curiously enough, the most of instrument had a second pinnule fitted on the opposite limb difficult part of the instrument to make was the plane mirrors. so that when the horizon below the sun was ill-defined, the The two aces had to be ground parallel and silvered in the old opposite horizon could be used. This was of greater use at way with mercury and tinfoil. The problem lay in the grinding of anchor off unexplored coasts where latitude had to be the glass, for if it was not absolutely flat, the instrument would determined. The size of the instrument was controlled by the be inaccurate. fact that the scales on the arc had to be calibrated by hand. The eyepieces are either of the Huygens or the When Jesse Ramsden (1735-1800) invented his dividing Ramsden type which work on diff e rent optical machine in 1771, this operation could be swiftly, accurately principles, although the results are almost the same. and economically carried out in a smaller area, so the size The main diff e rence is that the Ramsden can be used of the instruments shrank from a radius of approximately with cro s s w i re for measurements whereas the s 45cm (18") to one of approximately 20cm (8"). In Huygens may have cro s s w i res but only so as to this compact form, the octant was in use until the mark the centre. The object glass is achro m a t i c end of the 19th century. They were cheap to buy to avoid spherical and chromatic aberrations (they were offered in a catalogue at 30 shillings) which the four spherical surfaces of the and were less vulnerable than the brass meniscus or concave flint glass lens and the instruments, that ultimately replaced them, biconvex crown glass lens overcome when when used on board a small freighter or fishing bonded together. boat. The handsome quadrant or octant was The pocket sextant was a useful tool for the well established as a practical 'no nonsense' tool 19th century surveyor, which he used for a by about 1790. quick meridian bearing. A handy size for the Perhaps the onslaught of the Napoleonic pocket, they were not intended to have the wars and the enormous demand for accuracy of a navigational instrument. instruments for the hastily impressed battle fleet, plus the new mass-produced scales, Leonard Honey, 22 Heathview, caused this decline in aesthetics, but London N2 0QA t: 020 8883 8696 no nautical instrument had ever had a e: l.honey@virgin.net Full details of wider appeal. Wherever there was a a range of reproduction scientific need for maritime victualling, octants instruments, ASTRO kits, sundials, were sold, frequently with the chandler's orreries etc) can be found on trade label in the box or on the instrument. w: www.green-witch.com Leonard’s sextant. CES Month YEAR xx www.ices.org.uk

Courtesy of Leonard Honey, a knowledgable chap.