Edited by A. N. Stimson, Head of Navigation Section, Department of Astronomy and Navigation, National Maritime Museum, Greenwich, England.
The sextant has come to be widely recognized as a universal nautical symbol. Indeed, the sextant, in conjunction with the compass, has been the basic navigational tool for more than two centuries. The mariners' most prized possession was often his sextant. Witness the drama portrayed by the handwritten account found with a 19th century English sextant.
"This sextant was salvaged from the pilot house of the Norwegian steamship Victory by her master, after being sunk by gunfire from a German submarine 35 miles north of Ushant at 1:00 PM on July 6, 1917. The master, 2nd mate, nine crewmen, and a stowaway were rescued from an open boat by the USS O'Brien at 5:30 A.M. on July 7th, 20 miles west of Ushant and were later landed at St. Nazaire, France. As a token of his gratitude for the rescue the master gave this sextant to the Captain of the O'Brien."
A rare hydrographic surveying sextant also known as a "Quintant"
because of the increased size of its scale to 165°. This
fine example, signed "Cary, London" is engraved on gold and
platinum. Such instruments were used to take triangulation
sightings when making soundings for Admiralty charts. Note
incorporation of a bubble level on the index arm and the
tripod pillar stand. (jpeg 18K)
It is only in the last 20 years, with the advent of satellite navigation and inertial guidance, that the demise of the sextant has been heralded. Yet, despite its obsolescence in the computer age, the simplicity, accuracy, and relatively low cost of the sextant will ensure its survival.
In tracing the evolution of the sextant and searching for remaining examples, the first question a collector is likely to ask is "How old is it?" In attempting to date an early instrument one must consider the state of communications in the 12th through the 18th centuries. The word of new inventions and discoveries was slow to travel. For this reason it was not uncommon for an innovation to actually be "reinvented" many times over! Moreover, such was the scientific understanding of the average Renaissance man that new inventions were often looked upon as unnecessary complications of methods tried and true. Inertia to change was great. As recently as 1925 for example, the German Maritime Ministry, Deutsche Seewarte, was still certifying octants for use at sea.
Unless an instrument is specifically dated, the margin of error in dating can be as much as 20 years - more in older instruments. Bear this in mind as we explore the history of these fascinating instruments.
The earliest attempt at navigation was undoubtedly simple coastal piloting. Mariners would venture no further than the sight of land. The limitations of such navigation held trade and exploration to a minimum for thousands of years, while openwater sailing was reserved for the incredibly brave or foolhardy.
The early maritime cultures of the Chinese, Phoenicians, Polynesians, and Vikings certainly made open-ocean travel a reality. Yet we have no tangible proof of their having used navigational instruments.
The knowledge required of a mariner in those instrumentless times was set forth in the Sanskrit Mu'allim of 434 A.D. "He knows the course of the stars, both regular, accidental, and abnormal, of good and bad weather: he distinguishes regions of the ocean by the fish, the colour of the sea, the nature of the bottom, the birds of the mountains, and other indications. And the only aids he possesseth are his memory, helped by a pilot book, and a sounding lead or staff."
Quite apart from one another the Chinese, Egyptians, Babylonians, and Greeks had discovered that they could relate their position on the earth relative to the stars. The observations by their astronomers would give birth to celestial navigation as it progressed from the 15th century onward. Simply stated, one's position on earth could be ascertained relative to a star (fixed point) by measuring the angle of elevation (altitude) of the star from the observer (apex) and the earth (horizon).
During the 15th century the Portuguese began to explore the west coast of Africa using coastal piloting. As word of Marco Polo's adventures in China and the treasures of the Orient spread, ocean travel to India and China demanded improved navigation. Prince Henry the Navigator founded a navigational school for his officers where he recruited astronomers, cartographers, mathematicians, and craftsmen to expand the science of navigation, construct navigational instruments, and draw up accurate charts.
Perhaps the earliest instrument, of which a rare few still remain, is the astrolabe or "astrolage". The first astrolabes were non-marine and were constructed in the Islamic countries of the Middle East which had absorbed and applied the remnants of Greek science and technology beginning in the 9th century A.D. The earliest surviving example dates from the 10th century. The astronomer's astrolabe was a complex and costly affair. In essence it was a mechanical computer coupled with an alidade mounted on a two-dimensional planisphere which could be rotated over a plate on which a network of azimuths and altitudes dividing the heavens were engraved. Around the rim were scribed the hours of the day, the days of the year, and the signs of the zodiac. Engraved on the backplate was more data.
The sea astrolabe was an adaptation of the astronomical type. It was much simplified, as the alidade and degree scale were adequate for the mariner's use. It was made much heavier to keep it vertical on the rolling platform of a ship, and cut-out to reduce disturbance in the wind. The sea astrolabe was introduced about 1460, but did not see general use until the beginning of the 16th century. Its use persisted until after 1670, particularly in the fleets of the Spanish and Portuguese, where it was in evidence early into the 18th century.
For the serious nautical collector the astrolabe is perhaps the ultimate. Only 56 examples are known to have survived. One must be extremely cautious in collecting. A good many reproductions, primarily of the astronomical type of these instruments, were made in the Islamic countries in the late 18th and 19th centuries.
A contemporary of the astrolabe, which may actually have been a predecessor of the sea astrolabe, was the simple quadrant. Like the sea astrolabe, the mariner's quadrant was adapted from its earlier and more complex astronomical counterpart. In design and function it was remarkably simple. It consisted of nothing more than a triangular plate, the apex of which was fitted with a plumb bob, with a pair of sighting pin holes on one edge. On the lower limb a degree scale was scribed, over which the plumb bob swung. In use, the observer merely lined up the celestial body viewed through the pin holes while an assistant read the position of the plumb bob on the scale.
Apparently the quadrant was in use well before 1450, although that was the first recorded mention of the instrument. Early types were often embellished with the appropriate landmarks at the point on the scale where their corresponding readings would fall (e.g., 39º - Lisbon).
The sea quadrant never seemed to gain much favor with the English sailors, although its use by the Dutch persisted through the 18th century. Both brass and paper-covered wooden examples survive.
The first real ancestor of the modern-day sextant as a multipurpose nautical instrument was the cross staff or Jacob's staff. It was first described in 1342 by a Jewish scholar named Levi ben Gerson. The instrument, as its predecessors, was an adaptation from an earlier astronomical surveying device. It consisted of a frame (staff) over 30 inches long with scales engraved on all four sides. Perpendicular to the frame ran two or more transoms or "crosses" (hence the name). By lining up the horizon with one end of a cross and the celestial object with the other end, the observer had a simple trigonometric computer.
The cross staff represented a great leap forward in the art and science of navigation, since it embodied all of the functions for recording the altitudes of the sun, stars, moon, and planets, as well as terrestrial sights - a function lacking in the astrolabe and simple quadrant.
Most frequently the observer's latitude was found by "shooting" the sun, an expression popularized because of the resemblance of the instrument to the cross bows of the period. To this day the navigator's celestial observations are still referred to as "shots".
Cross staves were generally constructed of hardwood (to prevent warping), although an ivory example has been preserved. Very few have survived even though their use was in evidence early into the 19th century. Because of their simplistic design it is likely that most were discarded as useless once they were brought ashore. When the mariner took his sun shots with cross staff, the blinding glare of the sun often caused him to turn his back. By using two crosses and adjusting their angles of incidence, the observer could sometimes read the shadow cast by the sun on his instrument. But this procedure was fraught with error. The next logical step in the evolution of navigation instruments was the development of the backstaff or Davis quadrant.
Mid-18th century backstaff, or "Davis Quadrant" made by C. Elliott
in New London, New England in 1760. Both the sight vane and shadow vane are
missing. The horizon vane remains. Sold by West Sea Company, and
now in the Portland Maritime Museum Collection, Portland, Oregon. (jpeg 21K)
This ingenious device was first proposed by English captain John Davis in 1594. The name quadrant came from the fact that 90º could be measured although there was no full 90º arc on the instrument. The backstaff consisted of two triangles, the larger of which was calibrated to 30º of arc and the arc at the apex of the instrument which was calibrated to 60º.
Most backstaves were of English manufacture, but American and Irish examples do exist. The backstaff gained rapid popularity after its introduction, particularly with the English and Dutch sailors. Much care was lavished on its construction in order to avoid warping and to ensure the accuracy of its scales. Early instruments were constructed of walnut, lignum vitae, and fruitwood and were characterized by in-line scales. Later models included ebony and even mahogany in their construction and featured diagonal, interpolative scales.
Until the very early years of the 18th century a mariner's navigation consisted of sunshots to determine the latitude and dead reckoning, coupled with piloting, to estimate the longitude. Latitude, the distance north or south of the equator, is the horizontal component of the imaginary grid system encircling the earth, unaffected by the earth's rotation relative to the stars. Longitude, the distance east or west on the earth's surface, is the vertical component of these lines of position. It changes constantly, with respect to the heavens, as the earth rotates. Thus a key element in most methods of determining longitude is precise time keeping.
The onset of the 18th century saw new methods and instruments innovated for finding the elusive longitude. Among these, the lunar distance method found favor with the English, culminating in the perfection of the reflecting circle by Mayer, Borda, and Troughton toward the end of the century. Another method, longitude by change in compass variation, promised an easy solution in theory, but was not precise enough to be of any value in practice.
The search for the longitude generated some bizarre proposals. In one case Sir Kenelm Digby claimed that he had caused one of his medical patients to jump with a start, even though the two were separated by a great distance. This was accomplished by placing some specially invented "powder of sympathy" into a bucket of water and then adding a bandage taken from the patient's wound. This "fact" led to the suggestion that every ship should be equipped with a wounded dog. On shore, a diligent individual equipped with a standard pendulum clock and a powdered bandage from the dog's wound, would dip the bandage into water at the stroke of each hour causing the dog aboard the ship to yelp at the appropriate instant!
The impractical application of all these systems was becoming tragically obvious. Several instances of entire squadrons of British ships being lost due to imprecise navigation occurred in 1691, 1707, and again in 1711. These losses provided a final impetus to the British Admiralty to pass a bill "for providing a publick reward for such person or persons as shall discover the Longitude," in 1714. The amount of the reward was £20,000 - a phenomenal sum at the time - indicative of the importance placed upon perfecting an accurate means of navigating.
Finally in 1735, John Harrison, a Yorkshire carpenter, successfully constructed the first marine chronometer having some components of wood and weighing 125 pounds! Because of its precise timekeeping ability, the chronometer, in perfected form, was later to become an indispensable addition to nearly every ocean-going vessel afloat. As a result of his successful contribution Harrison eventually received the reward nearly 40 years later. In the interim, the modern era in navigation had begun.
A large, unsigned mid-18th century "Hadley Octant", also known
as a reflecting quadrant. This example measures 20½ inches
tall and dates circa 1760. Scale division techniques had not
been perfected to allow makers (and users) the benefit of smaller
instruments at that time. (jpeg 23K)
The increased activity in "the search for the longitude" also spurred innovative interest in other areas of navigation. In 1731 John Hadley demonstrated his new reflecting quadrant to fellow members of the Royal Society in London. His quadrant was based on the principle of light reflection and angles of incidence described by Robert Hooke, Isaac Newton, and Edmund Halley nearly a century earlier. The principle is that when an object is seen through a double reflection its angle from the eye is twice the angle between the reflecting surfaces. Thus Hadley's quadrant, reading to 90º, actually required an arc of only 45º, one eighth of a circle, or an "octant". Basically the instrument consisted of a triangular wooden frame with a swinging index arm pivoted at the apex. a mirror was fixed at that point which would move with the arm. A second mirror, half of which was transparent, was fixed to one limb with the sight attached to the opposite limb. A precise scale, calibrated in degrees, was scribed on the arc of the bottom limb of the triangle, across which the index arm moved.
Quite independent of Hadley, Thomas Godfrey, working in Philadelphia, had also devised an improved altitude-measuring device based on the same principle, The Royal Society recognized the equal contributions of both men and awarded them a prize of £200 each.
The improvements of the Hadley quadrant, or "octant" as it came to be known, over previous instruments was immense. Not only was it more accurate, it provided simplicity of operation, and the ability to "capture" the object being sighted for rapid, multiple sightings. The merits of the quadrant were immediately noticed by the British Admiralty and it was quickly put into commercial production. Even so, the instrument did not find popular acceptance and general use until about 1750.
The earliest Hadley quadrants, like backstaves, were constructed of walnut or other indigenous woods, the scales being engraved on boxwood. With the discovery and growing importation of exotic woods such as ebony and African mahogany in about 1750, the use of mahogany was quickly implemented, gradually giving way to the exclusive use of ebony.
Around 1760 ivory was introduced as the material for scales and nameplates because of its durability, ease of engraving, and light color which provided for easier reading. About the same time brass began to replace wood as the preferred material for the index arm, a trend that would eventually culminate in the elimination of wooden components altogether.
The early quadrants were characterized by large frames, 18 to 20 inches in length, sometimes 24 inches and larger! The size of the instrument was dictated by the fact that the scales had to be calibrated by hand - the larger the instrument the easier the division of the scale. The more accurate division of the scale, results of work by the Sissons and John Bird, and incorporation of a secondary vernier scale on the index arm, increased accuracy of instruments to within one minute of arc and created smaller, easier-handled instruments. Early on, colored glass shades were added to aid the mariner when taking his sights in the sun's glare or in hazy conditions. Another original feature was the "backsight" fitted below the horizon mirror. It enabled the observer to sight on a celestial object using the opposite horizon (over 90º) in cases where the fore horizon was indistinct. About 1780 the introduction of the tangential screw fine adjustment represented the last major change in the basic operation of the octants and sextants for the next 150 years!
While the "Hadley quadrant" was technically a "Hadley octant" because its arc was one eighth of a circle, the term "quadrant" is still applied to those large instruments produced prior to 1780. Fine examples are still to be found by the collector, but at premium prices. Later, and smaller octants are much more common and are priced accordingly.
Along with the development of the octant came the more familiar sextant. A common misconception is that the sextant is a relatively recent innovation in comparison to the octant. In actuality the sextant is very nearly contemporary with the octant. Another generalized assumption is that octants were constructed of wood and sextants of brass. Some very lovely examples of ebony and ivory sextants have been preserved from the late 18th century. Likewise, in the early to mid-19th century, numerous examples of brass octants were produced.
When Nevil Naskelyne, England's future Astronomer Royal, published his lunar distance method in 1764 the immediate need and popular demand for an instrument with the capability to record over 120º of arc was established. The result was the limited production of the sextant, comprising 60º of actual arc (1/6 of a circle) using the Hadley principle. The reflecting circle, already mentioned, was also refined during this period. The earliest known sextant is dated 1757 and was the work of the master of hand division, John Bird.
A large reflecting quadrant, possibly by Thomas Swann of Liverpool, circa 1775.
Note the introduction of a vernier scale on the index arm and the use
of ivory for the scales. A reinforcing brass "furniture" piece braces
the arm. (jpeg 31K)
Perhaps the greatest boon to the production of the sextant came in the period from 1768 to 1774 when Jesse Ramsden invented and perfected his dividing engine. With the advent of this tool, scales on instruments could be engraved swiftly and accurately at minimum expense. The size of an instrument was no longer a factor in its accuracy, allowing makers to produce smaller, easier-handled, and more economical models. As such, demand increased and business flourished, particularly during the period of the Napoleonic Wars near the turn of the 19th century.
One of the greatest concerns of the nautical instrument makers throughout history has been accuracy. Because of the severe conditions and extremes encountered at sea, a poorly constructed instrument was apt to shrink, expand, warp, or crack rendering a false, and potentially fatal reading. Numerous materials and innovations were tried in an attempt to ensure rigidity and stability of the octant and sextant.
Double Frame or "Pillar Sextant" signed "Walker, Liverpool",
circa 1830. By now virtually all such instruments were calibrated
on Ramsden's scale dividing engine or a version thereof. Note the
prominent brass screws holding the pillars together. (jpeg 18K)
Perhaps the most famous of these innovations was the pillar frame sextant patented by Edward Troughton in 1788. The frame was constructed of two parallel strips of sheet brass joined together by machined pillars secured with screws. This "double frame" sextant was made by numerous makers using slight modifications for more than one hundred years following. A variation of the double frame was the "bridge frame" sextant made by Ramsden and a few other makers late in the 18th century. Examples of both types of these early sextants are quite scarce and highly sought after by collectors.
Although numerous frame styles were tried including the "bell", lattice, straight, arched, and braced, the final solution to the problem of rigidity and stability was found by using bell bronze alloy in casting the frame. The most prevalent frame style going into the 20th century was three adjacent circles.
Typical mid-19th century ebony and ivory octant. The antiquated
backsight has been eliminated and the index arm sports a metal brace. A fine
adjustment tangent screw has been added to the vernier. (jpeg 21K)
Among the earliest accessorial improvements was optical enhancement of the image by means of a telescopic attachment. Evidently this innovation came very near the advent of the sextant itself, even though telescopes generally were not incorporated for use with octants until about 1830. Strangely, a similar such disparity was the use of a handle. Even the earliest sextants had handles, whereas their inclusion in the construction of octants came at about the same time that optics were added. Another feature unique to the sextant and lacking in the octant was the scale/vernier magnifier. Because of its smaller size and finer scale the sextant was read by means of a small magnifier affixed to the index arm. The octant required none.
The ultimate evolution of the octant. This lovely example signed
"Parkinson & Frodsham, London" dates circa 1860. Note the addition
of a handle and horizon shades on the instrument itself. As for
accessories, a peep sight and telescopic sight have been added.
At this point, the only thing that differentiates the octant
from a sextant is the arc of its scale. (jpeg 24K)
By 1850 the demise of the octant was imminent, even though its use persisted into the 20th century. The superiority of the sextant in terms of accuracy, compactness, and durability was indisputable.
The last half of the 19th century saw little change in navigational instruments in general and the sextant in particular. A good sextant would provide 50 years of service. As makers fulfilled the need of mariners and demand declined there was little incentive for improvement in manufacture or design.
An average late 19th century sextant signed "Negus, New York".
The sextant changed little in the century between 1820 and 1920. (jpeg 31K)
With the onset of World War I and the loss of hundreds of vessels and the construction of many more, nautical instrument manufacture was given a second wind. The advent of the drum micrometer sextant by the end of the War was the greatest single improvement in the sextant during this century. By employing a continuous tangential screw attached to a calibrated knob, an instantaneous reading visible in the low light situations of the navigating bridge was readily obtainable. A further improvement was the incorporation of a small battery-powered light for easier reading.
Actually this "new" invention was not new at all. There is an example of the drum micrometer incorporated into a sextant made by Ramsden that dates circa 1780. Before that it was an astronomical fitting dating back to the 17th century!
Just prior to World War II the long evolution of the sextant culminated in the invention of the ball recording sextant. It is ironic, and perhaps fitting, that the final form of the sextant was not a sextant at all but a much earlier ancestor, the true quadrant. Developed for use at night when no horizon was visible, the recording "sextant" used no reflecting mirror. Rather, the celestial object was viewed directly as with the true quadrant. Instead of using a plumb bob, a liquid-damped steel ball recorded the altitude of the object on a screen. A drum micrometer was used to determine the precise altitude reading. Because of its size, easy reading, and nighttime capability, the recording sextant found favor with airplane navigators, leading to "aircraft sextants" built on the same principle. Numerous examples still exist for the collector's choosing at inexpensive prices.
This has been a capsule look at the history of the sextant, and in no way constitutes a thorough treatise on the subject. Numerous topics such as the ring dial, circumferentor, surveyor's sextant, pocket or box sextant, and other instruments with related uses have not been discussed. A bibliography has been provided to aid the reader in further study.
Good hunting!
ca. 9th century A.D.: First astronomical quadrant
9th century A.D.: First astronomical astrolabe. Brass and/or wood
10th century A.D.: Earliest surviving example of astronomical astrolabe
11th century A.D. Directional properties of lodestone employed
1187: Earliest known description of the compass
1342: Cross staff described by Levi ben Gerson
1415: Prince Henry the Navigator establishes his navigation
institute
ca. 1450: First recorded mention of the mariner's simple
quadrant, although the instrument predates this period
ca. 1460: First mariner's astrolabe. Sheet brass construction
ca. 1500: Introduction of cast brass mariner's astrolabe
1555: Earliest surviving example of a dated mariner's
astrolabe
1594: Captain John Davis introduces backstaff
1631: Pierre Vernier describes his method for making an
improved scale reading
17th century: Backstaff constructed of walnut, lignum-vitae,
and fruitwood. In-line scale.
18th century: Backstaff constructed of ebony, rosewood, and
mahogany in addition to earlier materials. Diagonal scale.
1731: John Hadley introduces reflecting quadrant
1735: John Harrison's first chronometer
1756: Mayer's reflecting circle
1757: First "Hadley sextant" by John Bird. Brass, ebony and
ivory introduced as materials for constructing instruments. Brass
begins replacing wood as the preferred material, starting with
the index arm in quadrants
1768: Scale dividing engine by Jesse Ramsden
ca. 1770: Finite scale on vernier calibrated "20 - 0 - 20".
Earlier instruments calibrated "10 - 0 - 10"
ca. 1780:Instruments decrease in size with widespread use of
dividing engine in scale manufacture
ca. 1820: Elimination of backsight feature in the construction
of most octants
ca. 1840: Octants and sextants more similar in appearance with
the inclusion of handles, optics, and all-brass construction in
octants
ca. 1880: Cessation of the production of wooden instruments of
the navigational type
ca. 1918: Inclusion of the drum micrometer
feature in modern sextants
ca. 1940: Ball recording sextant, aircraft sextant