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The Sextant

The major problem with back-sight instruments was that it was difficult if not impossible to sight the moon, the planets or the stars. Thus, toward the end of the 1600's and into the 1700's, the more inventive instrument makers were shifting their focus to optical systems based on mirrors and prisms that could be used to observe the nighttime celestial bodies.

The critical development was made independently and almost simultaneously by John Hadley in England and by Thomas Godfrey, a Philadelphia glazier, about 1731. The fundamental idea is to use of two mirrors to make a doubly reflecting instrument—the forerunner of the modern sextant.

How does such an instrument work? Hold the instrument vertically and point it toward the celestial body. Sight the horizon through an unsilvered portion of the horizon mirror. Adjust the index arm until the image of the sun or star, which has been reflected first by the index mirror and second by the silvered portion of the horizon mirror, appears to rest on the horizon. The altitude of the heavenly body can be read from the scale on the arc of the instrument’s frame.

Hadley's first doubly reflecting octants were made from solid sheets of brass. They were heavy and had a lot of wind resistance. Lighter wooden instruments that could be made larger, with scales easier to divide accurately and with less wind resistance quickly replaced them.

Early Hadley octant. This mahogany octant was made about 1760 by the famous London maker, George Adams.

Hadley' octant of 1731 was a major advancement over all previous designs and is still the basic design of the modern sextant. It was truly a "point and shoot" device. The observer looked at one place - the straight line of the horizon sighted through the horizon glass alongside the reflected image of the star. The sight was easy to align because the horizon and the star seemed to move together as the ship pitched and rolled.

We have seen how navigators could find their latitude for many centuries but ships, crews and valuable cargo were lost in shipwrecks because it was impossible to determine longitude. Throughout the seventeenth century and well into the eighteenth century, there was an ongoing press to develop techniques for determining longitude. The missing element was a way to measure time accurately. The clock makers were busy inventing ingenious mechanical devices while the astronomers were promoting a celestial method called "lunar distances". Think of the moon as the hand of a clock moving across a clock face represented by the other celestial bodies. Early in the 18th century, the astronomers had developed a method for predicting the angular distance between the moon and the sun, the planets or selected stars. Using this technique, the navigator at sea could measure the angle between the moon and a celestial body, calculate the time at which the moon and the celestial body would be precisely at that angular distance and then compare the ship’s chronometer to the time back at the national observatory. Knowing the correct time, the navigator could now determine longitude. When the sun passes through the meridian here at Coimbra, the local solar time is 1200 noon and at that instant it is 1233 PM Greenwich Mean Time. Remembering that 15 degrees of longitude is equivalent to one hour of time gives us the longitude of 8 degrees, 15 minutes West of Greenwich. The lunar distance method of telling time was still being used into the early 1900’s when it was replaced by time by radio telegraph.

An octant measures angles up to 90 degrees and is ideally suited for observations of celestial bodies above the horizon. But greater angle range is needed for lunar distance observations. It was a simple matter to enlarge Hadley's octant, an eighth of a circle, to the sextant, a sixth of a circle, that could measure up to 120 degrees.

An early sextant by John Bird. The first sextant was produced by John Bird in 1759. This is a very early example of his work now in the Nederlands Scheepvaart Museum in Amsterdam. The frame is mahogany with an ivory scale. It is so large and heavy that it needed a support that fitted into a socket on the observers belt.

A brass sextant by Dollond. Here’s a fine brass sextant from the early nineteenth century by the master London instrument maker John Dollond.

In the first half of the eighteenth century there was a trend back to wooden frame octants and sextants to produce lighter instruments compared to those made of brass.

Ebony sextant. A very handsome example by H. Limbach of Hull of a sextant with an ebony frame. Ebony was used because of the dense wood's resistance to humidity. The scale and vernier were divided on ivory, or should we now say bone. The design was not successful because the wood tended to split over the long arc of a sextant.

Examples of sextant frame designs. A sample of variations in frame design. The challenge was to produce sextant frames that were light weight, low wind resistance and with a minimum change is dimensions with changes in temperature. As you can see, some of them are quite esthetically pleasing.

Probably the finest 18th century instrument maker was the Englishman Jesse Ramsden. His specialty was accurate scale division. Here’s a small brass sextant that Ramsden made shortly before his death in 1800. Ramsden's major achievement was to invent a highly accurate "dividing engine"—the apparatus used to divide the scale into degrees and fractions of degrees. His design was considered so ingenious that the British Board of Longitude awarded Ramsden a prize of 615 pounds—in 18th century terms, a small fortune. His "dividing engine" now resides in the Smithsonian Institution in Washington.

Ramsden pentant. To be correct, the instrument should be called a pentant, a fifth of a circle, rather than a sextant. This jewel is only 4 1/2 inches radius. The scale is divided on silver from minus 5 degrees to 155 degrees with each degree further divided in three to 20 arc minutes. As you can see, the scale is beveled at 45 degrees. Why set the scale at an angle to the frame - perhaps just to show that he could do it!

The development of more precise scale division was a milestone in instrument development. Certainly, it permitted more accurate observations but it also permitted smaller, lighter, more easily handled instruments. The sextant you see here is my all-time favorite.

Modern sextant, 1988

The standard of excellence for post World War II sextants was established by the C. Plath firm in Germany. Here's an example from 1988. Among its attachments are an unsilvered horizon glass that lets the observer see the full horizon as a straight line across the round horizon glass; an astigmatizer lens that distorts the image of a star into a straight line for precision alignment with the line of the horizon; a quick-release drum micrometer that reads to one-tenth of an arc minute. There’s also a battery-supplied lighting system for the drum micrometer and the bubble artificial horizon attachment. This attachment and a monocular telescope complete the kit. But, for all the fancy modern refinements, the optical system is exactly what John Hadley proposed in 1731.

The problem of finding your location when you can’t see the horizon to take a sun or star sight has challenged explorers, map makers and navigators for hundreds of years. Early in the 1730s instrument makers began developing artificial horizons for use with quadrants. Of course, the explorers and mapmakers working inland could not use the horizontal line to the natural horizon of the sea and so they needed an artificial horizon to establish a line of reference for measuring the altitude of celestial bodies.

Mercury artificial horizon. A very elegant three-piece explorer and mapmaker's kit by Carey of Pall Mall, London from 1880. The instrument is a pentant, a fifth of a circle capable of measuring angles up to 170 degrees; mounted on a collapsible aluminum stand. Around the base you can see the parts of the mercury bath artificial horizon. Mercury was poured from the iron bottle into the trough to form a shiny horizontal surface to catch the reflection of the celestial body. The triangular glass tent was placed over the trough to keep the wind from disturbing the surface.

A mercury artificial horizon in use. Here you see the famous American explorer, John Charles Freemont, using a sextant and mercury artificial horizon to find his position during his expedition of 1842 to map the Oregon Trail. The sextant had to be pointed downward to view the reflection of the celestial body on the surface of the mercury pool through the clear portion of the horizon glass while simultaneously adjusting the index system to bring the image reflected by the two mirrors alongside. The mercury artificial horizon was popular with explorers for more than a century but it was hard to use on shipboard with a rolling deck.

A little earlier, we were talking about the explorers' and mapmakers' need for an artificial horizon when they couldn't see the natural horizon. Well, there are two classes of modern navigators who absolutely need an artificial horizon - the aviators and the submariners. Aviators find the natural horizon so far below them that it is useless and furthermore, they are frequently flying above the clouds. Conversely, even on the surface, submariners are so low in the water that a sight to the horizon is unreliable. In fact, it is the unique needs of the aviator that has driven sextant innovation throughout the twentieth century.

For a while, balloonists of the late nineteenth century tried to use conventional sea-going sextants but their need for artificial horizon instruments soon became apparent.

Balloon sextants. The optical concept of these instruments is to the reflect the image of a bubble from a small spirit-level into the line of sight so that the bubble and the celestial body can be viewed simultaneously. The one at the top, from 1880, is derived from an instrument invented by Captain Abney many years earlier for use in chart making. The instrument in the middle is by Cary of London, 1900, and the one at the bottom is one of their later models with an electrical lighting system from 1910 - just about the time of the Wright brother's first powered flight.

The rapid development of heavier-than-air craft during World War I lead to airplanes with increasing range and greater need for accurate navigation instruments and techniques, all depending on artificial horizons.

Gyroscopic aircraft sextant. An early 1920's gyroscope sextant by a Parisian company with the descriptive name of La Precision Moderne. A spinning mirror, mounted on the top of an air driven gyroscope reflects an image of the celestial body into the line of sight, much as with the old-fashioned mercury artificial horizon.

One of the most important pioneering trans-Atlantic flights was by the famous Portuguese aviators, Sacadura Cabral, pilot, and Admiral Gago Coutinho, navigator, in 1919. They flew 11 and one half hours from Cape Verde Islands to Rio de Janeiro carrying an artificial horizon sextant designed by Admiral Coutinho.

The System Gago Coutinho. The design was based on two spirit level tubes – one to keep the sextant horizontal and the other to keep the sextant vertical. The sextant proved itself again in a flight from Lisbon to Rio de Janeiro in 1927 with Captain Jorge Castilho as navigator.

The Portuguese Navy, who had rights to the development, contracted with the prestigious German firm of C. Plath for production. In 1929 Captain Wittenman navigated the Graf Zeppelin around the world using a Coutinho sextant. With this spectacular record, the design was the hit of the 1930 Berlin Air Show. It was used by many of the major airlines of the world throughout the 1930’s. In addition to an artificial horizon, aircraft sextants needed a device to average the values of six or eight sights taken in succession to average out the small errors in aligning the sight and to compensate for the rapid movement of the aircraft. Here are some prewar examples.

Early bubble sextants with averagers

WWII Aircraft sextants

Of course, World War II was a powerful influence that produced an explosion of designs and a number of U.S. instrument makers Fairchild, Link, Pioneer and Agfa-Ansco made important improvements. C. Plath in Germany and Tamaya in Japan supplied the Axis

There has been very little evolution of hand-held celestial navigation instruments since the end of World War II. Faster flying aircraft lead to the development of periscope instruments that minimized wind resistance but Radio Direction Finding and then inertial guidance became the standard for aircraft navigation and celestial was no longer needed.

The early space flights used an especially designed sextant. In the remoteness of space there is no such thing as "horizontal" or "vertical". Instead, the instrument was designed to measure the angle between the edges of the earth or the angle between celestial bodies to determine the space craft's position in space. But again, electronic techniques for positioning in space became the standard.

Gemini IV sextant

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