The History of the Sextant (continued)

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.

Gemini IV sextant

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.

So, where are we? I can tell you with great precision, within about thirty meters, that we are at latitude 40 degrees, 12 minutes, 32 seconds North and 8 degrees 25 minutes 20 seconds West. Those of you in the back of the room probably are a little east of that. How do I know with such certainty?

Global Positioning System or GPS receiver. Instead of measuring angles of the celestial bodies above the horizon, it computes our position by measuring the time it takes for radio signals to arrive from three or four of the many man-made satellites that are in known positions in orbit around the earth.

A significant part of the romance of the hand held instruments for taking the stars that we have seen this evening is that they all soon will be obsolete, outmoded by GPS. Yes, there are still quite a few old-line navigators that refuse to give up their nautical almanac, their chronometer and their sextant for this new fangled electronic stuff. What if the batteries go dead or the thing falls overboard? But finally, there is the simple satisfaction of shooting a star, noting the time, reading the almanac and making the calculations to find out where you are.

Peter Ifland, Ph. D. in Biochemistry (U. of Texas)
Commander in the US Naval Reserve
Author of Taking the Stars: Celestial Navigation from Argonauts to Astronauts, The Mariners' Museum, Newport News, Virginia, 1998
and of numerous articles about navigation and navigation instruments