The History of the Sextant (continued)
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. Theres 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 cant 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.
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.
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.
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 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 1930s. 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.
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.
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?
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.
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