The iconic image of a sailor taking a sight with a sextant from the platform of a sailboat is a romantic vision to be sure—until you try it and the reality sets in. Lining up an erratic horizon with the sun while the boat swings thru a multitude of gyrations is a skill that takes practice and patience. Add a wee case of mal de mar and then you’ve got a real challenge.
However, once you become proficient and are able to navigate your way cross an open ocean, it is indeed a satisfying pleasure. You might then bask in the knowledge that you are, in essence, using the same instrument and technique that Captain Cook used on his world-changing explorations almost 250 years ago.
Simply stated, the sextant is an instrument used in navigation for measuring the altitude—or angle—of the sun above the horizon at sea. The object measured can also be the moon, planets or stars, or, a combination of these. With an accurate watch and navigation tables, one is able to calculate a single position’s latitude and longitude.
Sir Isaac Newton first envisioned the concept, but two other men independently developed the idea around 1730: Englishman John Hadley, and Thomas Godfrey, an American. The instrument was then called the “octant” because it measured 1/8 of a full circle, or, a 45-degree arc. During their famous westward explorations, Lewis and Clark used an octant for mapping.
The sextant is based on an arc of 1/6 of a circle, or, a 60-degree arc, and was developed in 1759 under the direction of Captain John Campbell with the help of London instrument maker John Bird. The difference between an octant and a sextant is the limit of their measuring scales, from 90 to 120 degrees of elevation, but both work on the same principle. Measuring the sun at its highest noon position only requires an arc of 90 degrees, but later, attempts at measuring other celestial objects at hours other than noon required a greater angle.
Featured in PatentWear’s Sextant design is George Davidson’s patent of 1866, an early attempt at using a level as an artificial horizon for taking sights. However, accelerations at sea proved too great and unpredictable for the concept to be useful except on a large and stable ship, leading to the realization that two levels would be required to be accurate: one horizontal and one vertical. In 1919, aviation navigation took a great leap forward when the famous Portuguese navigator, Admiral Gago Coutinho, designed and used this artificial-horizon sextant on his aeronautical system for a flight just short of twelve hours to Rio de Janeiro from the Cape Verde Islands. In 1929, Captain Wittenman used a Coutinho sextant to navigate the Graf Zeppelin around the world. With such a dazzling record, the design became the hit of the 1930 Berlin Air Show, and was used throughout the ’30s by major airlines throughout the world.
With the advent of modern GPS, chart plotters, and satellite navigation there is still a prudent rationale for learning traditional celestial navigation with a sextant. Beyond the romance of the past, there is the more practical and wise consideration of both Murphy’s Law and the Golden Rule of Blue Water Sailing: if it’s electronic, it will fail sooner or later—and usually when you need it most! Another sailing adage applies as well: hope for the best but prepare for the worst. In that light, one might as well hone those celestial sight skills, carry back-up paper charts, and share in the same delight of discovery that early explorers must have surely felt.