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Mounted on my wall at home is a sextant. It is not an especially old or valuable one. Made from brass and steel, it was manufactured by Heath and Co of London in the 1950s, and still works well. The advent of modern marine navigation aids have made it largely redundant. It never fails to attract the attention of visitors, whether they have an interest in the sea or not. Some are drawn to the precision of its engineering, others to the elegant shape, with its curving scale and polished mirrors. The design is so perfect for its task (measuring the angle of celestial objects above the horizon) that it has barely changed in two hundred and fifty years. If James Cook was to walk through my front door, he would recognise it instantly.

Mariners have used the sun and stars to navigate by from the earliest times. Those with fixed locations, like the Pole Star, or with predictable cycles of movement, like the sun, are especially valuable for crossing open water. This spurred the creation of devises to help sea captains. In the Pacific, Polynesian seaman used stick charts. The Ancient Greeks invented the astrolabe, which Islamic navigators later perfected. In the 15th century, the first Portuguese explorers began to venture out into the Atlantic and down the coast of Africa. They found that they needed a practical device to measure the angle of stars above the horizon, and developed the first quadrants to help them. These were simple quarter circles of wood or brass (hence the name) with a plumb line hanging from the centre. One sailor would site along the top of the quadrant at a star, while a colleague read off the angle where the plum line crossed a scale.

The quadrant was widely used by European sea captains for the next hundred years, although it did have a number of disadvantages. The use of a weight on a string meant a calm sea was needed for an accurate reading. The most valuable measurement for a navigator calculating his latitude was the altitude of the sun at noon, but a quadrant required the user to look directly at it. It was John Davis, an Elizabethan explorer from Devon who solved these problems. He developed the Davis quadrant, or back staff, which allowed the navigator to stand with his back to the sun, and use the shadow it made instead. A half-transom was slid along a graduated staff until its shadow fell on a slot through which the sailor viewed the horizon. The angle of the sun above the horizon was then read off the scale.

Davis’s invention lasted till the early part of the 18th century, when it was first replaced by John Hadley’s Quadrant in 1731, and shortly after by the even better sextant (meaning a sixth of a circle) following some suggested improvements from Royal Navy Captain John Campbell. Smaller, much more accurate, and handier to use than the back staff, the sextant required little more than a steady hand and a good eye. First the navigator focused on the horizon through a small telescope mounted on the frame. Just in front of this was a glass plate, one half of which has been mirrored. A swinging arm with a second mirror fixed to the top was used to find the sun, and then “bring it down” to the horizon. When the two were aligned, the arm was clamped, and the angle read off. Finally sextants had smoked glass plates to ensure the user was not blinded.

Simple, elegant and easy to use, the sextant had numerous advantages over the back staff. Most obviously it allowed the user to face the object they were observing. It could also be used more easily on a moving ship and, with the smoked glass filters swung aside, at night to observe the height of the moon and stars. Provided it is well maintained, a sextant should be accurate to within a half minute. Additionally, it can be used for measuring the angle between any two points, giving it a whole variety of other uses at sea, such as checking that an anchor is holding, calculating the relative speed of two ships, or for finding how far away an object is of a known height, such as a lighthouse.

The stars that helped the first navigators to sail across open water have now been supplemented by our own, man-made, celestial bodies. Global Positioning Satellites (GPS) provide the fixed points that locate ships, cars, planes and even mobile phones on the surface of the earth, all with a degree of accuracy that a mariner with a sextant cannot compete with. But I will still keep mine in good working order, just in case.

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