The weakness of earlier instruments
The astrolabe, the quadrant and the backstaff all required the observer to align an object in the sky with the horizon while the ship moved. With a quadrant, the plumb line swung. With the backstaff, the observer had to line up two shadows and a horizon at once. The accuracy that could be achieved was limited by the observer’s skill and by the motion of the vessel, and results were rarely better than about a degree under ordinary sea conditions.
The idea of reflection
The reflecting instrument solves this by letting the observer see two things in one view: the horizon directly, and the sun or star reflected by a mirror. When the mirror is rotated until the reflected object appears to touch the horizon, the angle through which the mirror has been turned gives the height. Because both images move together as the ship pitches, the observer need only keep the two in contact, which he can do even when the instrument itself is moving about.
The principle was described in the late 17th century by Isaac Newton in a paper that was found only after his death and was brought to light afterwards through his contacts with Edmond Halley. In 1730 and 1731 the English mathematician John Hadley presented a reflecting octant to the Royal Society, and in about the same period Thomas Godfrey, a glazier in Philadelphia, independently built a similar instrument. Both are credited in the history of the invention. The name “octant” refers to its arc of one eighth of a circle, which, because of the doubling effect of the mirror, could measure up to ninety degrees.
Hadley’s octant at sea
The octant was tested at sea in the 1730s and found to work better than the instruments it replaced. Its scale was read with a pointer, the index arm, and a pair of mirrors: the index mirror, which turned with the arm, and the horizon glass, which was half silvered so that the horizon could be seen through the clear half. Coloured shades were slid into place to protect the observer’s eyes when sighting the sun. Makers in England produced the octant in wood, then in brass, and it spread through the navy and merchant ships.
The sextant
A navigator often needed to measure angles larger than ninety degrees, notably the angular distance between the moon and the sun or a star, in the lunar distance method. The solution was to increase the arc to a sixth of a circle, which gave a range of up to about 120 degrees. In the late 1750s, the London instrument maker John Bird made a sextant for the Royal Navy on a suggestion from Captain John Campbell, and others followed. Makers such as Jesse Ramsden developed dividing engines, devices for cutting the scale evenly in brass, so that scales could be made more accurately and in greater numbers. Later sextants included a telescope, a vernier or micrometer drum for fine readings and several filters.
The sextant remained the standard sea instrument for more than two centuries, and its form changed little from the late 18th century.
From sight to position
A sextant sight on its own is only a measurement, and navigators needed methods to turn it into a position. The simplest was the noon sight: measure the sun’s height at its highest point, look up its declination in the almanac and calculate the latitude. The Nautical Almanac, first published in Britain in 1767, supplied the necessary data. By adding a time from the marine chronometer, the navigator could also find longitude, using a sight of the sun, a star or a planet taken at a known time.
In the 19th century, methods moved forward. An American captain, Thomas Sumner, reported in 1837 that a single sight of the sun placed the ship somewhere along a line, and that two sights at different times gave a position where the lines crossed. The French naval officer Marcq de Saint-Hilaire published a more convenient version in 1875, called the intercept method, in which the navigator assumes a position, calculates what the sextant reading should be and then plots the difference. Later tables of precomputed values saved the labour of long trigonometry, and books of sight reduction tables became part of every navigator’s kit.
Routine at sea
On a ship in the great age of sail and steam, celestial navigation had a daily rhythm. At dawn and dusk, when stars and horizon were both visible, the officers took star sights. Through the day, they took sun sights, and at noon, they determined the latitude and compared the day’s run with the one before. The result was marked on the chart, and the master and officers talked about the day’s progress.
The method was demanding but accessible. On the clipper tea races, for instance, captains pushed their ships hard and used the sextant to make the most of favourable winds and currents. At the other extreme, in 1916, Frank Worsley navigated the small boat James Caird across the Southern Ocean to South Georgia, after the loss of Shackleton’s ship in the Endurance expedition, taking a few sights under extremely difficult conditions.
Decline and survival
Radio, radar, hyperbolic systems and finally satellite positioning replaced the sextant as the primary means of finding position. Large ships today rely on electronic fixes, and the sextant is not required equipment on many of them. Even so, courses in celestial navigation continue to be taught in some maritime training, because they give a method that does not depend on power or external signals. The sextant is also appreciated by yachtsmen and historians as a link with the earlier tradition.