July 2016

Maritime navigation: 18th century innovations

A good Navigation is that art which instructs the mariner in what manner to conduct a ship through the wide and trackless ocean, from one part to another, with the skipper had to master greatest safety, and in the shortest time possible. –John William Norie celestial navigation.

Sailors need reliable navigation to accurately travel from point A to point B.

Early navigation techniques before the 1700s relied on celestial readings. Naviga- tors would espy a particular star or the sun, and measure the angle between the horizon and the sun or star to determine latitude.

Latitude was the angular distance (in degrees) of the North Star (Polaris), or the south celestial pole star (South Star), above the horizon. Using latitude sailors were able to estimate a ship's global positioning relative to north or south. Sextant, Wikipedia creative Without reliable navigation, early sailors were confined to the guiding landscapes commons of coastal sites and dared not venture out to open sea without risk of getting lost or swallowed in unpredictable winds and currents.

A good skipper had to master celestial navigation.

Longitude (or use of meridian lines) was a trickier problem to solve because it Longitude was a trickier required comparing the time-of-day difference between the sailor's starting loca- problem to solve tion and ending location. But there was no reliable clock with which to accurately capture time of day. A reliable clock would allow accurate calculation that gave a measure of east and west, rather than just rely on a measure of north and south that latitude offered. So longitude was useful to discover. Together with latitude, longi- tude could afford sailors a more precise global position.

So without a clock, an accurate one at that, latitude was all those early sailors had to go by. And navigation devic- es were primarily simple, like the cross-staff, quadrant or octant.

But they improved over time and, since the days of Captain Cook, a seaman with a clear horizon needed a sextant and a chronometer to ascertain his position.

The sextant was an instrument with a calibrated arc one-sixth of a circle, or 60 degrees, and is still very much in use in navigation today.

What about this revolutionary angle-measuring device, and how did it evolve?

Sextant, Wikipedia creative commons

1 Maritime navigation: 18th century innovations

Let’s go back to early astronavigation, the sailors’ dependency on observing the angle between the horizon and a chosen heavenly body to figure out latitude. To find the way home from a voyage, navigators sailed north or south using the bearing of the sun or star, and veered left or right using calculations to maintain a constant angle to the heavenly body. To figure out the right angular alignment, navigators in the 16th and 17th centuries used an astronomer's quadrant. But the quadrant was susceptible to wind. Blustery sea conditions meant rolling waves and high winds, so angle determination was no easy matter. An alternative, the astrolabe became popular for its reliability under adverse seas. By hanging it down perpendicu- lar to the ocean, and perfectly lining up its two pinholes, the astrolabe measured with accuracy angular heights of the Polaris or South Star. When a ray of sunlight passed through the top pinhole, it angled through the lower pin- hole and a sailor was able to read the angle’s measure from a scale along the circumference of the astrolabe. Then came the cross staff, in use by early European navigators, who saw its features grow elaborate over time. This device worked by placement to the eye to observe the sun. With an adjustable crossbar, a sailor could line up the sun or star and read angular altitude (degrees) on a scale on the staff. But nothing beat the sextant of the 18th century. An advanced angle-measuring device, the sextant was used in conjunction with astrological devices that deter- mined the position of the sun, moon and stars (latitude), and with a time-telling device (such as the chronometer) to establish, by celestial calculation, longitude. The sextant came about when intuitive astrologers and instrument makers began to experiment with optical sys- tems that had mirrors and prisms. The history of the sextant points to a precursor in a bi-mirrored instrument that was independently, and simultaneously, developed around 1731 by Englishman John Hardley and American Thom- as Godfrey. Today, navigation by sextant still uses two mirrors, one half-silvered to allow the passage of light so one can look at the horizon through this mirror. Readings are determined by a scale on the arc of the sextant, relative to the angular altitude of the sun or the star to the horizon. Still very much a complex mathematical method, needing calculations, corrections, tables, and astronomy, the sex- tant determines with accuracy the angle between heavenly bodies and the horizon. The longitude challenge

As revolutionary as the sextant was, the Harrison clock, from which the modern chronometer emerged, was a major advance in timekeeping and resolving longitude.

From very early days, navigation and timekeeping went hand in glove. An officer or senior crewman in charge of the main deck navigated the ship and posted lookouts, with the aid of ‘watch’ periods across 24 hours. Audible cues in the form of a loud bell rang once after the first thirty minutes of each watch, twice after an hour, thrice after an hour and a half, and so forth, to alert crewmen of time.

The chronometer, when it was discovered, gave a better measure of time and was significant for longitude. Using a chronometer, a sailor could determine the exact time-of-day difference between their starting location and ending location, and used calculation to establish longitude.

Chronometer, Wikipedia creative commons

2 The importance of understanding longitude (or median lines) was that it qualified a comprehensive global position determined by both latitude and longitude, measured in degrees. Lines of latitude measured from the sun deter- mined the ‘true’ north and south and ran parallel to the equator; longitude was a measure of east and west and was based on a principle of time difference.

But before English clockmaker John Harrison pulled it off in the 18th century, there was no accurate measure of time. No clock existed that was precise enough over periods of time or at varying levels of temperature and humidity for sailors to determine an exact time of day.

John Harrison, Wikipedia creative commons

Harrison was a genius with a passion; he wouldn't let an idea go. When the British government threw down the gauntlet and passed the Longitude Act in 1714 as a scientific challenge, the Brits were investing to dominate the oceans over Spain, Netherlands, France and other nations—yes that impulse to lead. Working-class clockmaker Harrison, with little formal education, picked up the gauntlet. He took it upon himself, his very life work, to address the longitude challenge and earn the £20,000 reward on offer.

Harrison’s first experimental timekeeper was the H1 chronometer. The H1 ran without lubrication and compensated for changes in temperature and humidity. It boasted two interconnected swinging balances that rendered it impervi- ous to the ship’s rolling. Harrison’s invention became a significant milestone in clock-making history, a true longitude find.

Refinements arose over the years in the H2, H3—this one took Harrison 19 years to develop—and finally the sea watch, his H4 chronometer that looked like a large pocket watch. The English Board of Longitude sent the H4 to the West Indies on a couple of trials, and it performed without fail on immaculate timekeeping.

British naval officer Captain James Cook, who had sailed the ocean on instinct and latitude, got hold of the H4 and was elated by it. He used it with immense satisfaction on his third voyage to the South Pacific.

But the English Board of Longitude stayed unimpressed with Harrison and his chronometer sophistications. With the board obstinate that Harrison hadn’t quite met his brief for the prize, and Harrison arguing otherwise, a fall-out natu- rally ensued. Harrison finally earned generous compensation in 1773. Notwithstanding, this lavish turn of events was due to the intervention of King George III, an intercession that compelled Parliament to rule in the clockmaker’s favour.

Thanks to Harrison’s timekeeping revolution, we revel today in the findings of famed explorers like Captain Cook, who used the chronometer for timekeeping and to measure longitude, on top of latitude, as he circumnavigated the globe.

Nothing surpassed this timekeeping invention that solved the longitude challenge, until the late 20th century when Americans won hands down over the British and the rest of the world. The US Department of Defence developed the global positioning system (GPS).

Published June 2016, The Australia Times Magazine

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