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Navigation and maritime technology in the 1600s

Sextent

The sextant measured latitude using a system of mirrors to reflect the sun and measure the sun’s height.
Chris Jorgensen

Like the first manned space flights, early ocean voyages were journeys into the unknown.

Before the invention of the sextant, compass and chronometer, navigation during classical and medieval times was basic at best. The very first sailors relied on their proximity to land—steering by the sun during the day and by the stars at night. To estimate distance and direction, they used their knowledge of capes and harbours, seasonal winds and currents.

Medieval sailors like Christopher Columbus employed a slightly more advanced method of deduction called dead reckoning. A navigator located his position by measuring the course and distance he had sailed from a known point, such as a port. He then measured his course and distance from that point on a chart, and marked it with the new position. Each day's final position would be the starting point for the following day's course-and-distance measurement.

Distances were unreliable. While latitude, the distance north-south, could be calculated by relating the sun's position to constellations, the calculation of distances east-west was an inexact science.

To find their way to the East Indies and back again, navigators aboard ships like the Duyfken used other methods of navigation, such as the sounding lead.

On their return voyage, to determine whether they were in the vicinity of the Cape of Good Hope at the southern tip of Africa, navigators would lower into the water a lead weight attached to the end of a rope to sound the depth of the ocean.

They knew that south of the continent lay a shallow seabed, about 200 metres deep. They also knew they would find on the seabed at this point a certain type of seaweed, called trompen. Using these indicators, along with latitude and the presence of seabirds in numbers, they were able to determine the point at which the ship should change course and sail north across the Atlantic toward the island of St Helena, and to Europe beyond.

Apart from the sounding lead, the Duyfken and other VOC (Dutch East India Company) ships employed several other navigational aids.

Invented sometime in the twelfth century, the magnetic compass was not used on board ocean-going vessels until the 1300s. In 1483, a German monk on a pilgrimage to the Holy Land described in colourful terms the method of steering by compass.

... they have as compass a Stella Maris 'star of the sea': the North Star near the mast and a second one on the topmost deck of a poop. And beside it all night long a lantern burns, and they never take their eyes off it, and there is always a man watching the star the compass rose, and he sings out a sweet tune telling that all goes safely and with the same chant directs the man at the tiller how to turn the rudder.

At about the same time, remarkably accurate maps of Mediterranean coastlines, called portolan charts, were gaining widespread acceptance. Thought to date back to classical times, the maps drawn on vellum (stretched calf skin) reflected the accumulated experience of generations of Mediterranean sailors. The charts, from the Italian portolani (sailors' charts), also recorded the early sailors' growing knowledge of the Atlantic Ocean.

Navigation received a further boost in the 1400s, when the Portuguese, encouraged by Prince Henry the Navigator, began exploring the west coast of Africa. The Portuguese colonised the Canary Islands in 1420 and the Azores in 1433, and began investigating the wind systems of the north Atlantic and the Orient.

Meanwhile, they established a school of navigation in the Portuguese port of Sagres to investigate the use of astronomical instruments in navigation.

Dead reckoning, an inaccurate form of navigation, had by this time received additional support. Using a wooden quadrant (a quarter circle with a plumb line suspended from its apex), navigators could check the height of the pole star above the horizon and thus obtain their latitude. The Portuguese were the first to navigate using latitudes.

Astrolabe and planisphere

Astrolabe (left) and planisphere
http://ourmaritimehistory.web-log.nl/

In 1481, navigation was further improved with the introduction of the brass astrolabe (a circular instrument that measured the altitude of the sun or the stars) to calculate latitude, and mathematical methods of working out the distance sailed north and south using degrees of latitude.

In the early 1500s, the cross-staff was introduced to determine latitude by measuring the diameter of the sun, or the distance between two celestial objects. Based on an Arabian instrument called the kamal, the cross-staff was vastly more accurate than the astrolabe.

Following Columbus's voyage to north America in 1492, Spain was expanding into the New World, and soon adopted the Portuguese methods of navigation. At the same time, the portolan charts began to take account of the new discoveries and incorporate the latitude of places (their location north or south of the equator) and a scale of latitude.

Before the Duyfken sailed to the East Indies, Dutch cartographer Gerard Mercator had devised a map on which a straight line represented a line of constant compass course. Mercator's Projection was, however, in its infancy and was used only sparingly by sailors of the period.

By the mid-1600s, navigators from all seagoing European countries were using altitude observations to measure latitude, expanded charts and the compass. Although they could calculate latitude accurately, longitude remained an enigma.

The most common method of measuring east or west movement was the log and line. Sailors at the stern of the ship would unreel a line, one end of which was attached to a flat piece of wood, and time its length against a sand glass. The theory was that the length of line played out over a certain time equalled the speed of the ship. Knots tied into the line at regular intervals were designed to help the calculations.

The inability to calculate longitude accurately resulted in not only unnecessarily long voyages, shipwrecks and commercial losses, but also in accidental discoveries, such as those made by the Dutch in Australia.

Sailors would have to wait until English clockmaker John Harrison (1693–1776) solved the longitude puzzle forever with the invention of the first reliable marine chronometer, the H4, in 1759.