Navigation

C

Introduction of the Compass and Chart

The magnetic compass began to appear in navigation in about 1100 ad. Early compasses consisted of a piece of naturally occurring magnetite attached to one end of a wooden stick, then floated in a pool of water. The magnetized stick oriented itself to Earth's magnetic field, rotating until the end supporting the magnetite pointed north. The compass revolutionized navigation. It provided navigators with a fixed reference point regardless of their location, the boat's heading, the wind direction, or the state of visibility.

The first true nautical charts did not appear until about the year 1300. The earliest charts were portolans, Italian-made books of maps, which came into use at the end of the 13th century. Portolans contained centuries of seafarer observations, such as locations of harbors, distances, depths, and descriptions of coastlines.

D

Improved Tools for Celestial Navigation

Astrolabes, many of them beautifully crafted like jewelry, measured the angle between the zenith (point directly overhead) and the celestial body. An astrolabe consists of a circle or section of a circle, marked off in degrees, with a movable arm pivoted at the center of the circle. Some of the world’s most important explorers used astrolabes on the expeditions to faraway lands. Christopher Columbus used an astrolabe in his historic voyage to the Americas. Ferdinand Magellan and his crew relied, in part, on an astrolabe during the first circumnavigation of Earth from 1519 to 1521. Many Arab travelers used astrolabes to navigate the desert.

In the late 16th century many navigators abandoned the astrolabe in favor of the wooden cross-staff. This T-shaped device consisted of a wooden pole about 1 m (3 ft) long that was fitted with a sliding crossbar. It performed the simpler task of measuring to the star or the Sun up from the horizon, rather than down from the zenith. Sailors held the cross-staff to their eye, oriented the crossbar vertically, then pulled the slider toward them until the Sun was at one end of the crossbar and the horizon on the other. They read a degree scale marked on the intersection of the crossbar and the staff to determine the altitude of the Sun. Staring directly into the Sun damaged the eyes, so navigators switched to the back-staff. This similarly constructed device permitted navigators to take the same measurement with their backs to the Sun.

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E

Mercator Projections

Chart-making became a science in the great age of exploration between about 1550 and 1700. In 1537 Flemish mapmaker Geradus Mercator designed a map specifically for navigation. A Mercator map reflects the curvature of Earth. But a map is flat and Earth is spherical, so while the equatorial regions appear normal on the map, the high latitudes are greatly distorted. Any line cutting two or more meridians at the same angle is represented on a Mercator map as a straight line. Such a line, called a rhumb line, represents the path of a ship following a steady compass course. Charts and maps continued to become more accurate and detailed, and by 1700, charts included compass variation and ocean currents.

F

Quest for Longitude

The greatest advance came a century later in response to a challenge by England’s Royal Society to solve what was called “the longitude problem”—how to measure longitude accurately. Each year England lost hundreds of ships in wrecks because the navigators miscalculated their longitude. To determine longitude, navigators had to make a series of complex calculations, which could take many hours to complete and even then could be inaccurate. In an effort to slow the losses, the Royal Society offered a huge monetary prize to anyone who could devise a way to accurately determine longitude at sea. Part of the solution called for developing a more accurate instrument for measuring altitudes. Out of this came the sextant, developed independently in 1730 by English mathematician Joseph Hadley and by American inventor Thomas Godfrey. In 1735 English watchmaker John Harrison completed the solution when he developed the chronometer, the first reasonably accurate portable timepiece. With a quality chronometer and a good sighting instrument on board, navigators had a much better chance to find their way.

In the 19th century, European academics improved the mathematical methods of making calculations in celestial navigation. In 1884 European countries agreed to make the meridian of longitude that ran through Greenwich, England, the prime meridian. Until this time, each of the major European countries had claimed that the 0° meridian ran through their own capital city.

G

Precision in Measuring Speed

Accurate speedometers came into popular use in the latter half of the 19th century. Until this time, most sailors estimated their speed with a chip log, a float affixed to a rope knotted at intervals of 14.4 m (47.25 ft). Navigators threw the float off the boat and counted the knots as they hit the water. The navigators timed their count, using a 28-second sandglass to ensure consistency. The number of knots that ran out in 28 seconds equaled the boat’s speed in nautical miles. The term knot, meaning one nautical mile per hour, originated with the chip log. If the first knot appeared as the sand ran out, the boat’s speed equaled one nautical mile per hour, or one knot. In 1861 English marine instrument maker Thomas Walker refined the patent log, and many sailors used this device to measure speed instead of the chip log. To measure speed using a patent log, a sailor counted the revolutions of a small rotor towed behind the ship’s stern. A rudimentary form of the patent log dates back to the 17th century, but few ships used the instrument until Walker improved it in the 19th century.

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