Secrets of Ancient Navigation
by Peter Tyson
Oak and triple bronze must have girded the breast of him
"Aye, mate." One can almost hear the weary assent of countless a hoary sailor
upon hearing these words of Horace, almost see the rheumy eye staring distantly
as if at some ghost ship on the horizon that only he can see. For the old
poet's words ring only too true. In the three or four millennia of seafaring
before John Harrison came along, how could mariners know where they were going?
The sea is literally without landmarks to guide by, a vast, featureless
emptiness ready and more than willing to swallow up the lost and unlucky,
leaving no trace save the awful memories of those who survived them.
who first committed his frail bark to the angry sea
The first seafarers kept in sight of land; that was the first trick of
navigation. Follow the coast. To find an old fishing ground or the way through
a shoal, one could line up landmarks, such as a near rock against a distant
point on land; doing that in two directions at once gave a more or less precise
geometric location on the surface of the sea. Sounding using a lead and line
also helped. "When you get 11 fathoms and ooze on the lead, you are a day's
journey out from Alexandria," wrote Herodotus in the fourth century B.C. The
Greeks even learned to navigate from one island to the next in their
archipelago, a Greek word meaning "preëminent sea." They may have followed
clouds (which form over land) or odors (which can carry far out to sea).
But what if land were nowhere nearby? The Phoenicians looked to the heavens.
The sun moving across the commonly cloudless Mediterranean sky gave them their
direction and quarter. The quarters we know today as east and west the
Phoenicians knew as Asu (sunrise) and Ereb (sunset), labels that
live today in the names Asia and Europe. At night, they steered by the stars.
At any one time in the year at any one point on the globe, the sun and stars
are found above the horizon at certain fixed "heights"—a distance that
mariners can measure with as simple an instrument as one's fingers, laid
horizontally atop one another and held at arm's length. The philosopher Thales
of Miletos, as the Alexandrian poet Kallimachos recorded, taught Ionian sailors
to navigate by the Little Bear constellation fully 600 years before the birth
Now to Miletos he steered his course
The Norsemen had to have other navigational means at their disposal, for in
summer the stars effectively do not appear for months on end in the high
latitudes. One method they relied on was watching the behavior of birds. A
sailor wondering which way land lay could do worse than spying an auk flying
past. If the beak of this seabird is full, sea dogs know, it's heading towards
its rookery; if empty, it's heading out to sea to fill that beak. One of the
first Norwegian sailors to hazard the voyage to Iceland was a man known as
Raven-Floki for his habit of keeping ravens aboard his vessel. When he thought
he was nearing land, Raven-Floki released the ravens, which he had
deliberately starved. Often as not, they flew "as the crow flies" directly
toward land, which Raven-Floki would reach simply by following their lead.
That was the teaching of old Thales
Who in bygone days gauged the stars
Of the Little Bear by which the Phoenicians
Steered across the seas
Heeding the flightpaths of birds was just one of numerous haven-finding
methods employed by the Polynesians, whose navigational feats arguably have
never been surpassed. The Polynesians traveled over thousands of miles of
trackless ocean to people remote islands throughout the southern Pacific.
Modern navigators still scratch their heads in amazement at their
accomplishment. Like Eskimos study the snow, the Polynesians watched the waves,
whose direction and type relinquished useful navigational secrets. They
followed the faint gleam cast on the horizon by tiny islets still out of sight
below the rim of the world. Seafarers of the Marshall Islands built elaborate
maps out of palm twigs and cowrie shells. These ingenious charts, which exist
today only in museums, denoted everything from the position of islands to the
prevailing direction of the swell.
Charts have aided mariners ever since the Alexandrian astronomer Ptolemy
created the first world atlas in the second century A.D. The redoubtable
Ptolemy even plotted latitude and longitude lines on his atlas's 27 maps,
though the farther one got from the known world centered on the Mediterranean,
the dangerously less reliable they became. Even before Ptolemy, there were
sailing directions—the Greeks called them periplus or "circumnavigation"—that were compiled from information collected from sailors far and wide. One of
these, The Periplus of the Eritrean Sea, a document written in the first
century by a Greek merchant living in Alexandria, described trading routes as
far east as India. By the 10th century, Italian-made portolans supplied
detailed directions, distances, depths, and coastal descriptions, and by the
13th century, sea maps with scale and bearings began to appear.
Statue of Ptolemy.
The greatest advance in navigation came with the compass. The Chinese
apparently knew about the powers of magnetism as early as the third millennium
B.C., when, historians tell us, one army defeated another after the battlefield
had become enveloped in dense fog by using a device known as a "point-south
carriage." This was a standard carriage for carrying royalty with a small,
rotating figure mounted on the front, which by magnetism always pointed south.
(The Chinese chose to have the arrow point south rather than north.) But no one
seems to have manipulated the lodestone for sea navigation until early in the
present millennium. The first mention of the compass in the West comes from the
Englishman Alexander Neckham, who wrote in 1187 that "sailors use a magnetic
needle which swings on a point and shows the direction of the north when the
weather is overcast." Despite its usefulness, the compass took a long time to
come into wide use, as many seamen thought it operated by black magic. (Hence
the invention of the binnacle, in which sea captains could hide their recondite
instrument from the suspicious eyes of the crew.) In the meantime, sailors
relied on natural forces they could readily comprehend.
One of these was currents. From time immemorial, journeys have been made or
broken by these undersea winds. The western-trending currents of the Indian
Ocean, for one, are likely responsible for the Indonesian-based race of
Madagascar, an African island 4,000 miles from the nearest bit of Indonesia.
Similarly, the clockwise currents in the North Atlantic helped doom one of the
greatest land scams in history: Erik the Red's colonization scheme for the
island he cleverly dubbed "Greenland." Of the 25 ships that sailed west from
Norway in the year 990, only 14 arrived. The father of those North Atlantic
currents—the Gulf Stream—was named by none other than Benjamin Franklin.
While deputy Postmaster-General of Great Britain in the 18th century, Franklin
noticed that his mail ships to the American colonies took longer than whaling
ships. Questioning whalers, he learned of a powerful current originating from
the Gulf of Mexico—hence his name for it—and sweeping northeast into the
North Atlantic (and, incidentally, giving the British Isles a climate
positively balmy for such a northern latitude).
Like currents, trade winds have always been important to mariners. Those
blowing heads on yellowed old maps were not mere decoration. In the Indian
Ocean, for example, Indian traders over the ages have ridden the northeast
monsoon to Africa in the cool, dry winter and taken the southwest monsoon back
to the subcontinent in the hot, wet summer. To make their annual voyages from
Tahiti to Hawaii, a journey of several thousand miles, the Polynesians hitched
a ride on the prevailing south-easterly wind, setting a starboard tack and
For millennia, as sailors from the Phoenicians to the Polynesians knew, the
heavens remained the best way to find one's north-south position. Increasingly
sophisticated devices were designed over the centuries to measure the height of
the sun and stars over the horizon. The gnomon or sun-shadow disk operated like
a sundial, enabling the user to determine his latitude by the length of the
sun's shadow cast on a disk floating level in water. The Arabian kamal
was a rectangular plate that one moved closer or farther from one's face until
the distance between the North star and the horizon exactly corresponded to the
plate's upper and lower edges. The distance the plate lay away from the face—measured by a string tied to the center of the plate and held at the other end
to the tip of the nose—determined the latitude.
In the Middle Ages, sailors relied on the astrolabe, a disc of metal that one
held suspended by a small ring. The disc had a scale with degrees and a ruler
for measuring the height of an astronomical body. Other medieval mariners
preferred the cross-staff, a T-shaped device whose base was held up to the eye.
One measured the sun's height by pulling the slidable top of the T toward one's
eye until the sun lay at the top and the horizon at the bottom. Since blindness
resulted from frequent use, the explorer John Davis invented the back-staff in
1595, which enabled one to get the same measurement with one's back to the sun.
The sextant was the most advanced of these devices, allowing users to determine
their latitude to within a sea mile or two, even from a swaying deck.
In the years after the sextant was invented in 1731, many held out hope that
it would aid in east-west navigation as well—that is, in finding longitude.
Sailors could employ the sextant to figure longitude using the lunar-distance
method, but with the astronomical tables of the 18th century, the process could
take several hours to work out one's position—not remotely good enough for
sea travel. In the end, it was the dogged clockmaker, John Harrison, who solved
the longitude problem with his chronometers. And today, the precocious
step-child of these highly accurate clocks, the Global Positioning System, has
finally proved the Roman dramatist Seneca right, when he wrote in the first
There will come an age in the far-off years
When Ocean shall unloose the bonds of things,
When the whole broad earth shall be revealed . . . .
Peter Tyson is Online Producer of NOVA.
Photos: (1,2) NOVA/WGBH; (3) Visuals Unlimited/Owon Gingerich.
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