Navigation
Before Netscape
Edwin M. Knights charts the history of early navigation.
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Though
the Chinese had much earlier success with navigational
aids, European explorers were soon well armed with an
impressive array of instruments.
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SOMETIMES
IT'S BETTER to be a bobolink. Or a pigeon. Or even a tuna. These
are but a few of the creatures that have directional abilities
that humans only wish they had. Put us in a dense fog and we
go in circles.
Early sailors were very much aware of their limitations, hugging
the shorelines, looking for familiar landmarks, and going ashore
at night. As they began to venture out to sea, they learned
to recognize other signs of nearby land, such as certain types
of birds and fish, seaweed and other drifting objects that had
been washed out to sea. They could also take soundings of water
depths, keep track of time and measure their speed. The better
navigators became expert in estimating a ship's position by
observing the sun, moon, stars and planets. The poorer navigators
just disappeared.
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The
astrolabe was another essential tool in celestial navigation.
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Celestial
Navigation
The art of studying the sky to guide ships across the ocean
is called celestial navigation. The stars and other celestial
bodies are so far away they seem to be located on the surface
of an imaginary sphere, with its center located at the center
of the earth. The Phoenicians and Greeks were among the first
to attempt sailing at night. They didn't realize it, but landlocked
nomads were already using the stars to find their way at night
across the desert sands on their ships of the desert, camels.
Archeological findings show that the Vikings and Polynesians
also made some long, remarkable voyages.
Navigators found they could determine a ship's latitude quite
easily by checking the height of the noon sun. The Greeks also
invented the cross-staff and the astrolabe, used to measure
the altitudes of celestial bodies. The cross-staff was a stick
about a yard long, with a shorter sliding stick set at right
angles. The navigator pointed the staff at a spot halfway between
the horizon and the sun or a star. The crosspiece was then adjusted
until the sights at its ends were in line with the celestial
body and the horizon. A scale along the stick showed the angle
of the body above the horizon.
The astrolabe was usually made of bronze, with a pointer pivoting
from its center. One sailor held the astrolabe by means of a
small ring at its top, while another knelt facing the instrument's
rim. The kneeling person aimed the pointer at the star and read
the angle from markings on the disk.
Many factors caused a ship to drift from its intended course.
Among these were the wind, waves, swells and ocean currents.
Clearly there was a need for a better way of staying the course!
The
Magical Lodestones
Just when the Chinese found the first lodestone depends upon
which reference you want to believe, but it was well over 2,000
years ago. These interesting rocks were referred to as "tzhu
shih", or loving stones, as they liked to kiss. In the sixth
century bc, a Greek writer noted that pieces of iron were attracted
to certain rocks. This phenomenon, which we now know as magnetism,
intrigued the Chinese greatly. They found that if they floated
a piece of lodestone on a small piece of wood, it always aligned
itself in a north-south direction. They developed the first
compass, which was a spoon-shaped lodestone resting on a bronze
plate. The handle of the stone always pointed south. They also
pierced a couple of short sticks with a coarse iron needle that
had been magnetized by rubbing on a lodestone and they found
it also always pointed north-south.
The Chinese used the magnetic compass to orient buildings, tombs
and even furniture to positions they felt would harmonize with
nature and they found many other applications for the compass,
including medical treatment. There is evidence that Chinese
sailors were using magnetic compasses as early as the sixth
century bc. Europeans didn't use them until almost 1,200 years
later.
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The
cross-staff was used to measure the altitude of celestial
bodies relative to the horizon.
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What's
a Lodestone?
The lodestone got its name from the north star, Polaris, which
was the seamen's lodestar, showing them the way. It consists
of magnetite, a massive oxide of iron ore. Not every piece of
magnetite can become a lodestone; it requires a certain crystalline
structure. And something else must occur to give the stone its
magnetic personality. This happens when the stone is struck
by lightning. The huge electrical discharge, lasting but a split
second, has been shown to transform the right types of magnetite
into lodestones.
Lodestones weren't practical to use as compasses, but the Chinese
had already found they could transfer magnetism to an iron needle
and float this in a small container. Europeans kept improving
the magnetic compass, but they didn't know why it pointed north.
Most scientists felt it was attracted to the pole star. In 1269,
an Italian scholar from Picardy named Peregrinus wrote a paper
stating a magnet had two poles, north and south. He noted that
similar poles repelled and opposite poles attracted each other.
A Buddhist astronomer reported that a compass didn't point directly
at the north as early as 720ad, but Europeans didn't find this
out until long after Columbus made his epic voyages.
Chris
Flunks Celestial Navigation
Columbus, who was from Genoa, sailed by dead reckoning, as did
most everyone else in the Mediterranean. Since most of the sailing
was east or west at about the same latitude, there wasn't much
incentive to practice celestial navigation. He did try it a
few times, with rather poor results. Determining longitude requires
a very accurate timepiece; at the equator, every error of one
second means an error in longitude of about 1/4 mile. Thus Columbus'
ampoletta, or sand glass, was of no use. It had to be corrected
every midnight (if it were clear) using a little instrument
appropriately called a nocturnal, which worked by measuring
the rotation of the stars around Polaris. It wasn't until 1728
that John Harrison came up with a chronometer that would keep
accurate time at sea.
Many years later it was determined that the magnetic compass
really pointed to a spot in Hudson Bay about 11.5 degrees from
true north. This compass error, sometimes called the declination,
was methodically recorded on maps in which isogonic lines show
the boundaries of the differences in variation from true north.
Making
the Compass Work at Sea
Using a compass aboard a ship, especially a sailing ship, required
a special mounting to compensate for the pitching, rolling and
yawing of the vessel. This was achieved by designing a housing
known as a binnacle, which would maintain the compass relatively
level with the horizon. Early models of the compass didn't retain
their magnetism very well, so each ship had to carry its own
lodestone to "recharge" the compass needle. The needle's shape
was changed to a parallelogram; later several parallel magnets
were mounted to a compass card pivoting around a jeweled bearing
and surrounded by fluid.
Improving the compass did not necessarily increase its reliability,
however, because as more and more iron was used in ship construction,
more errors were introduced in compass readings.
And still nobody understood why this area close to the North
Pole attracted a magnet. Did the earth have a lodestone in its
core? Then it was found that the direction of the magnetic field
was slowly changing, followed by more confusion resulting from
the finding that iron loses its magnetism when heated to 1,000
degrees. Since the interior of the earth is very hot, how could
it contain permanently magnetized iron?
Why
is the Earth Magnetized?
One theory is that the core consists largely of molten iron
and that convection caused by the heat from the core causes
liquid iron to move in a rotational pattern. Sir Joseph Larmor
proposed that the Earth was a dynamo in 1919. In the middle
of the 20th century geologists made a remarkable finding - some
ancient rocks showed the earth's magnetic field had pointed
in the opposite direction. It seems likely the polarity of the
earth has shifted around several times. There are predictions
that the magnetic polarity of the earth may reverse again in
1,500 to 2,000 years. This is going to upset a lot of birds,
fish and bumblebees, if they are still around. Findings that
the magnetic fields of some other planets are not aligned similarly
with rotation also bring into question the dynamo theory.
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The
standard compass became less practical with the increased
popularity of steel-made ships, as the steel threw off
the compass' ability to find magnetic north. The gyroscope,
developed in the ealry 1800s, allowed for development
of a compass that overcame this problem.
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The
Gyrocompass
The first gyroscope is credited to J.G.F. Bohnenberger in 1817,
and J.B.L. Foucault, a French physicist, explained its inertial
principles in 1851. The harnessing of electricity enabled G.M.
Hopkins to develop the first electrically driven gyroscope in
1890. By now most ships were being made of steel, creating major
difficulties for a magnetic compass. This was of particular
concern in naval warfare, and two great naval powers, Germany
and the US, hastened to develop military applications for the
novel gyroscope.
A gyroscope will always point in a single direction in space.
If force is applied, it reacts to it at right angles. Dr. Hermann
Anschutz of Germany patented the first north-seeking gyrocompass
in 1908. The same year, Elmer Sperry, an American, developed
the first ballistic gyrocompass. The gyrocompass, aligned in
a north-south axis, maintains this direction no matter what
course is followed by a ship or plane. It is unaffected by metal
and now navigators had an instrument that would point to true
north. One master compass can control several "repeater" compasses
or provide information to an automatic pilot or helmsman. Sperry
went on to perfect the gun battery controls and gyro stabilizer
systems for ships during WWI. His company produced "Metal Mike,"
the first gyro pilot system for ship's steering and later developed
numerous variants and electronic amplification. Using Sperry's
artificial horizon and the aircraft directional gyro made possible
the first recorded all blind flight in 1929. Elmer Sperry had
amassed 360 patents by the time of his death in 1930.
Leading
the Way
When HMS Resolution sailed forth in 1772 to test the current
states of the art in navigation, including chronometers, the
constellations and planets in the skies above still bore, as
they do today, the ancient names bestowed upon them by celestial
navigators centuries ago. Today's traveler has high-tech navigational
aids, but the Boeing 747 still carries a magnetic compass on
its instrument panel, corrected for the magnetic effects of
surrounding electronic equipment. The magnetic compass deserves
a place of honor in any Instrumentation Hall of Fame. And the
wondrous gyrocompass, conceived almost a century ago, filled
the need to find true north for those high in the sky or far
beneath the surface of the sea. Meanwhile, fish and mammals,
birds, bees and even bacteria just take off and head in the
right direction. While some birds definitely use stellar constellations
during their migrations, other organisms may have single-molecular
magnets of manganese or iron that act as built-in compasses.
Perhaps humanity had them, too, until the magnetic polarity
of the earth reversed.
Perhaps that is why we walk in circles in the fog.
This
article originally appeared in our October/November
2001 issue.
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The 1650s