There's a tantalizingly brief gap between several medieval events and the European Age of Exploration. China closed itself to outsiders in 1368 after a century of extensive contact that included the famous journeys of Marco Polo. China's great voyages to Asia and Africa, that probably saw them round the southern tip of Africa and see the Atlantic, ended in 1431. The last ship to visit the Norse colony in Greenland sailed in 1406. Columbus sailed in 1492.
Why did people risk their lives in tiny cockleshell boats to travel to the far side of the planet? Ships today carry mostly mundane cargo: grain, cars, lumber, coal, oil. The value of the ship is usually far more than the cargo. The Exxon Valdez spilled 10 million gallons of oil, worth roughly $10 million. $10 million wouldn't begin to replace a supertanker. But in the 1500's cargos of spice, silk, or fine porcelain were worth many times the value of the ship. A successful voyage could set you up for life. To this day "my ship came in" is an expression for good fortune. In the 1500's, "my ship came in" often meant becoming an instant millionaire.
Everyone learns that Magellan was first to circumnavigate the earth, and that fifty years later Sir Francis Drake became the second on his raid to attack the Spanish in the Pacific. But who was third? That information is hard to find. It turns out it was one Sir Thomas Cavendish, about ten years after Drake. Here are the first few circumnavigations.
- Ferdinand Magellan (Spain) 1519-22
- Sir Francis Drake (England) 1577-80
- Sir Thomas Cavendish (England) 1586-88
- Simon de Cordes (Holland) 1598-1600
- Oliver Van Noort (Holland) 1598-1601
- George Spilberg (Holland) 1614-17
- James LeMaire and William Cornelius Schouten (Holland) 1615-17
Sometimes the lack of something is almost as informative as its presence. There's a reason why most of these voyages are forgotten. They were mostly voyages of piracy, aimed at harassing Spain, rather than for discovery. Drake and his fellow pirates would now be called state-sponsored terrorists. This pattern is very similar to the early days of space exploration, where superpower rivalry, rather than discovery, was the motivating force. Not until the mid-1700s were there circumnavigations largely aimed at exploration.
A geographic oddity is that almost all early voyages were from east to west around South America, despite the fact that the easiest way to sail around the world is from west to east, with the wind at your back. The reason was that sailing around South America allowed expeditions to enter the Pacific secretly, take the Spanish by surprise, and then run for home. Spain tried and failed to establish settlements at the Straits of Magellan. In those days it was simply impossible to keep such posts supplied by ship, and the climate in that region was too poor to grow adequate crops.
William Dampier (between 1679 and 1711) seems to have been the first to circumnavigate more than once. In fact he did it three times. The odds of surviving a circumnavigation were very poor in early voyages. The prevention of scurvy (acute vitamin C deficiency) was not discovered until around 1800. Captain Cook's voyages of the late 1700's, with no deaths from scurvy, were considered phenomenal.
The Dolphin (1764-66 and 1766-68) was the first ship to circumnavigate the globe twice. It took almost 250 years after Magellan for shipbuilding technology to be able to build a ship capable of surviving two voyages. Most previous voyages abandoned ships at sea or broke them up for the wood because they were simply falling apart.
By the 1600s a globe-girdling network of European trade routes was in place. Ships were crossing the Atlantic regularly, and sailing from Europe to the Far East. The Spanish offloaded goods on the Atlantic coast of Mexico, shipped them by caravan to the Pacific, and loaded them onto ships that regularly sailed between the Philippines and Mexico. But it was rarely necessary for any single ship or person to circle the globe. Thus, there were only about 25 circumnavigations to 1800. Giovanni Carreri (1693-98) sailed to Mexico, crossed overland, then booked passage across the Pacific and back to Europe, in so doing becoming the first known commercial round-the-world passenger. They probably lost his luggage.
When explorers began travelling far from Europe, they discovered to their horror that compasses often pointed quite far from true north. Queen Elizabeth offered a substantial prize to anyone who could solve the problem. The court physician, William Gilbert, began experimenting with magnets and in 1600 published De Magnete, considered the first great work on magnetism and also the first great work on geophysics.
By experimenting with spheres of lodestone, a natural magnet, Gilbert deduced the overall form of magnetic fields and concluded that the Earth had two magnetic poles. By mapping the magnetic field at enough places, the angle between a compass needle and true north, called the variation, could be predicted for any place on Earth. If you knew roughly where you were, you could correct your compass and find true north even on a cloudy night.
|The map at left shows why compasses don't point exactly north. The north magnetic pole is not at the geographic pole, but hundreds of kilometers away in northern Canada. Wisconsin is presently in a region where compass variation is almost negligible, but in Maine, compass needles point 20 degrees west of north and in Seattle, 20 degrees east.|
Not only does the Earth's magnetic field vary in space, it also varies in time; quite rapidly, in fact. It changes measurably in a human lifetime. Europeans were shocked by compass variation because around 1600, purely by luck, compass variation in Western Europe was very small. As far as Europeans knew, compasses pointed north, period. By 1800 the variation in London and Paris was over 20 degrees.
In a world without accurate maps, you don't need, and really cannot use, accurate compasses. Almost certainly other people had noticed compass variation, but if all you need is a rough estimate of direction, it probably didn't matter. Only when Europeans began trans-oceanic navigation out of sight of land did compass accuracy become crucial.
The distance north or south of the equator, your latitude, is easy to find in principle. The heavens appear to rotate about imaginary points called the celestial poles. The elevation of the celestial pole above your horizon is your latitude. At the north pole, the north celestial pole is at an elevation of 90 degrees: straight overhead.
Distance east and west, or longitude, is another matter altogether. Everybody on earth at a given latitude sees the same sky during a 24-hour day. The only difference is the time that they see it, and the key to longitude turns out to be time. When it's noon in New Orleans (90 degrees west) it's midnight in Calcutta (90 degrees east). It's 6 P.M. in London (0 degrees) and 6 A.M. in Fiji (180 degrees). So if you have a clock that keeps accurate time and reads the time of your home port, you can determine local time from the sun and stars and calculate your longitude.
All you need is a clock capable of keeping time to an accuracy of minutes over a voyage lasting months or years, on a rolling ship where the temperatures might be tropical heat or arctic cold. The circumference of the earth is 25,000 miles at the equator. That means one hour of time difference corresponds to over 1000 miles, one minute to 17 miles, and one second to a quarter mile. A minute of error in your clock might be no big deal on a clear day in nice weather and good visibility; it could be a killer if you're trying to avoid a rocky coast at night in a storm.
There are really two kinds of longitude measurements needed. You need accurate longitudes of ports and landmarks so you can draw accurate charts and reset your clocks in port. These can be determined with some leisure. The other kind of measurement is longitude on shipboard, so you know where you are on those nice charts.
The first attempts at determining longitude were astronomical. If you observe an eclipse in Calcutta and in London, a comparison of the data will allow an accurate determination of longitude. This is easy to see for the Moon, where the Earth's shadow is cast directly on the Moon and every observer sees exactly the same thing, but it will work also for eclipses of the Sun. Eclipses are relatively rare, but once a good measurement is made, the longitude of the observer is fixed. This approach works for establishing longitudes of major ports, but eclipses are too infrequent to be of much use at sea.
On the other hand, not far away is a bright planet with lots of bright moons: Jupiter. At least one of Jupiter's moons passes in front of Jupiter or behind it just about every day. The Dutch astronomer Roemer began developing predictions for the moons of Jupiter in the early 1600's. To his annoyance, he found that predictions were behind schedule when Jupiter was near the Sun in the sky, and ahead when Jupiter was opposite the Sun. The total discrepancy was about 16 minutes. He finally realized that the discrepancy was due to the time it took light to cross the Earth's orbit. This was the first indication that light had a measurable speed. Unfortunately, measuring the eclipses of Jupiter's moons never became a practical means of determining longitude.
The final solution turned out to be mechanical: a really good clock (chronometer). To make a device of the required accuracy required first-rate machine tools and high-grade steel for the spring. The drive to develop a reliable chronometer pushed the frontiers of machining and metallurgy to the limits. In 1714 the British government has established a prize of 20,000 punds (roughly a million dollars in today's purchasing power) for a marine chronometer accurate to within two minutes (half a degree of longitude). A clockmaker named John Harrison devoted his life to the challenge, starting in 1728 and culminating in 1761 with his final clock. But it took until 1772, the personal intervention of the King, and an act of Parliament for him to claim the prize. He died in 1776.
Captain James Cook took Harrison's chronometers on his celebrated voyages and praised them highly. He also, since his voyages overlapped the American Revolution, took along a letter of free passage from Benjamin Franklin, who was not about to let a mere war interfere with some good science.
The search for longitude was one of the pivotal technical and scientific problems of the 17th and 18th centuries. The financial stakes were huge. Attempts to solve the problem astronomically led to enormous improvements in astronomical measurement and numerous astronomical discoveries. Attempts to build accurate clocks led to refinements in machine tools and steelmaking. And - this is the key - with good machine tools and steel, you can make anything.
Since the origin for longitude is arbitrary, maps produced in different countries used various definitions. If the country had a large astronomical observatory, that was often used as the starting point. The two leading claimants for a global definition of longitude were Greenwich Observatory near London and Paris Observatory. By the late 19th century, Britain was so far ahead in the global race for colonies, and such a leader in naval forces and chartmaking that it was able to impose Greenwich as the zero of longitude. In 1884 an international commission agreed to adopt Greenwich Observatory as the global zero meridian of longitude.
|Long before 1882 the United States had expanded to the Pacific and tended to draw state
and territorial boundaries as straight lines of latitude and longitude. But how to define
longitude in an era when there was no global standard? Close inspection of all the
north-south boundaries in the western U.S. shows that they all lie about 7 minutes west of
the nearest degree of longitude (Top, at right).
The District of Columbia was originally laid out as a tilted square (bottom, at right; the portion in Virginia was later returned). The north-south center line is at longitude 77 degrees 7 minutes, and all state boundary longitudes were defined from that longitude. The map of the U.S. is a relic of the days when there was no global longitude definition. (Incidentally, the longitude line doesn't correspond to any major Washington landmark, though it does pass just west of the White House and Washington Monument.)
If you live in one place and never travel, you can set your clock and forget it. Noon is the time when the Sun is highest or due south. If you travel east or west slowly you can just reset your watch periodically as it gets out of sync with local clocks. But when railroads made it possible to traverse several degrees of longitude in a day, the result was chaos. At every station, travelers found their watches were a few minutes off. Different railroads frequently kept different clocks in the same station. Station agents had thick books to convert times between different railroads.
In the 1870's the nation's railroads finally began dividing the country into time zones where station clocks were all set to the same time. Even so, different railroads defined their zones differently. Finally, in the 1880's, Congress defined nationwide time zones.
The idea had opponents. Some felt it was impious for man to define time differently from natural solar time. On the designated date for the time change, crowds gathered to watch the local clocks reset. The clocks were reset and - well, that was pretty much it. It was one of the greatest anti-climaxes in history.
Until recently, Saudi Arabia lacked time zones. Solar time, used in calling Muslims to daily prayer, was official. Saudi Arabia now uses a standard time for civil and business purposes but religious time is still solar. China, with an east-west extent similar to the U.S., has a single time zone. Clock time has never had the significance to Chinese culture that it has in the West, and it simply doesn't matter to the Chinese if the sun rises or sets at a clock time that might strike Americans as odd. In some places (Newfoundland is the closest) the time zones are a fraction of an hour out of sync with adjacent time zones.
Issac Newton hypothesized that, if the Earth were not perfectly rigid, it would bulge at the equator. If the Earth were a fluid, the bulge would be about 1/200 of its diameter; the equatorial diameter of the Earth would be a bit greater than its polar diameter.
One way to test this hypothesis is to measure a degree of latitude carefully near the equator and near the poles. 'Down' anywhere on Earth means perpendicular to the surface; on a non-spherical Earth that generally does not mean toward the center of the Earth
The figure above shows how latitude is defined on the real, non-spherical Earth, and why apparently simpler definitions don't work:
Because lines perpendicular to the surface do not pass through the center of the Earth, degrees of latitude differ slightly in length, with those near the equator being just a bit shorter. Later it was also discovered that careful measurements of gravity at different locations could be used to find the true shape of the Earth.
To this day, careful determination of the shape of the Earth provides important clues to the Earth's deep interior. But that's not why it became a focus of international rivalry in the 1700's. To make the necessary measurements, a nation has to:
There's a word for a country that can do all this: superpower. In the 18th century, the two European superpowers were the British and the French, and they competed for military advantage, colonies, and scientific glory. They sent expeditions to map the shape of the Earth partly for scientific purposes but equally to show who was the biggest baddest dude on the block. This rivalry was a direct forerunner of the Apollo and Star Wars programs.
A French expedition managed to gain access to the hitherto closed Spanish Empire and did a survey in Ecuador (a measure of diplomatic clout). But when the data were analyzed, the French got the utterly unexpected result that the Earth was actually longer along its polar axis. That was so at variance with theory that the response just about everywhere was 'that can't be right', and indeed the French, to their deep embarassment, had made an error in their data analysis
Open any advanced mathematics reference and you will see functions and methods bearing names like Laplace, Legendre, and Gauss. Almost every major mathematician of the 18th and early 19th century worked on geodesy, the science of measuring the Earth's shape, and many of the most important techniques in advanced mathematics came out of this study.
Knowing the exact shape of the Earth is still important. Ballistic missiles could not navigate accurately without it; more benignly, Global Positioning Systems (GPS) could not locate points accurately without extremely accurate knowledge of how subtle variations in the shape of the Earth affect satellite orbits.
As the British began to consolidate their hold on India, they began to realize the need for accurate surveys and maps. They launched the Great Trigonometrical Survey. After a preliminary survey across southern India, the Survey launched a grand scheme to survey a line from the southern tip of India north to the Himalaya. This effort under directors Colin MacKenzie, William Lambton and George Everest, took over 50 years.
Colonialism has a bad name nowadays, but give the British their due. British officials and soldiers posted to India had a substantial chance of not coming home, or of being permanently disabled. Illness, heat, injuries and battle casualties all took a heavy toll.
Rank, of course, has its privileges, and in the days before air conditioning, being on the shady side of the ship was reserved for the most privileged. Since the journey was mostly east-west, that meant being on the left, or port side on the outbound journey and on the right, or starboard side coming home. Port Out, Starboard Home: "posh" accomodations (that's where the word comes from).
As the Great Trigonometrical Survey pushed north, locations as determined by two methods began to diverge. There didn't seem to be any external factor that could affect triangulation, but latitude as determined by astronomical techniques depends on accurately levelled instruments. The obvious source of a problem was the mass of the Himalaya, which exerted a sideways gravitational pull. (In fact, the British were aware of the problem but didn't expect that even stations many kilometers from the mountains would be affected.) Surprisingly, when the effect of the Himalaya was calculated, it turned out to be three times as large as the observed effect. Something deep in the Earth must be offsetting the effect of the Himalaya. The crust of the Earth is lighter than its interior and floats. Under mountain ranges, the crust is thicker, and the crust floats higher. This balancing process is termed isostasy.
One offshoot of the British mapping effort in India was the discovery of Mount Everest. With Nepal and Tibet forbidden to foreigners, the British could only survey the Himalaya from afar. They picked six survey points and measured the directions and altitudes of every peak visible. Then they plotted all the lines of sight on a single map. Where many lines crossed was the true location of a peak.
One day the Indian assistant came into the Survey office and announced he had found the highest mountain in the world. It was an apparently unremarkable peak but seemed low only because it was far away. The peak was named Mount Everest after the director of the Indian Survey.
The irony is that the British Survey, unlike many colonial officials, had insisted on retaining local place names. With Nepal and Tibet off-limits, the local name of the peak was unknown. The peak is called Sagarmatha in Nepali and Qomolongma in Tibetan.
A ship's compass, mounted on a structure called a binnacle, has two large iron balls on either side, mounted on an arm. The balls can be moved in and out to compensate for the magnetic fields of everything else on the ship. An object of known direction is sighted, and the balls adjusted until the compass reads the correct direction.
To magnetize lodestone, you need a powerful source of natural magnetic fields, say a really powerful electric current. Lightning. Even ordinary rocks can be magnetized noticeably by lightning strikes because they have small amounts of magnetite in them. While doing field work in Ontario I once sat on a rocky knob and found all my compass azimuths were 10 degrees in error. I moved a few feet away and got correct readings. Mountain peaks that are struck repeatedly can throw compass needles 180 degrees off.
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