Editor's Note: This is the third in a series of columns detailing Kent Mountford's trip on the sailing ship, Royal Clipper, tracing the route of the early voyages from Europe to the Chesapeake. For the first legs of his voyage, read "Past is Prologue," March & April 2008.

As I came on deck early one morning, the 1,000-foot cliffs of Hierro Island's shoreline rose over the western horizon. Geologically the youngest of the Canaries, Hierro, called Ferro in earlier centuries, was the last outpost of land seen by mariners as they started their voyages to the New World on the trade winds.

Near Hierro, sailors who tried their compasses for accuracy found that "magnetic north" closely agreed with bearings taken on Polaris, or North Star, which is in almost a direct line with the axis of Earth's rotation. This makes Polaris appear motionless in the Northern Hemisphere and all the other stars seemingly rotate around it.

Polaris is not exactly aligned with true north as drawn on the globe, which was well-known by the 16th century. The importance of Hierro was that here, the position of the North Star and compass agreed. Because of variations in the Earth's magnetic field, this is not always the case. Elements of the land mass (as well as masses under the sea) draw the compass away from true north. What was not well understood at that time was that variation also changed systematically over a period of years, a couple of seconds of arc at a time.

Because Hierro seemed to be the zero point on the globe for magnetic variation, it was selected as the place to begin counting longitude West to East in the 17th century. In the time of Ptolemy, the zero point had been in Alexandria, Egypt, for political purposes, then Jerusalem for religious ones. Long after Hierro, Paris, then finally Greenwich, near London, became the international Prime Meridian, where one can stand with one foot in the Western Hemisphere and the other in the Eastern.

When Christopher Newport on Susan Constant passed near Hierro in January 1607, those aboard the ship attempted to track the westerly progress to determine longitude on their voyage and eventually for the Chesapeake. They were not very successful.

I was learning to track our latitude-distance north and south relative to the equator and pole. But it was the Royal Clipper's electronics that kept me straight on longitude.

Like voyagers in the 15th-19th centuries, we were following the wind and quickly left behind all visible traces of other shipping. Commercial trade no longer follows these routes.

Because of this, the sea on this route, contrary to popular expectation, is not littered with human debris. I had expected more trash, given that in the center of seas-where great gyres of wind and current spiral together-trillions of pieces of discarded material form huge garbage accumulations. A London newspaper, The Independent, reported that an area twice that of the Unites States in the Pacific contains an estimated 100 million tons of mostly plastic rubbish.

But in our trans-Atlantic part of the gyre, we saw clear, blue ocean. At night, luminescent plankton, punctuated by the glow of phosphorescent jellyfish, flowed away with our waves.

We passed one soccer-ball size fishing float and one free-floating, unidentified data transmission buoy. I saw no trash in the water, save that discarded by our own ship before landfall.

What I did see were hundreds of Atlantic flying fish skittering across the sea, sometimes hundreds of feet, gliding down to immerse quivering tails and boost themselves for a second flight. These creatures are often swept high enough by wind currents that they come aboard ships. The flying fishes were larger (from perhaps 5 inches to a foot) as we approached the equator.

They travel with ocean currents as well, including the Gulf Stream, which has brought them as far north as Wood's Hole, MA, and the Gulf of Maine.

There is a record of flying fish in 1878 (although no preserved specimens) from the lower Chesapeake Bay and even the mouth of the Potomac River. I have a few times thought that I saw them off Smith and Tangier islands.

There were also porpoises in the open Atlantic, mostly in the mornings, and one day whales, their great glistening backs rising and falling as they swam away.

Shearwaters, birds with wingspans of 45 inches, accompanied us, gliding on the wind.

We were sailing to find the trade winds, named because they enabled trade to flow across this great trackless ocean in an orderly manner. They are driven by the great circulating gyre of wind, and to a lesser degree currents, which runs clockwise around the North Atlantic. They blow harder in the cold months and thus, as autumn comes, mariners await the stiffening winds to speed their journeys.

Part of this gyre system, the Canary Current, sweeps down past the Madiera Islands. On voyages of 30-40 days these winds provide a boost anywhere from 144-480 nautical miles(166-552 statute miles) during the passage. This was critical when the crew was coming down with scurvy or running out of fresh water. The assist from this current could get a cumbersome ship to the Caribbean Isles almost four days earlier.

As we left Hierro, our winds were very light, which meant burning expensive oil to keep moving. Capt. Klaus Muller, who had a schedule to keep, was also responsible for controlling costs. The ship tracks how much oil is saved for the owners when winds are sufficient to move the ship. Under power, Clipper burns 3 to 5 tons a day, but when the wind is right, fuel consumption drops as much as 70 percent to just what's needed to run generators, the reverse osmosis units making fresh water, winches and-in the tropics-air conditioning for the passengers.

The captain's strategy, as in centuries past, was to run farther south, deviating from course, in search of stronger winds nearer the equator. "The trades will be steadier once we get about 200 (nautical) miles farther south," he said. "We will find the wind."

At that time, my 16th century replica navigator's backstaff showed our latitude to be 28 degrees 51 minutes north. The ship's three GPS systems said 27 degrees 50.7 minutes north. I was off by 36 miles, a touch more than half a degree, and about what was expected for these old wooden instruments.

We did find the winds but encountered a problem. Illness and death were part of almost every crossing in the ages of discovery and colonization. Death aboard modern ships is not unknown. One of Royal Clipper's passengers became gravely ill, and the decision was made to radio the Canaries and turn back. We were met by a rescue boat, which took the passenger, then set off again. (The illness turned out to be complications from a recent surgery, and the woman, we learned, completely recovered.)

Next morning aboard Clipper, I stood on the bridge with First Officer Nikolay Sadovikov. He said he'd been talking the previous night with Russian friends aboard other ships at sea when he saw what appeared to be a rain squall off the ship's port side. His Russian compatriots suggested that it might be something else. He flipped on Clipper's big searchlights and saw the air filling with dust and thousands of insects! A huge Saharan dust storm was coming across the sea from Mauritania, on West Africa's coast about a thousand miles away. Sadovikov altered course to escape the cloud. Imagine a thousand locusts and other bugs in the main salon just in time for breakfast. As it was, many passengers talked about finding large "grasshoppers," various moths and small stink bugs.

As we worked southward almost 200 nautical miles closer to the equator, the trade winds had crossed thousands of miles of warming sea, picking up immense quantities of moisture. The sun warmed the air each day which rose, creating large tropical clouds that sometimes condensed into rain. We could see these showers around the ship, sheets of rain creating dark bands slanting above sea. Sometimes, dawn found the decks scoured clean with fresh water from torrential rains. What a Godsend this would have been in the age of discovery: life-giving water.

While clouds tempered the increasing strength of the tropical sun, which was more direct the farther south we traveled, this would have posed a real problem for navigators using 16th and 17th century instruments. I required a bright sun before, during and after solar noon, and on some days, clouds threatened to obscure the sun and take away the bright sun spot my sight vane required to measure altitude.

Missing a noon sight, until the time of accurate sextants and comprehensive tables, reduced one to attempting a night altitude of Polaris, and calculating latitude from that. When Clipper was just 13 or 14 degrees north of the equator, Polaris was vanishingly close to the horizon and often not visible through the rim of salt haze hovering over the sea. The horizon itself (black on black!) could not easily be seen with any accuracy, so a different instrument, the astrolabe, was used. When suspended to get the instrument exactly vertical on a rolling ship, I found it impossible to align this dim star with the two holes through which I must sight to measure altitude.

Crossing the sea in search of a destination on the far side depended on maintaining a clear understanding of where the ship was at any time. Losing that reckoning threatened a safe arrival. In my re-enactment of the 17th century experience, I had no real measure of East-West distance traveled.

Had I been truly dependent on my own figures, I would have made hourly estimates of speed and kept a record of direction sailed. My crew would have kept these figures for me day and night, recording speed and course on a wooden tally or "traverse board" which I, as navigator, would be responsible for reckoning.

Speed was determined by guess and by golly, often with just a chip of wood tossed over-side and the time of its travel along the ship's side counted in seconds.

Proper mariners used a triangular board called "the chip," which was weighted to float upright and resist towing through the water. It was tethered to a long rope that was wound on a free-running spool. As the line paid out through a crewman's hand, he counted spaced knots in the rope until a small glass-like an egg-timer-ran out. He would close his hand and the number of knots which had passed was the ship's speed "in knots." These little hourglasses were expensive and easily broken. Many mariners had none and might, while the line ran out, murmur some rhyme or religious saying to meter the passage of time.

By and large, mariners had only the dimmest idea how far they had come. They had no measure of currents, or how an oblique wind might have set the ship aside her intended course. Thousands of people died through the centuries as a result of such errors.

We think of the compass as immutable and always pointing north, but the magnetic pole is different and changing from the geographical North Pole to which our charts are ruled and by which we navigate. As 17th century mariners already knew, Hierro in the Canaries was one place where north by the compass was very close to north based on the observation of Polaris. But that was not true as they forged west across the ocean.

We had come 1,740 nautical miles (2,001 statute miles) west from Hierro, and were in an astounding 18,000 feet of water. Bottom was an abyssal plain forever bathed with impenetrable darkness 3.4 miles beneath us. Still the earth was creating mysteries.

One of my shipboard colleagues, Englebert Wolny, brought his GPS to me one morning and pointed out that it differed from the ships' compass by almost 20 degrees. We went in the wheelhouse and on the large sailing charts saw that we were close to one of the lines plotting positions of equal compass variation. The earth far beneath us was indeed distorting the magnetic heading of the compass 18 degrees.

I had been trying to estimate noon using a 17th century replica fly-needle compass with a sundial vane on its face. If I believed that instrument, I was reckoning noon, when the sun's shadow points true north, considerably in error. Instead, I cheated by using the ships' computer almanac to estimate when noon would occur and tried to start my backstaff readings in advance of that time, to be sure to sight the actual peak altitude of the sun. I continued them afterward to assure the angle was really decreasing.

Wolny had a good modern optical sextant, and one morning we ran a series of altitudes to compare my 16th century technology with his instrument. Considering that I was using a glued-up stick, with its arcs hand ruled and divided, the two instruments ancient and modern were linear with each other and the error was about 43 minutes, or about 0.72 degrees, of arc.

Leaving the abyssal plain, now bowling along in the trade winds, we crossed onto what oceanographers have named Research Ridge, a seamount that rises to a depth of 1,800 feet. This shallowing of the ocean floor beneath us was a signal of the landfall to come.