I've spent a lot of time anticipating the 400th anniversary of John Smith's Chesapeake explorations. In that time, I've relived, to some extent, many parts of those two voyages. The deeper into this quest, however, the more complex it became to really put oneself in the place of these intrepid fellows.

In 2007, Ian Bystrom and his crew, rowing and sailing the Sultana Project's plausible shallop re-creation, probably came as close as one could. They never accepted a towline or cheated on the physical demands of their voyage. While they did have modern clothing, camping and foul weather gear, sunscreen and pretty much any diet they chose to assemble, this does not diminish the difficult physical challenges they encountered.

But, like me, the replica shallop's crew had the incredible advantage of modern charts and the U.S. satellite-based global positioning system. Both they and I knew our positions within several yards at almost any time.

Smith had no such assistance and, in fact, had only an approximate idea where he-or the Chesapeake-was situated on the globe. He and Capt. Christopher Newport, who sailed with him across the Atlantic, needed two pieces of information to fix themselves in a geographic position: 1. their distance north and south on the globe and 2. longitude, their distance along a line from Europe and England.

This second piece in the puzzle would elude mariners and cartographers in all but the crudest sense for another century and a half.

Late in 2007, I had the opportunity to navigate using a set of replica 17th century instruments along a course close to that followed by the Virginia Company Adventurers across the Atlantic Ocean.

The earth spins on its axis in frictionless space at a constant rate, thus for our purposes-as well as Smith's-a day, is a day, is a day. When the sun rose astern of a vessel westbound across the Atlantic, it passed through local noon and set at local sunset. Noon, absent a clock of any sort, was most easily assessed with a magnetic compass.

Portable compass/ sundials had been sold in Paris by Jean Fusoris 1365-1436, long before Columbus' time, and by the 17th century, travelers often carried them, including Smith, who also used his to mystify the fierce Powhatan Chief Opechancanough, perhaps saving his life.

Some compass versions had a folding gnomon or shadow vane, which when the compass dial was oriented north in line with its needle, would tell the local sun-time hours of the day with reasonable accuracy.

Local noon was declared when the north-pointing needle and the shadow of the gnomon coincided, and this time of day was vital for the navigator as it was the point in the sun's transit at which English mariners would declare that a new day had begun aboard.

As soon as noon was sighted on Royal Navy ships, the navigator would inform the officer in charge, who would announce "make it noon." The ship's bell was struck 8 times, and a sand hourglass-actually a half-hour glass-was turned. This process was repeated each time the sand ran out. Time was thus kept with approximate accuracy with one important exception. Each day, the ship moves west at the pleasure of the winds anywhere from 75 to more than 200 miles, and the sun, in fact, is rising later and later.

For westbound ships transiting the Atlantic not far from the equator, the sun rises an hour later with each 15 degrees of longitude traveled, and by the time one reaches the Chesapeake on other side of the ocean-local time is five hours off from stationary pendulum clocks ticking away back in London. Really knowing where one was in terms of longitude was only a rough approximation until accurate, portable wind-up clocks were developed in the mid-18th century.

Smith reflected this uncertainty in the first versions of his map, which were printed with no longitudes, or east/west references. It was not until the 1620s that mariners had made enough trips to Virginia that repeated estimates of longitude agreed closely enough and the publisher added numbers to the map.

Latitude, the distance on our globe between the equator and the poles was well understood. We have Eastern civilizations to thank for the set of skills necessary to figure latitude. They appear sometime about the 6th century, some say in Baghdad, others in India and still others attribute the discovery to the 8th century Persian mathematician al-Fazari.

An early, portable tool for determining latitude was the astrolabe, a circular device inscribed with degrees about its circumference, or "rete." This device was suspended by a thumb ring-held exactly vertical even on a rolling ship-by gravity. A sight vane or "rule" rotated like a propeller about the central pivot with a little peep hole at each end. Sighting through these, one could measure an angle against the rete. This technique was used to measure the height of the sun, the north star or other celestial bodies. Initially used by Islamic nations to determine times for prayer, astrolabes were later found to be helpful navigating on land or sea.

Conceptually, if one stands at the north pole on earth, Polaris or the North Star is right overhead, and the earth spins about it like the axis of a top. We also know that in winter, the sun doesn't rise at all that far north because it is shining close to the equator and below one's horizon at the pole 24 hours a day.

Conversely, if one is sailing along the equator near the winter solstice (about Dec. 21) the sun is pretty close to being overhead. The North Star is almost at the horizon. The 90 degrees between these extremes represents the range of latitude for the Northern Hemisphere.

As one sails north, Polaris appears higher in the northern sky each night. Its angle above the horizon is very close to one's latitude. Every day, even at noon, as one sails north, the sun appears lower in the southern sky.

Even after heavy metal astrolabes were developed, the little peep holes were difficult to align at sea-imagine trying to find tiny Polaris among all the other objects in the sky while trying to get the angle exactly right on a pitching deck.

The astrolabe was introduced widely in Europe by the 11th century and the "Canterbury Tales" author, Geoffrey Chaucer (1343-1400), even wrote a how-to astrolabe manual for his son. This is almost certainly the instrument Christopher Columbus used during his trans-Atlantic voyage in 1492.

In the 1300s, an inventor-likely the mathematician Jacob ben Makir of Provence, France, developed a calibrated staff to make angular astronomical observation. The staff included a fitted cross piece-also known as a transom or transversal-that slid smoothly along a scale. Others trace it to the Chaldeans around 400 B.C.

The cross staff was held with the end of the staff against a precise spot on the observer's cheek, with one's eye just in alignment. The lower arm of the transom was sighted to be in line with the horizon and simultaneously glancing up and down, by sliding the transom in and out, the upper end was brought to align with the sun. On the calibrated staff scale could be read the angle between the two tips and the observer's eye. This was equal to the altitude of the sun above the visible horizon.

Like the astrolabe, the cross staff was first used on land. Its use as a maritime tool has been traced to the German astronomer Johan Werner in 1514. This was a tricky operation on a plunging ship and took enough time looking directly into the sun that many mariners developed cataracts or became blind in the eye that was used.

Capt. Will Gates of the Maryland Dove recounts the apocryphal tale, only slightly tongue in cheek, that this was why pirates had one eye patched. In an attempt to solve this problem, a brass fitting with a piece of smoked glass was mounted on one end of the transom but, on the moving ship this shading lens, like dark glasses, would only help when the sun's alignment was exact.

The problem was onerous enough that brilliant minds of the time came up with an elegant solution: the Backstaff or Davis Quadrant, after its inventor John Davis, who published his discovery in a book, "Seaman's Secrets" in 1595. This invention is constructed with two partial arcs of a circle, which allow any angle up to 90 degrees to be measured. The two arcs add up to a quarter-circle and the instrument is thus considered a "quadrant." Each of the two partial arcs is supplied with a sliding vane, one through which the sun shone and the second for the observer's eye.

Davis really thought "out of the box" developing his backstaff because to use it one faces away from the sun. The observer stands with the sun exactly at his back and aligns a horizon vane on the staff with the interface of sea and sky. The shadow vane is then slid along the curve of its arc until its shadow covers the horizon vane. The shadow vane is pierced with a small, accurately bored hole which admits a tiny spot of sunlight, and this appears on the shadow vane. The observer, meanwhile, is pressing his eye close to a hole on a sight vane on the other arc, which when moved slowly up or down its arc, changes the position of the spot of sun until its alignment with the horizon is exact.

Introduced only 11 years before the Jamestown adventurers would leave on their voyage, this instrument was state-of-the-art for its time. It's advantage was obvious, but we do not know if this or a cross staff was used for the Virginia Adventure crossings.

Several scholars testing with these ancient instruments conclude that aside from its optical hazard, the cross staff was actually more accurate. And, while it was popular for more than 100 years, the backstaff itself was eventually prohibited on Dutch East India vessels after 1731 because of its inaccuracy.

By then, more sophisticated instruments had been developed and all three of the earlier devices that had enabled the exploration of the globe fell into disuse. Another advantage was that each could be made by a moderately skilled hand in accordance with then-published instructions, and sometimes by the men who would later trust their lives to them at sea.

There is a survival at sea account, in fact, during the World War II years where a man adrift in a life raft used three pencils to construct a simple instrument and was able to track his latitude in the eastern Atlantic until his rescue. While he could not control the drift of his rubber raft, he tracked his latitude sailing exactly as his predecessors had done centuries earlier.

Smith practiced this "latitude sailing navigation" during his exploration of the Chesapeake and it enabled most of the remarkable accuracy he achieved in his map.

My circle of Smith scholars thought at one point in recreating Smith's voyages that he might have used a cross-staff or backstaff, but horizons, either in line with the noontime sun, or with the sun directly at the mariner's back are relatively hard to get on the Chesapeake because of intervening points of land, islands or river banks.

It's likely that Smith and his crew used a more simple and robust instrument: the quadrant. This was a wooden quarter circle with peep sights along one edge and on which were engraved a scale of angles (and other information) and which had only a cord-suspended pendulum as a moving part. That weight could be improvised from a musket ball or stone, and a new cord could be plaited from plants in a Chesapeake forest.

The skills Smith and his mariner counterparts used to cross oceans, avoid harm and map continents fascinate me, and I long wondered how well these apparatus would work in modern trials. To my amazement, I'm not alone and there are a number of trials that have been made. Today's boaters should be grateful that one can purchase a pocket-size GPS unit for less than $100, which makes all this unnecessary.

Grateful, but never smug. Every boater on the water today should still take a page from Smith's careful piloting because he maintained a very good dead reckoning position wherever he went on the Chesapeake and its many rivers. This is as good a practice today as it has been during all the intervening years, because modern navigation aids are not infallible.

Our GPS failed once while my yawl Nimble was making a tricky landfall on an inhospitable shore. A rain squall had overtaken us and visibility in moments went from miles to feet, taking away all the visual cues.

My wife, Nancy, was aboard as navigator and the minute we had trouble with the GPS she marked our position, time, compass course and speed on the chart. We carried our estimated position along on the chart-not easy in driving rain on wet paper-and adjusted our course to clear the dangerous and unseen headland before us.

We also carefully watched the depth, because from the many soundings on our chart we could estimate how far off the shore we were should a current carry us away from our estimated course and against the beach with its dangerous surf.

Navigation historian Dr. Robert Hicks says, only half in jest, that Smith's most frequently used tool during his exploration voyages was the sounding lead. It's certain that he often had one of his men forward plying that lead and calling out depths as they entered any uncertain situation.

Most modern mariners watch depth as well, although using sonar-based depth sounders, which generally provide a continuous readout of the bottom profile.

This instrument, too, is subject to surprising failures if some problem with its sensitivity software develops, or marine growth coats the underwater transducer that sends and receives the reflected signals which it converts to depth. Sometimes the foam from a propeller's wash or large breaking seas will confuse the sensor. Nimble's depth sensor has given me fits under the most urgent conditions for several years.

During the episode described earlier, our depth sensor was working. Even so, as we approached that hazardous shore but it was still great comfort to see the headland's high banks off our port side when the sweeping rain parted for a moment. We changed our course, then in safely deep water, and started tracking our dead reckoning position as we headed into a safe harbor.

Boaters should always be responsible and always be ready to take navigation into their own hands.

In contrast, last summer, I was moored at a boat yard dock when two men roared up on a jet ski. The owner was despondent, having lamented that he had no idea from whence he had come, or what direction he must return. He had no chart, no money, no GPS and was almost out of gas. He lay, face down, on the dock moaning, "this isn't happening to me." I assured him it was. A woman on the dock gave him $10 and pointed out the nearest gas dock. I got out my charts and pointing out landmarks, showed them where to go. I also made them promise to send the lady her money. They neither returned the money, nor even sent her thanks.