The great idea that led Columbus to conquer the ocean in 1492 was that the world was small enough to reach the riches and spices of the East by sailing west.

Columbus always would remain convinced that he had opened a new trade route to the Indies.

But within a decade, Amerigo Vespucci had twice crossed the same ocean and returned with the conviction that he had seen a new continent and that the world was much bigger than anyone had imagined.

While Vespucci had no claim of arriving first on the lands that would later be graced by the name America, a take on Amerigo, his great discovery was the truth.

"The discovery of the New World is an accomplishment not consummated in the expeditions of Columbus," wrote Argentine naval historian H.R. Ratto. "The glory of Vespucci is, indubitably, to have formulated the concept of the American continent."

Vespucci understood what others had not because his open mind let him perceive contradictions, and his drive for perfection led him to search for answers.

"He was a realist who relied upon observation, was eager to receive new truth and willing to adjust his mind to it," wrote Frederick Pohl in "Amerigo Vespucci: Pilot Major." And "he never swerved from his single purpose, which was to add to man's knowledge of the Earth and to man's ability to steer his course upon the face of it."

Vespucci's quest led him to explore 6,000 miles of undiscovered coastline — more than any other explorer in history. It led him to devise a more practical way of determining longitude that sailors would depend on for three centuries.

Vespucci had a thirst for knowledge. He learned everything about navigation and astronomy he could from experts; because of that he was able to see where they were wrong.

Vespucci (1454-1512) was born into a prominent political family in Florence, an Italian banking and cultural center. His father pointed him to a career in commerce.

Vespucci's education was entrusted to his uncle, Giorgio Antonio, who was a Dominican friar and renowned scholar.

"He instructed Amerigo in the teachings of Aristotle and Ptolemy, firing his nephew with the particular ambition of adding to the sum of human knowledge, especially in astronomy, cosmography and geography," Pohl wrote.

Vespucci did undertake a career in business, overseeing operations for the Medici family in Seville, Spain, including the supplying of ships preparing to cross the ocean.

Even before Vespucci decided to follow in the wake of other explorers, he was fascinated with the great unknown questions of geography.

"We learn from the records of his life that he already was making a collection of all the charts, maps and globes that he could find," wrote biographer Frederick Ober.

Vespucci was slow to accept conventional wisdom and fast to reject it when he found evidence to the contrary.

"I am one of those followers of Saint Thomas, who are slow to believe," he wrote to Lorenzo di Medici in one of three letters that provide most of what we know about Vespucci's voyages.

On his first journey, Vespucci set sail in 1499 from Cadiz, Spain, and became the first European to explore the coast of South America and discover the Amazon.

In a letter written upon his return in 1500, Vespucci wrote, "It is said that there are not more than 77 languages in the world; but I say that there are more than a thousand, as there are more than 40 which I have heard myself."

Contrary to the accepted belief that land south of the equator, called the Torrid Zone, was uninhabitable, Vespucci found the air fresh, the climate temperate and the region well-populated.

"Rationally, let it be said in a whisper, experience is certainly worth more than theory," he wrote.

Vespucci's passion for understanding the world and for a place in history trumped his desire for riches.

Spanish historian Martin Fernandez de Navarrete wrote in the early 1800s that Vespucci "did not bring home many pearls. . . . The desire to push on for discovery was greater than for the acquisition of riches."

Vespucci reached for greatness and had the determination to overcome great obstacles.

In between his first and second voyages he wrote:

"In the endeavor to ascertain longitude I have lost much sleep, and have shortened my life 10 years, but I hold it well worth the cost, because if I return in safety from this voyage, I have hopes of winning fame throughout the ages."

The challenge of figuring longitude at sea had stumped navigators and mapmakers since Ptolemy first plotted latitude and longitude lines on an atlas in A.D. 150. The key was to know what time it was, both onboard and in the home port. Without accurate timepieces, this proved virtually impossible.

Sailors were limited to dead reckoning — guessing their east-west speed often by throwing a chip overboard at the bow and estimating the time it took for the stern of the ship to pass it. Alternatively, they could compare the time of an eclipse at sea to the time it occurs at home as listed in an almanac, though the infrequency of observable eclipses made this impractical.

Vespucci, who had long struggled with the problem, hit upon an approach in 1499. Instead of using eclipses as a point of comparison, he saw the logic in using conjunctions, the common occurrence of the moon passing before a planet.

The famed Capt. James Cook said in the 18th century, "The method of lunar distance from the sun or stars is the most priceless discovery which the navigator ever could have made, and must render the memory of the first discoverer of this method immortal."

Having sailed the northern coast of South America on his first voyage, Vespucci believed he would find a passage to the East farther to the south. His second voyage along nearly the entire eastern coast of South America made him believe he had "arrived at a new land, which . . . we observed to be a continent."

"He had discovered a new continent in the only way it was possible at that time to discover it, by extensive exploring coupled with well-grounded deductions in line with a remarkably near-to-accurate conception of the circumference of the earth," Pohl wrote.

BY JED GRAHAM