Harry found himself hoping that Colon would lose his argument yet again. For if Colon by some chance swayed de Santangel, Harry knew that he was going to have to stand on his feet and shout Colon down, betraying years of effort by himself and his friends. Harry prayed that this convoluted dilemma could be resolved in some way that spared him making such a dreadful decision.

Without preamble Colon began to speak. It was a case he had set out many times before, and he told it clearly, with a deep, strong voice. But his tone was laced with impatience that he was forced to go through all this again, and his Spanish was poor, his Italian accent thick, and he stumbled over his words.

He began by quoting the authorities of antiquity. He leaned heavily on the great Ptolemy, who in the second century had presented a clear and consistent model of the world, around which the sun and stars rotated on crystal spheres. Colon knew Ptolemy through a three-centuries-old Latin translation of a book called the Almagest that had been preserved by the Arabs.

He referred too to the work of Pope Pius II, who before his elevation had written a book developing Ptolemy's ideas, and a cardinal called Pierre d'Ailly whose Image of the World had presented strong views on the relative sizes of Asia and the Ocean Sea. He read extracts from Roger Bacon's Opus Majus, which was a synthesis of Ptolemy's vision with more recent work by Christian and Muslim geographers. Bacon argued that there was no significant land mass to be found save those already known: Asia, Africa and Europe.

Ferron scowled as Grace's own engineer was quoted against her case.

Summing up these ideas, Colon presented a map drawn for him by one Martin Behaim, with a relatively narrow Ocean Sea and only a scattering of islands between Europe and Asia. If Colon's arguments were correct then a voyage west across the Ocean Sea to the riches of the east, thus closing a circle around the curve of the earth, ought to be a trivial matter.

Colon was interrupted by a weary-looking geographer from Talavera's party, who presented rebuttals he had been forced to make many times before. All this was supposition, he said, addressing de Santangel. 'Nobody would argue with the bulk of Ptolemy's ideas, so lucidly set down in a greater age than this, and preserved through the ignorant copying-down of the Arabs. Of course the earth sits at the centre of the universe; that much is obvious to a child.

'But as to the size of the earth, even the great Ptolemy is only one authority among a range with differing views. Centuries before Ptolemy was born the Greek Eratosthenes computed the earth's circumference from the angle of shadows cast by the sun, and derived a figure somewhat greater than Ptolemy's. These "authorities" like d'Ailly have clearly been selected by the gentleman because their estimates of the earth's figure all lie at the lower end of the range of accepted possibilities, the mean of which lies, I should judge, somewhere around four times bigger than that shown in the gentleman's charts. And if that is so, if we were so foolish as to set off a boat into the western sea, the crew would never return – and nor would our money!' That won a laugh.

But Harry knew he had a point, as he and Geoffrey had indeed selected many of the more helpful authorities and fed them to Colon.

Colon, however, was undeterred. He said simply that nobody could prove his figures right until the earth was sailed all the way around, but nobody could prove them wrong either.

He went on to the next leg of his case, that a voyage across the Ocean Sea would be no more than an extrapolation of journeys of exploration that had already been made – mostly by the Portuguese, he pointed out to the Spanish court. He spoke of developments in shipbuilding, and of the discovery of hitherto unknown islands in the Ocean Sea, from Madeira and the Canaries to the Azores.

And he spoke of earlier attempts to sail west across the Ocean Sea. He had collected many such tales in his years on Porto Santo and in Portugal. One Diogo de Tieve had sailed steadily south-west from the Azores, coming to a place where the condition of the sea and the circling of land birds indicated that land lay to the west, but he had had to return before it was reached. A Genoese merchant called Luca de Cazana had mounted several voyages that sailed west from the Azores for a hundred leagues or more, but in vain. Colon said that even unsuccessful explorations returned useful information about the state of the winds and the ocean currents. Colon himself had sailed far down the coast of Africa, and, he claimed, knew the Ocean Sea as well as any man alive.

Now he moved on to older legends. He referred to stories of the voyages of Saint Brendan and other Irish monks who had supposedly sailed far to the west across the north of the Ocean Sea, encountering mountains of ice and other strange phenomena. He quoted from the sagas of the Vikings, who, as was well known, had built churches in a western land so rich and warm that vines grew readily, so extensive it had never been explored properly.

Colon turned at last to what he believed to be the strongest plank of his argument: his own personal experience. He spoke of evidence he had seen with his own eyes, relics from the lands across the sea fortuitously washed east: bits of carved wood, branches of unfamiliar species of trees, even war canoes carved from single logs. It had been such bits of evidence, Colon said, which had been the seed of his own quest to go west.

Concluding, Colon spoke of the riches of the east, and quoted at length the biography of Marco Polo, who had crossed China in the days of the Mongol Peace. And Colon spoke of his dream of a grand alliance of western Christendom with the armies of the Khan and the wealth of the domain of Prester John, all linked by Colon's mighty new trans-oceanic trade routes: a grand alliance that would fall upon the great Islamic states from the east and wipe them from the earth, at last liberating Jerusalem from the hands of the infidel.

One by one the court's experts rose to counter aspects of Colon's case.

Harry half listened. Of course, he thought, there was a hope, just a grain of it, that Colon might actually be right. Much of Colon's evidence was faked by Harry, Geoffrey and Abdul. But not all of it.

And if anybody could succeed at this venture it would be Colon. He was a vision of doggedness, of determination. Where others had tentatively probed a little way west before running for home, if Colon ever got his ships he would keep right on going come what may.

Yet, Harry thought sadly, Colon was at the same time an unimpressive figure. His poor education showed in the stumbles in his Spanish, and in his sometimes weak grasp of scholarship. Snobs about the new printing presses mocked men like Colon, who now found learned material much too easy to get hold of, and regurgitated it, half-digested, in pursuit of their own pet theories. Such nostalgics pined for the good old days when books were so rare and expensive that scholarship was properly kept in the hands of the privileged few.

But, Harry thought, his soul stirring, if the prophecies were right, these flawed arguments by a flawed man might yet result in a bold stroke that would transform the fortunes of mankind for ever.

The last speaker was Diego Ferron. 'This is all fanciful. Legend. Tittle-tattle from drunk sailors. It is the mission of this court to prosecute holy war. But the way to do that is to drive east, straight for the exposed belly of Islam, not to follow some fool's errand to the west. The monarchs' money should be spent on weapons, not ships, not thrown into the endless Ocean Sea in pursuit of a dream…'

De Santangel heard him out. Then he got to his feet.

And just as he did so there was a murmur at the back of the court. Harry turned.