The Moghul

that permitted taking the elevation of the sun by its shadow. But even with the astrolabe there was the problem of catching the sun precisely at midday. And on a rolling ship the error in reading it could easily be four degrees.
    For longitude, a ship's location east or west on the globe, there were no fixed references at all; but as a mariner traveled east or west, the sun would come up somewhat earlier or later each day, and precisely how much earlier or later could be used to compute how far he had gone. Therefore, calculating longitude depended solely on keeping time extremely accurately—something completely impossible. The best timekeeping device available was the hourglass or "sandglass," invented somewhere in the western Mediterranean in the eleventh or twelfth century. Sandglass makers never achieved real accuracy or consistency, and careful mariners always used several at once, hoping to average out variations. But on a long voyage seamen soon totally lost track of absolute time.
    Since they were unable to determine a ship's location from the skies, mariners also tried to compute it from a vessel's speed and direction. Speed was estimated by throwing a log with a knotted rope attached overboard and timing the rate at which the knots in the rope played out—using a sandglass. Margins of error in computing speed were usually substantial. Direction, too, was never known completely accurately. A compass pointed to magnetic north, not true north, and the difference between these seemed to vary unaccountably at different locations on the globe. Some thought it had to do with the lodestone used to magnetize the needle, and others, like the Grand Pilot of the king of Spain, maintained seamen were merely lying to cover their own errors.
    For it all, however, longitude was the most vital unknown. Many attempts had been made to find a way to fix longitude, but nothing ever worked. Seamen found the only real solution to the problem was "latitude sailing," a time-consuming and expensive procedure whereby a captain would sail north or south to the approximate latitude of his destination and then sail due east or west, rather than trying to sail on the diagonal. King Philip III of Spain had offered a fortune to the first man who discovered how to tell longitude at sea.
    Hawksworth spent days poring through the piles of tables, many of which were strewn about the floor of the room and damaged from mildew and rot. Next he carefully copied the symbols off the walls of the circular building and matched these with those on several of the charts. Were these the names of the major stars, or constellations of the zodiac, or . . . what? The number was twenty-eight.
    And then it came to him: they were the daily stations of the moon.
    As he continued to sift through the documents, he realized that the scholar who wrote them had predicted eclipses of the moon for many years in advance. Then he found a book, obviously old, with charts that seemed to provide geometric corrections for the distortion caused by the atmosphere when sighting stars near the horizon, something that always had been troublesome for navigators.
    He also found other writings. New. Some appeared to be verses, and others, tables of names and numbers. Sums of money were written next to some of the names. But none of it meant anything without the Persian, which he could not read. And Shirin had never returned to the observatory, at least not when he was there.
    Until two days ago.
    At the observatory that morning the sky had been a perfect ice blue, the garden and orchard still, the air dry and exhilarating. No workmen were splashing in the moat beyond the wall that day. Only the buzz of gnats intruded on the silence. He had brought a bottle of dry Persian wine to make the work go faster, finding he was growing accustomed to its taste. And he had brought his lute, as always, in hope Shirin would come again.
    He was in the stone hut, cleaning and sorting pages of manuscript, when she appeared silently in the doorway. He looked up and felt a sudden rush in his chest.
    "Have you uncovered all of Jamshid Beg's secrets?" Her voice was lilting, but with a trace of unease. "I've found out that was our famous astronomer's name. He was originally from Samarkand."
    "I think I'm beginning to understand some of the tables." Hawksworth kept his tone matter-of-fact. "He should have been a navigator. He could have been a fellow at Trinity House."
    "What is that?"
    "It's a guild