The problem seemed overwhelming. Since the great earth was curved, no line on its surface was straight, and once at sea there was absolutely no way to determine exactly where you were, which way you were going, or how fast.
The least uncertain measurement was probably latitude, a ship's location north or south of the equator. In the northern hemisphere the height of the polestar was a reasonably accurate determinant of latitude, although it was a full three degrees distant from the northernmost point in the sky. Another measure of latitude was the height of the sun at midday, corrected for the specific day of the year. The problem lay in how to measure either of these elevations accurately.
A hundred years before, the Portuguese had come across an ingenious Arab device for telling the elevation of the sun. It consisted of a board with a knotted length of string run through the middle. If a mariner held the board vertically and sighted the horizon at one end and some object in the sky at the other, the length of the string between the board and his eye could be used to calculate the elevation of the object. In a short time a version appeared in Europe—with a second board replacing the string—called the cross-staff.
However, since locating both the horizon and a star was almost impossible at any time except dawn or dusk, this device worked best for sighting the sun—save that it required staring into the disc of the sun to find its exact center. Also, the cross-staff could not be used when the sun was high in the sky, which was the case in equatorial waters. Another version of the cross-staff was the astrolabe, a round brass dial etched with degree markings and provided with a movable sight 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.