When Christopher Columbus arrived in America in 1492, he thought he had reached the shores of Asia. He probably would have been able to avoid the error and anticipate the achievement if he only had an accurate timepiece and an accurate star chart. With the two combined, he could have reliably charted his course. Sailors had long been able to determine latitude (north-south position). By using a quadrant to measure the altitude of the sun or the polestar--the brightest star that appears nearest to either celestial pole at a certain latitude. But latitude is only half the data needed. Longitude and latitude intersect to pinpoint position. While on land longitude could be measured and checked against reference points, at sea navigators had to relate the positions of the stars and other objects in the sky with time to determine where they were on the ocean. The necessary celestial measurements...