In April 1916, as Shackleton and five other men took to the sea in a tiny lifeboat, their only hope for survival was to land at the island of South Georgia, some 800 miles away.

Few challenges posed by the James Caird journey were more daunting--or critical--than navigation. This task fell to Frank Worsley, the captain of the lost Endurance, who had experience in making landfalls on small islands in the Pacific. To plot the course to South Georgia without any landmarks, Worsley drew upon a handful of tools including a sextant.

Taking an accurate sextant sight is not easy even in calm sailing, and for sailors Frank Worsley's successful navigation of the Caird has an almost mythic dimension. As high seas pitched the small boat, Worsley was held upright by two companions while he sighted the sun between thick clouds; the horizon could only be estimated. Then, crouched in the bottom of the boat, he worked out the math with the stub of a pencil and consulted his blurry, waterlogged tables and his Nautical Almanac. In the course of the 17-day, 800-mile journey, Worsley was able to take only four sextant readings. Yet even a degree of error could cause the boat to miss her landfall.