When the voyaging canoe Hokulea leaves Hawaii this month on a three-year global odyssey, its crew will navigate primarily using ancient methods.

To prepare, its navigators will chart a course on a map. Aboard the sailing canoe, they rely only on memory of that course.

Their wayfinding relies on the movement of the sun, the position of stars and a feel for ocean currents.

The crew constantly gauges their speed to figure distances. Debris floating on the open ocean hints at land. Navigators rely on seabirds to lead them to islands.

Only a few navigators in the world are capable of sailing so precisely across thousands of miles without instruments. To them, their art remains a feat of mental math, applied astronomy, intuition, endurance, mental fortitude and keen observation.