How do you navigate the seas without a compass?

THE MOST IMPORTANT moments of a day on Hōkūleʻa, a 62-foot-long deep-sea canoe, are sunrise and sunset. That’s when the navigator can know for sure where the sailboat is headed. In between, the swell—the direction of the waves—helps hold course, but “you have to have known what direction it’s coming from based on where the sun rose,” says Kaʻiulani Murphy. At night, the stars are an important guide: “The sky kind of gradually changes with your latitude,” Murphy says. But on cloudy days, it is impossible to find a true path without sunrise and…