For centuries, determining longitude was an extremely difficult task for sailors, so difficult that it’s been thought improbable — if not impossible — for animals to do it. But migratory sea turtles have now proved capable of sensing longitude, using almost imperceptible gradients in Earth’s magnetic field. That turtles and other migratory animals could detect these small changes was considered unrealistic, but experiments on animals released in out-of-the-way locations repeatedly described them finding home with unerring accuracy and efficiency, explicable only as a product of both longitudinal and latitudinal awareness. The mechanism found in turtles might also exist in birds.