 This is a mathematical problem of solving a triangle.If a navigator knows how long the ship has sailed on the erroneous course, he can calculate its current distance from its intended course, and estimate how long it must sail back on a new bearing until it recovers its old course.In the Corsica-to-Genoa example, there is an implied triangle "ACD", with one side given ("AC" = 70 miles on actual NW course), a 45° angle at "A" (angle of difference between actual course NW and intended course N) and another angle of 90° at "C" (angle of difference between actual course NW and return course NE).