The sixth edition follows faithfully in the tradition of its honored predecessors which, over a period of more than forty years, have provided exactly what is needed for beginning navigators to learn the subject on their own. It explains what is being done and why, without getting into mathematics beyond adding and subtracting, and occasional multiplying and dividing. The method taught is H.O. 249 because it is simple, inexpensive, and accurate enough for use even by experts.