In 1766 Nevil Maskelyne, fifth Astronomer Royal, compiled and published the first edition of The Nautical Almanac and Astronomical Ephemeris, designed to make practicable the determination of longitude at sea by the method of lunar distances. To mark the occasion of the two-hundredth anniversary of this event the staff of H.M. Nautical Almanac Office presents an objective assessment of the method of determining longitude by the measurement of lunar distances, in terms of modern knowledge, outlook, notation and computing facilities.

Reference to contemporary publications has been deliberately avoided; the story of the development of methods and techniques for the observation and reduction of lunar distances requires, and deserves, a separate study.