Lunar distances and the Nautical Almanac

Abstract

The Royal Observatory was founded in 1675 specifically for “… the rectifying the tables of the motions of the heavens, and the places of the fixed stars, so as to find out the so much desired longitude of places for the perfecting the art of navigation.” Ninety-one years later Nevil Maskelyne, the fifth Astronomer Royal, was able to compile The Nautical Almanac and Astronomical Ephemeris for the year 1767 which made possible the determination of longitude at sea to an acceptable precision. The key to the successful solution of this problem is the method of lunar distances, and this method is discussed both historically and scientifically. The objects of this paper are: to examine the contribution to this achievement made by the Royal Observatory and its Astronomers Royal; and to speculate on the reasons for, and the effect of, the adoption of a nominal precision of 1″ (quite unnecessary accuracy) in the tabulations of lunar distances and in the associated calculations. All the main historical events and developments are well-known, and the historical detail has been curtailed accordingly.