In these days of accurate hydrography and cartography the navigator seldom has to worry about his charts. True, there may be changes in soundings and the depth of certain areas of the ocean may remain to be correctly determined and there still may be a few reefs and islands shown as P.D. There is never any doubt, however, that the latitude and longitude of coasts and islands, as charted, are substantially accurate.
Prior to the laying of the first transatlantic cable, the method employed to establish secondary meridians was the observation of moon culminations, and in 1866 the French government organized several parties which in the next several years visited many points in America, Polynesia, Japan, China, and India to permanently establish secondary meridians.
In commenting on the objections to this method the late Professor Asaph Hall of the Naval Observatory said:
It should be noticed that the moon’s motion among the stars being nearly thirty times slower than the rotation of the earth on its axis, an error in the observation or in the position of the moon will appear in the resulting longitudes multiplied by a factor nearly equal to thirty.