By the routine methods of navigation the latitude and longitude are in general not determined at any one time independently of one another. The determination of the latitude by the meridian altitude of the sun necessitates a knowledge of the Greenwich time (at least approximately) so that we can calculate its declination at the time of culmination. The same is the case with the planets. The only method in general use by which the latitude may be obtained, without the time, is that derived from the culmination of a star. But as the culminations of the principal stars, such as are used in navigation, are infrequent, this involves tedious waiting. Furthermore, the use of a night horizon is unsatisfactory and often impossible. Practically the altitude of a star can be determined only under three conditions, at twilight, at dawn and with sufficient moonlight. Generally, therefore, we can only utilize the culmination of a star when the horizon is sufficiently illuminated by the moon.
A Method of Determining Latitude at Sea without Time