Longitude by chronometer highest point, diminish the accuracy of determining longitude by chronometer as a method of celestial navigation. Accuracies of less than error in position are difficult to achieve using the "longitude by chronometer" method. Other celestial navigation methods involving more extensive use of both the Nautical Almanac and sight reduction tables are used by navigators to achieve accuracies of one nautical mile (1.9 km) or less. This only calculates a longitude at the assumed latitude though a position line can be drawn. The observer is somewhere along the position line. Time sight is a general method for determining longitude by celestial
Mercator 1569 world map that OE=OD, here approximately 54°. This is a measure of the rhumb line distance between the points on the sphere corresponding to A and B on the spherical Earth. Since each degree on the equator corresponds to 60 nautical miles the sailing distance is 3240 nautical miles for this example. If B is in the second quadrant with respect to A then the upper rose is used and if B is west of A then the longitude separation is simply reversed. Mercator also gives a refined method which is useful for small azimuths. The above method is explained in Legend