Lunar Distance Method of Determining Longitude 

Apianus proposed to determine longitude by using a lunar distance method. While his theory was sound, the 16th century did not possess the instrumental, theoretical and mathematical tools to make it work. Not until the late 18th century were these tools available. In this illustration from Cosmographia we see the cross-staff proposed for making lunar distance measurements. Although the cross-staff was a useful instrument for measuring latitude in navigation, it was not sufficiently accurate for the lunar distance method. Successful lunar-distance longitude determination awaited better lunar tables and the development of the sextant.

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