Lunar distance method The lunar distance method was used for longitude at sea during the 1700s and was regarded by astronomers as the most promising method for solving the problem of longitude. It required observation of the moon’s motion relative to the stars to calculate longitudinal position at sea. In the sky sphere, the zenith point is analogous to the pole of the longitudinal triangle on the earth, and the horizon is analogous to the Equator. This method was not always fit for purpose, as it required an understanding of spherical trigonomety in conjunction with the use of tables and a consistently clear night sky. Prominent English astronomer Neville Maskelyne experiemented with this method and published it in The British Mariner’s Guide in 1763. He adamantly (and mistakenly) believed that the lunar distance method would solve the problem of longitude. Source Maskelyne, N. 1763. The British Mariner's Guide: Containing Complete and Easy Instructions for Discovery of the Longitude at Sea and Land, Within a Degree, by Observations of the Distance of the Moon from the Sun and Stars, Taken with Hadley's Quadrant: to which are Added, an Appendix, Containing a Variety of Interesting Rules and Directions, Tending to the Improvement of Practical Navigation in General. And a Sett of Correct Astronomical Tables. Published by author.
