Bowditch on Solving Lunar Distances
Lunar distance was a complex but important means of determining longitude at sea. It remained useful throughout most of the 19th century as many ship captains could not afford expensive chronometers. Using lunars, longitude could be found with nothing but tables and a octant or sextant. Bowditch was important in navigational history in part because he came up with easier methods for solving lunar distance sight reductions.
This passage is from Bowditch's New American Practical Navigator, 1868, p. 231.