“The stars might lie, but the numbers never do.”
—Mary Chapin Carpenter, “I Feel Lucky”
Ancient sailors had two problems. First, they had to know their latitude—that is, their north-south position on a map. Second, they had to know their longitude, or east-west position. Determining latitude by observing the North Star was relatively easy and had been done as early as 300 BC. Longitude presented a more difficult problem because it relied on the use of a relatively accurate clock, or chronometer. Unfortunately, there were no sufficiently accurate chronometers (especially that would fit and work aboard a ship) until the early eighteenth century.
Before the invention of the chronometer, a sailor could only ...