How did sailors find their longitude at sea?

Sailors solved the longitude problem by using time. The Earth turns 360 degrees in 24 hours, so each hour of difference between local time and the time at a reference point such as Greenwich equals 15 degrees of longitude. Local time was easy to find from the Sun. The hard part was knowing Greenwich time while far out at sea. In the 18th century two methods emerged: carrying an accurate marine timekeeper, and working out Greenwich time from the Moon and stars. [1] [2]

Latitude could be found fairly easily from the Sun, but until the 1760s there was no reliable way to find east-west position out of sight of land. Mariners relied on dead reckoning, plotting each change of course and speed. Crude measurements and hard-to-measure ocean currents added errors that compounded the longer a ship went without sighting a landmark, sometimes with disastrous results. [2] [3]

Britain's Parliament passed the Longitude Act in 1714, offering up to £20,000 for a practicable and useful solution. Clocks of the time, driven by weights and regulated by pendulums, worked in church towers but were useless on pitching ships. Temperature changes made metal parts expand or contract, and humidity affected lubricants. [3]

John Harrison developed the first marine timekeepers in the 1750s and 1760s, though his first, H1, was finished in 1737 and weighed 75 pounds. He replaced weights with springs and pendulums with balance wheels, used laminated strips of dissimilar metals to resist temperature change, and used jewels and self-lubricating wood to reduce friction. His fourth, H4, resembled a large pocket watch and ticked five times a second. The Board of Longitude judged a trial voyage to Jamaica, begun in 1761, insufficient. After a second trial to Barbados in 1764, it met in February 1765 and accepted that H4 had kept time within the limits of the 1714 Act. [3] [4] [2]

Larcum Kendall made a copy of H4 that went with James Cook on his second voyage, and Cook reported that it exceeded expectations. Later makers such as John Arnold and Thomas Earnshaw streamlined production, and by the 1780s a boxed Earnshaw cost as little as £65. The term chronometer was first applied in 1780 by Arnold. [3] [2]

The alternative used simpler technology. With a sextant, a navigator measured the angle between the Moon and the Sun or certain stars, then performed lengthy calculations correcting for atmospheric and optical distortions. Nautical almanacs, with tables prepared by astronomers and by human computers, many of them women and children working from home, let the navigator look up Greenwich time for a set of readings. [2]

The method was cheap, accurate and reliable, and widely used from the 1760s. Chronometers were expensive and fragile, and if one stopped, Greenwich time was lost. Both methods remained in use until the late 20th century, when satellite navigation such as GPS largely replaced them. [2]