---
title: "Longitude (book)"
chunk: 1/2
source: "https://en.wikipedia.org/wiki/Longitude_(book)"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:33:05.967266+00:00"
instance: "kb-cron"
---

Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time is a 1995 best-selling book by Dava Sobel about John Harrison, an 18th-century clockmaker who created the first clock (chronometer) sufficiently accurate to be used to determine longitude at sea—an important development in navigation. The book was made into a television series entitled Longitude. In 1998, The Illustrated Longitude was published, supplementing the earlier text with 180 images of characters, events, instruments, maps and publications.

== Problem of longitude ==
Determining longitude on land was fairly easy compared to the task at sea.  A stable surface to work from, known coordinates to refer to, a sheltered environment for the unstable chronometers of the day, and the ability to repeat determinations over time made for great accuracy.
For calculating longitude at sea however, early ocean navigators had to rely on dead reckoning, or if in sight of land, coastal navigation, which involves triangulating several bearings of the same land feature from different positions. Once out of sight of land, longitude became impossible to calculate, which sometimes led to tragedies in stormy or foggy conditions.
In order to deal with not being able to calculate longitude, captains would sail to the known latitude of their destination, and follow the line of constant latitude home.  This was known as running down a westing if westbound, or easting if eastbound. In Farley Mowat's book Westviking, he gives examples from the Norse Sagas of Vikings using this practice to sail reliably from Norway to the Faroes, then Iceland, then Greenland, then North America, and then back to Ireland, with very primitive instruments.
Determining latitude was relatively easy in that it could be found from the altitude of the sun at noon with the aid of a table giving the sun's declination for the day. Latitude can also be determined from night sightings of  Polaris, the northern pole star.
Navigating purely by latitude was of course vulnerable if the sun was clouded over at noon, and caused problems as it prevented ships from taking the most direct route, a great circle, or a route with the most favourable winds and currents, extending voyages by days or even weeks.  This increased the likelihood of short rations, scurvy or starvation leading to poor health or even death for members of the crew and resultant risk to the ship.
Errors in navigation also resulted in shipwrecks. Motivated by a number of maritime disasters attributable to serious errors in reckoning position at sea, particularly spectacular disasters such as the Scilly naval disaster of 1707 which took  Admiral Sir Cloudesley Shovell and four ships of his fleet, the British government established the Board of Longitude in 1714.

"The Discovery of the Longitude is of such Consequence to Great Britain for the safety of the Navy and Merchant Ships as well as for the improvement of Trade that for want thereof many Ships have been retarded in their voyages, and many lost..." and announced the Longitude Prize "for such person or persons as shall discover the Longitude."
The prizes were to be awarded to the first person to demonstrate a practical method for determining the longitude of a ship at sea. Each prize, in increasing amounts, was for solutions of increasing accuracy.  These prizes, worth millions of dollars in today's currency, motivated many to search for a solution.
Britain was not alone in the desire to solve the problem. France's King Louis XIV founded the Académie Royale des Sciences in 1666.  It was charged with, among a range of scientific activities, the improvement  of maps and sailing charts and advancement of the science of navigation.  From 1715, the Académie offered one of the two Prix Rouillés specifically for navigation.  Spain's Philip II offered a prize for the discovery of a solution to the problem of the longitude in 1567; Philip III increased the prize in 1598.  Holland added to the effort with a prize offered in 1636. Navigators and scientists in most European countries were aware of the problem and were involved in finding the solution.  Due to the international effort in solving the problem and the scale of the enterprise, it represents one of the largest scientific endeavours in history.

== Knowing longitude ==