---
title: "Longitude (book)"
chunk: 2/2
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"
---

As Dava Sobel explains, "To know one's longitude at sea, one needs to know what time it is aboard ship and also the time at the home port or another place of known longitude—at that very same moment. The two clock times enable the navigator to convert the hour difference into geographical separation. Since the earth takes 24 hours to revolve 360 degrees, one hour marks 1/24 of a revolution or 15 degrees. And so each hour's time difference between the ship and starting point marks a progress of fifteen degrees of longitude to the east or west.
"Every day at sea, when the navigator resets his ship's clock to local noon when the sun reaches its highest point in the sky, and then consults the home port clock, every hour's discrepancy between them translates into another fifteen degrees of longitude. One degree of longitude equals four minutes of time the world over, although in terms of distance, one degree shrinks from 60.15 nautical miles or 111 km [Earth's circumference being 21,653.521 nautical miles, or 24,901.55 statute miles at the Equator], to virtually nothing at the poles.
"Precise knowledge of the hour in two different places at once—a longitude prerequisite so easily accessible today from any pair of cheap wristwatches—was utterly unattainable up to and including the era of pendulum clocks. On the deck of a rolling ship such clocks would slow down, or speed up, or stop running altogether. Normal changes of temperature encountered en route from a cold country of origin to a tropical trade zone thinned or thickened a clock's lubricating oil and made its metal parts expand or contract with equally disastrous results. A rise or fall in barometer pressure, or the subtle variations in the Earth's gravity from one latitude to another, could also cause a clock to gain or lose time."

Before the 18th century, ocean navigators could not find an accurate way of determining longitude. A practical solution came from a gifted carpenter, John Harrison, who solved one of the most difficult problems of his time by creating an accurate chronometer. The best scientists of the time, including Sir Isaac Newton, thought it impossible. Harrison spent four decades perfecting a watch that would earn him compensation from Parliament and longitude rewards thanks to the recognition and influence of King George III of Great Britain.

== Recognition ==
British Book of the Year, 1997
Modern Library 100 Best Nonfiction, 26th on the Readers List
American Academy of Arts and Letters Harold D. Vursell Memorial Award, 1999
Le Prix Faubert du Coton
Il Premio del Mare Circeo
Royal Society Prizes for Science Books, 1997 (Shortlisted)
American Library Association Non-fiction Outstanding Books for the College Bound and Lifelong Learners, 1999
American Library Association Outstanding Books for the College Bound and Lifelong Learners, Science and Technology, 2004
In recognition for Longitude, Sobel was named as a fellow of the American Geographical Society.
Historians of science and technology have raised some criticisms of the book, particularly for its creation of heroes and villains, suggestion that astronomical and timekeeping methods of finding longitude were rivals rather than complementary, simplification of the processes of technological innovation and Anglocentrism.

== Film adaptations ==
Nova Online: Lost at Sea, the Search for Longitude 
Longitude (TV series)

== See also ==

== References ==

== External links ==
Booknotes interview with Sobel on Longitude, January 17, 1999.