Science

Longitude: 8 key ideas

by Dava Sobel

The core ideas of Longitude, each in under a minute, written in our own words. Want the full depth? Get the book.

1. IDEA 1 OF 8

   Sailors could not find longitude

   Sobel explains that for centuries sailors could work out their latitude but not their longitude, their east-west position.

   This caused shipwrecks and huge losses of life. Without a way to know it, even skilled captains could get lost far from land.

2. IDEA 2 OF 8

   A prize was offered

   Sobel describes how the British Parliament passed the Longitude Act in 1714, offering a large prize for a practical solution.

   Many scientists and inventors tried to win it. The reward showed how serious the problem had become.

3. IDEA 3 OF 8

   Time could reveal position

   Sobel explains that knowing the exact time at home and on board would allow sailors to calculate longitude.

   But clocks of the time could not keep accurate time at sea.

4. IDEA 4 OF 8

   A carpenter built a clock

   Sobel tells how John Harrison, a self-taught clockmaker, spent decades building increasingly accurate marine clocks.

   His fourth, H4, was small and remarkably precise. The clock kept steady time at sea, in spite of the rolling of ships and changes in heat.

5. IDEA 5 OF 8

   Astronomers preferred the stars

   Sobel describes the rival lunar distance method, which used the Moon's position among the stars.

   Astronomers favoured it over Harrison's clocks. This method depended on careful observation and tables, so it was hard for ordinary sailors to use.

6. IDEA 6 OF 8

   Harrison faced resistance

   Sobel describes how the astronomer Nevil Maskelyne and the Board of Longitude delayed giving Harrison his reward.

   Harrison struggled for recognition. The delay left him bitter, even after he had shown that his clocks worked.

7. IDEA 7 OF 8

   Recognition came late

   Sobel explains that Harrison eventually received money, partly through the intervention of King George III.

   Yet he never received the official prize. The long struggle shows how hard it can be to win credit for a new idea.

8. IDEA 8 OF 8

   Clocks changed navigation

   Sobel shows that marine chronometers became standard equipment on ships.

   They made sea travel much safer. Knowing the exact time at a fixed place let navigators work out their position east or west. Ships could then sail with less guessing.