Time was of the essence - chronometry

In Cook’s time, navigators could work out their longitude in two ways. The ‘lunar distance’ method involved measuring the position of the moon and doing complicated calculations. The other method involved comparing local time with the time somewhere else in the world. 

British navigators used the time at Greenwich in England as their standard. They calculated local time by the sun’s height, but finding out Greenwich time was more difficult. They could take clocks set to Greenwich time on their voyages, but pendulum clocks weren’t accurate at sea.

In 1735, John Harrison made a spring-driven marine chronometer. The spring was balanced by weights to compensate for the ship’s movement. Cook tested chronometers on his second and third voyages, and found them reliable.