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.

[2] In Farley Mowat's book Westviking, he gives examples from the Norse Sagas of Vikings using this practice to hop reliably from Norway to the Faroes, then Iceland, then Greenland, then North America, and then back to Ireland, with very primitive instruments.

This increased the likelihood of short rations,[3] scurvy or starvation leading to poor health or even death for members of the crew and resultant risk to the ship.

"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.

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.

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.

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.