Longitude - Page 1


This is the story of longitude. Why it was so important - literally a matter of life and death - and how the 'longitude problem' finally came to be solved.

The voyage of HMS Centurion

Our story starts aboard the British ship HMS Centurion. It was May 1741, and the Centurion was somewhere off the coast of Chile. Her situation was desperate. Having taken a terrible battering rounding Cape Horn, the Centurion had lost touch with the other five ships in the squadron. Many of the crew were seriously ill with scurvy, and a number had already died. Their only hope was to reach land, repair the ship, and put the crew ashore to recover.

But their orders were explicit. To keep out of sight of the mainland, and not to alert the Spanish, with whom Britain was at war. So they decided to head for the tiny island of Juan Fernandes, 400 miles off the coast of Chile. Now the Centurion, under Captain Anson, knew where the island was. The problem was that they didn't know where the ship was. More precisely, they knew their latitude, but had no means of working out their longitude.

Why latitude was fairly easy to calculate…

Basically at that time you worked out your latitude by observing how high the sun was at local noon (the point when the sun is highest in the sky). Typically ships used a simple device called a cross staff, basically a long piece of wood with graduations on it, with a sliding cross piece. You placed the end of the cross staff to your eye, and you slid the cross piece to and fro, until the bottom end appeared to touch the horizon, with the other end touching the sun. You then read off the position of the cross piece on the main cross staff, which would tell you the angle of the sun above the horizon. Typically this instrument would allow you to work out your latitude to within 15 or 20 miles. The cross staff was the precursor of the modern sextant, which does the same job more accurately.

This photograph shows the kind of Cross Staff that was used aboard the Mayflower. Picture credit: flickr.

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