Latitude

Use some mapping software, like Google Earth, centre yourself on the middle of the Pacific Ocean and zoom out. A planet will appear that you can barely recognise. From that angle our world is almost entirely blue, save for a scatter of islands across the face of the deep. The continents of the Americas and Asia appear only as a fringe of land around the edge, and even Australia and New Zealand barely intrude. Yet in 1743 Admiral George Anson, aboard HMS Centurion, set out into this unimaginable vastness to hunt down a single Spanish ship. She was the Nuestra Senora de Convadonga, better known as the Acapulco galleon, and each year she travelled between the Spanish Philippines and Mexico. She was also the richest treasure ship in the world.

At first glance, finding a needle in a haystack appears simple compared with the task set for the crew of the Centurion. How were they to track down the only other western ship in the Pacific at the time, amongst all of those millions of squares miles of ocean? Yet, in spite of the odds, the ships met and fought each other, on the 20th June 1743. Anson triumphed, captured the Spanish ship and her fabled treasure, and returned home to a hero’s welcome. But Anson’s feat was not unique. Eighteenth century naval history is full of campaigns and battles in which either individual ships or fleets found and fought each other in empty stretches of sea, apparently against the odds. How was this possible?

The answer lies in the state of navigation at the time. Cartographers have long divided up their charts with a grid of lines to aid navigation. Those that show the latitude run in parallel bands between the equator and the poles. The ones that show longitude run north/south, passing through both poles. Finding a ship’s latitude is relatively straightforward. A sextant, a view of the sun at noon and a simple adjustment for the date is all that is required, and mariners have been performing this task reliably for centuries. Knowing longitude, on the other hand, is a much trickier proposition. So significant was the problem that in 1714 the British Government offered a prize of up to twenty thousand pounds (close to three million today) for the person who could find a practical way of calculating longitude onboard ship. It was only with the invention of Harrison’s marine chronometer, the first modern timepiece, that the problem was solved. But chronometers were rare and very expensive instruments that did not come in to common usage until the nineteenth century.

So ships and fleets crossed the oceans using the only reliable navigational tool that they had. They would first travel down the coast line on one side of a body of water. Once they reached the latitude of their destination on the far side, they would cross; sticking to that line of latitude as far as wind and currents would permit. But in time of war, this method of navigation was fraught with risk. If your enemy knew where you were bound, he could also make a good guess as to the route you would take. George Anson, in 1743, had no need to search all of the Pacific Ocean to find his treasure ship. His enemy’s destination was the port of Acapulco, and Acapulco lies at a latitude of seventeen degrees north. He only had to search along that line until he found his galleon.

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Philip K Allan is an author of historical naval fiction. His debut novel, The Captain’s Nephew is available now from all good on-line retailers, or click here to learn more