Calculating a ship's position using a cross-staff, lead weight and log line Finding the way across thousands of kilometres of ocean was one of the greatest challenges of any long-distance ocean voyage. For this, the ship's captain relied almost entirely on the pilot. It was his job to work out the ship's position, speed and direction. The pilot used several instruments and techniques to help him. It was the captain who decided on the ship’s course. He issued orders to the helmsman (who steered the ship) and the boatswain (leader of the crew) to adjust the rudder and the sails to maintain her course.
The captain plotted the course of the ship on a chart (a map of the sea and coastlines). During the 16th century, the Spanish had made detailed charts of the Caribbean Sea. Each had a compass rose, showing north, south, east and west, and a scale bar. This showed how far a distance on the map (a few centimetres, say) was in the real world—several hundred kilometres, perhaps.
Using dividers to measure distanceTo measure how far the ship had sailed, the captain used dividers. He placed the points first on the chart, one on the ship’s position the previous day, the other on her current position. Then he held them carefully against the scale bar, giving that same distance in kilometres.
The magnetic compass was invented in China around 200 BC. It was not used for navigation by Chinese mariners until the 11th century. The practice reached Europe a few decades later.
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