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.

Charts

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.

To 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.

Stars

Sailors used the stars on cloudless nights to find their way. If they were sailing north of the equator, the Pole Star always sat in the north in the Northern Hemisphere skies. It could be found by using the two “end” stars of the Plough, a familiar constellation, as pointers. In the Southern Hemisphere, the Southern Cross constellation could be used in the same way.

Calculating latitude

To work out a ship’s latitude—how far north or south it had travelled—the navigator calculated the position of the sun at noon, when it was at its highest. To do this, he used an instrument called a backstaff. He stood with his back to the sun and looked through what was called a sighting vane. Lining up the shadow cast by a part of the backstaff called the shadow vane with the horizon, he could then measure off the angle of the sun above the horizon. The higher the sun at noon, the closer they were to the Equator.

Astrolabe

At night the navigator could use an astrolabe to work out latitude by measuring the angle between the Pole Star, his eye and the horizon. He turned the pointer on the astrolabe until it lined up with the star, then read off the angle marked by the pointer on the outside rim of the instrument.

Calculating longitude

To calculate longitude—how far east or west the ship had sailed—was, before the invention of the marine chronometer (a time-keeping instrument) in the 18th century, much more difficult than calculating latitude.The best way was to keep detailed records of speed and direction. To find out the ship’s speed, a log line, with a weight attached to the end, was thrown from her stern. The line, which had equally spaced knots tied in it, was allowed to reel out as the ship moved forward.

Using a sandglass, a sailor could then time the number of knots that were reeled out in one minute. Multiplying this by 60 gave the speed in knots per hour—still the unit of speed that ships use today. Using the compass, it was possible then to calculate the distance travelled east or west.

Lead weight

To prevent a ship running aground, a lead weight was used to measure the depth of water. Attached to a marked line, it was thrown overboard. Sticky tallow (animal fat) on the base of the lead weight would pick up mud from the sea bed.

​​​​​​​From the type of mud picked up on its base, a skilled navigator could tell how close the ship was to shore.