Navigation

Changes to the ships themselves were not the only problems addressed by explorers. Navigation techniques on these early voyages were rudimentary to say the very least. The speed of the ship was calculated by having one member of the crew throw a chip of wood over the side of the vessel. By judging how far the ship travelled before the object hit the water helped determine how fast the ship was moving. This technique was later changed by attaching a wooden float to a line - known as the "logline" - where knots were tied at measured intervals. When the wooden float was tossed overboard, the speed was calculated by counting the number of knots that slipped through the fingers of the sailor holding the logline. This process, incidentally, gave rise to the calculation of a ship's speed in knots. For the most part, a good captain would rely on the ship's log, his lookout, and his leadsman to determine the location of the ship.

Most ships were equipped with a magnetic compass that was kept on deck in a binnacle that could be illuminated at night by means of a lamp burning olive oil. Although the compass was in wide use, most captains did not know why its needle pointed north. In fact, many captains preferred to keep the existence of a compass on board a ship secret because superstitious crew members would think that the ship was being guided by sinister forces. Nearby was the sand hourglass, which was the only reliable method of keeping time on board the ship - provided that a member of the crew did not heat the hourglass with the lamp, making the sand run faster, in order to shorten his shift.

An early compass

A quadrant t

 measure latitude

A traverse board

Clearly, the most important navigational tool available to sailors was the Stella Maris. Holding the astrolabe to his eye and sighting along its diameter, the sailor could read the height of the North Star on the scale. If, for example, the star measured forty degrees above the horizon, the sailor would know that he was at a latitude of approximately forty degrees north. If the star measured thirty degrees above the horizon, the sailor would know that he was at approximately thirty degrees north, and so on. The quadrant, a quarter circle measuring 0 to 90 degrees marked around its curved edge, was a common instrument to assist in determining latitude. Its straight edges had tiny holes or sights on each end. A plumb line hung from the top. The navigator lined up the sights on the Pole Star and the plumb line would hang straight down over the curved area at a particular point. This would indicate the height of the star in degrees latitude.

Portuguese explorers encountered some unexpected difficulties with this system as they travelled closer to the equator. They soon discovered that as the North Star dropped closer to the north horizon it became difficult to fix exact latitudes. Rather than navigating by the North Star, sailors then turned, quite literally, to the sun. By observing and measuring the height of the mid-day sun, a pilot could calculate the ship's latitude and, with the aid of tables, determine the distance and direction to be sailed to arrive at the intended destination.

The problem of determining longitude was not adequately solved during this period. It was only in the eighteenth century, with the invention of the chronometer, that a solution was found. Until that time, most sailors relied on dead reckoning. This meant that the pilot had to estimate the ship's speed with a logline, which was a line with knots in it and a weighted wooden float attached to the end. The wooden float was thrown from the stern and the number of knots pulled off the reel by the drifting log determined the ship's speed. Time was measured with one-minute glasses. This information combined with the known direction of the compass helped to determine progress along longitudinal lines. Time, distance, and direction were measured each time the ship changed tack due to wind direction. This zigzag plotting was calculated with a traverse board. Dead reckoning also included observations of the surroundings. Cloud formations, and wave patterns and directions, as well as birds and floating debris were all taken into account. Clearly, dead reckoning was a complicated process and not the most reliable method a sailor could use to determine his course.