The economic history of portugal
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3.2. Navigation ProblemsPortuguese navigators were confronted with some unexpected difficulties concerning their use of the astrolabe when they travelled closer to the equator. It became more and more difficult to indicate the height of the North Star in degrees latitude the closer the sailors approached the equator. The farther north a sailor travelled, the higher the Pole Star appeared in the sky. The farther south he sailed, the lower the star seemed in the sky. The star disappeared completely when navigators reached the equator. This problem was solved by observing and measuring the height of the mid-day sun which enabled the navigator to determine the ship’s latitude as well as the distance and direction which it was heading toward. Measuring longitude posed another great problem because observations of the sun and stars were of no immediate help. It was not until the 18th century that a solution was found, namely the invention of the chronometer. Until then the sailors had to rely on dead-reckoning. As already mentioned before, the log and sand glass combination gave the speed of the ship, with the course being obtained from the compass. Each time the vessel changed its tack due to wind direction, time, distance and direction had to be recalculated. 
A traverse board18 A traverse board helped determine this zigzag plotting. Every half hour of a four hour watch, pegs were inserted to demonstrate course and estimated distance covered. The navigator interpreted this information and transferred it to his portolan chart. Longitude could now be estimated by the east-west distance travelled. Observations of the surroundings were also included in dead reckoning as well as wave patterns and cloud formations. This description explains how complicated and unreliable the process of determining a sailor’s course was. Another reason why determining longitude presented such great difficulty was that the world maps in the 16th century were very inaccurate concerning the longitudes of places. For a better understanding, consider the following example, the east-west length of the Mediterranean erred by 19° which amounts to about 1,100 miles.19 The cross staff navigational technique was confronted with some obvious handicaps when used at sea. Waves caused for movement of a vessel and hence the line in sight constantly changed. This tool could only accurately be used in very calm water. It was better to use a land based sight when implementing this technique. Another problem that existed was the difficulty in finding the Polar Star, which was an extension of the line formed by the vertical side of the Big Dipper opposite the handle. 
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