78 MAPS AND THEIR MAKERS 



partly also because the latitudinal range was relatively small, 

 and the degree of accuracy of their observations not high. 



When the Portuguese embarked upon the waters of the 

 Atlantic and made their way southwards along the African 

 coasts, they encountered different conditions. There was no 

 body of traditional seafarer's lore to draw upon, as regards 

 winds and currents; familiar landmarks were lacking on the 

 coasts, which were often characterless for considerable stretches, 

 and fringed by unseen hazards — moreover a hostile population 

 discouraged unnecessary approaches. Added to these was the 

 possibility of being driven off course into the ocean. They were 

 also ranging through many degrees of latitude. In these 

 circumstances the pilots turned to the determination of latitude, 

 at first by observing the altitude of the Pole Star. Later, as the 

 vessels pressed further southwards, and the Pole Star sank 

 lower in the sky, latitude was obtained from the midday 

 altitude of the sun with the help of tables of declination. These 

 observations were made with the astrolabe, gradually simplified 

 from the landsman's type, and with the quadrant, a less 

 cumbrous instrument. 



Since the Pole Star does not coincide with the celestial pole, 

 it was necessary to apply a correction to its observed altitude 

 in order to obtain the latitude. The correction depended 

 upon the time of the observation, which could be obtained 

 from the position of Ursa Major in its orbit round the Pole. A 

 set of simple instructions, known as 'The Regiment of the 

 North', was therefore drawn up, which gave the correction to 

 be applied for certain positions of the 'Guards'. 



For the corrections to be applied to the sun's midday 

 altitude, a primitive table had probably been worked out by 

 1456. Later Jose Visinho, utilizing the work of the Jewish 

 astronomer Abraham Zacuto, calculated a table for each day 

 of the leap year March 1483 -February 1484, and this was used 

 by Bartholomeu Dias in his famous voyage. Later still Zacuto 

 assisted in preparing a perpetual almanac for the voyage of 

 Vasco da Gama. 



It will be seen that these scientific aids were provided 

 relatively late in the fifteenth century; the first recorded use 

 of the quadrant at sea dates from 1460. It is not until the early 



