114 THE OCEAN RIVER 



equator is known, its noontime altitude may be corrected by 

 this amount in order to determine the latitude. Similar meas- 

 urements of latitude are made from the moon's altitude. If the 

 polestar is used it is even simpler. The altitude of the polestar, 

 which lies almost on the axis of the earth, is almost equal to 

 the latitude. But the polestar, not exactly on the axis, revolves 

 round the pole; hence certain corrections must be made, and 

 these depend on its place in the circle of revolution. At present 

 the greatest error arising from this is about 1 ° . At the time of 

 Columbus, however, the polestar was about 3V2° away from 

 the true pole. 



The movements of the polestar were well known to the 

 ancients during the first centur}^ B.C., and they had various 

 methods of making the polestar correction. One of these was 

 to refer to the tables of the astronomers — but these were not 

 always available. Another method, developed later, was the 

 use of a simple hand calculator called the nocturnal. Sailors in 

 Columbus's day also had a simple rule for finding the polestar 

 correction from the position of the other stars in the Little 

 Dipper. Two of these stars, Kochab and Gamma, known as 

 the Guards, are referred to in the Regimento de estrolabio e 

 do quadrante, published in 1509.^ The figure of a little man 

 was marked on the nocturnal, and when held at the proper 

 angle his head pointed above the North Star, his arms were 

 east and west of it, and his feet beneath it. The position of the 

 Guards could then be used for finding the true position of the 

 pole. And here is the Regimento as it was used: 



This is the Regiment of the North Star 



When the Guards are on the West Arm the North Star stands 

 above the Pole one degree and a half. 



^ S. E. Morison: "Columbus and Polaris," American Neptune, Vol I, 1941. 



