22 CONCEPTIONS OF COLUMBUS 



The Source of Error in the Calculation 

 OF Columbus 



In the argument presented, there are two points 

 which call for further comment. 



As stated above, we do not know what estimate 

 Columbus used for the distance between Lisbon and 

 the Los Idolos Islands. My defense for introducing 

 a modern measurement to supply this gap is that we 

 have ample evidence in the portolano charts of the 

 ability of fifteenth-century seamen to estimate dis- 

 tances by dead reckoning. The portolano charts 

 were made by checking direction and distance. ^^ 

 1 1 should be borne in mind that these charts were the 

 most accurate maps produced before or during the 

 time of Columbus. Moreover, the measurements 

 of the portolano charts were based on the sea, not on 

 the land^^ — a point of special significance when con- 

 sidered in relation to the problem before Columbus. 



The second point is that the astronomical determi- 

 nations of the positions dealt with are wrong. This, 

 however, is the essential factor in the whole discussion. 

 The error of Columbus in believing that he had veri- 

 fied the old estimate of 56 ^< miles to the degree 

 springs directly from the wide inaccuracy of these 

 determinations. Columbus himself used the best 

 information available in his day. Why the observa- 

 tions should have been so far in error is not for this 

 study to discuss; but the unquestionable fact is that 



35 Lelewel, op. cit.. Vol. 2, p. 45. 



36 Ibid., pp. 47-48. 



