18 MAPS AND THEIR MAKERS 



practical purposes, which corresponded approximately with 

 what the Greeks called 'chorography', there developed very 

 slowly the science of 'geography', that is, the mapping of the 

 whole known world on scientific methods, what we should call 

 cartography. 



Unlike the determination of latitude, for which the axis of 

 the earth provides an established reference datum, the problem 

 of longitude long baffled the astronomers, for no meridian is 

 marked out as an initial one, in the way that the Equator 

 serves as an initial parallel. Since the earth makes one revolution 

 in a day, more or less, it was early recognized that simultaneous 

 observations of a celestial phenomenon such as a lunar eclipse 

 would, through the difference in the local times at the moment 

 of observation, give a value for the difference of longitude 

 (1 hour=15° of longitude). Without the requisite astronomical 

 tables or accurate portable time-keepers, the method was 

 impracticable, though a few attempts were made to observe 

 eclipses for this purpose. On all early maps, until the seven- 

 teenth century, longitudes were arrived at by transforming 

 distances into their angular values in relation to the circum- 

 ference of the globe. For this it was necessary to arrive at a 

 value for the circumference, which, divided by 360, would give 

 the length of a degree. This was done with considerable 

 accuracy, perhaps owing to the cancelling out of errors, by the 

 Greek astronomer Eratosthenes, who measured the meridian 

 arc between Alexandria and Syene. The figure he arrived at for 

 the circumference of the earth was 252,000 stadia, which 

 assuming he employed the short stade, was the equivalent of 

 24,662 miles, a result only some fifty miles short of the reality. 

 From this result it followed that a degree was equivalent to 

 68.5 miles. Unfortunately, this quite accurate figure was not 

 accepted by his successors, with important results for the 

 history of cartography. 



The Greeks had also attacked the problem of the projection 

 of the earth's surface on to a plane in order to arrive at an 

 orderly arrangement, or graticule, of parallels and latitudes, 

 with reference to which positions could be located. The draw- 

 ing of parallels was relatively simple, at least in the restricted 

 area for which observations were available. Eratosthenes 



