THE CONQUEST OF TIME AND SPACE 



It follows that meridian observation of the sun, owing 

 to the necessary inaccuracy of such observation, is not 

 the ideal method. In point of fact the sun may be 

 observed for this purpose to much better advantage 

 when it is at a considerable distance from the meridian, 

 since then its altitude above the horizon at a given 

 moment is the only point necessary to be determined. 

 The calculation by which the altitude of the sun may be 

 translated into longitude is indeed more complicated in 

 this case; but while spherical trigonometry is involved 

 in the calculation, the tables supplied by the Nautical 

 Almanac enable the navigator to make the estimate 

 without the use of any knowledge beyond that of the 

 simplest mathematics. 



MEASURING A DEGREE OF LATITUDE 



While these observations tell the navigator his exact 

 location in degrees of latitude and longitude, such 

 knowledge does not of course reveal the distance trav- 

 ersed unless the precise length of the degree itself is 

 known; and this obviously depends upon the size of the 

 earth. Now we have seen that the earth was measured 

 at a very early date by Greek and Roman astronomers, 

 but of course their measurements, remarkable though 

 they were considering the conditions under which they 

 were made, were but rough approximations of the 

 truth. Numerous attempts were made to improve 

 upon these early measurements, but it was not until 

 well into the seventeenth century that a really accurate 

 measurement was made between two points on the 



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