THE CONQUEST OF THE ZONES 



plane of the earth's center. Therefore while all points 

 on any given meridian of longitude are equally distant 

 in terms of degrees and minutes of arc from the merid- 

 ian of Greenwich, the actual distances from that merid- 

 ian of the different points as measured in miles will 

 depend entirely upon their latitude. 



At the equator each degree of longitude corresponds 

 to (approximately) sixty miles, but in the middle lati- 

 tudes traversed for example by the transatlantic lines, 

 a degree of longitude represents only half that distance; 

 and in the far North the meridians of longitude draw 

 closer and closer together until they finally converge, 

 and at the poles all longitudes are one. 



It follows, then, that the navigator must always 

 know both his latitude and his longitude in order to 

 estimate the exact distance he has sailed. We have 

 seen that a single instrument, the sextant, enables him 

 to make the observations from which both these essen- 

 tials can be determined. We must now make further 

 inquiry as to the all important guide without the aid 

 of which his observations, however accurately made, 

 would avail him little. This guide, as already pointed 

 out, is found in the set of tables known as the Nautical 

 Almanac, 



THE NAUTICAL ALMANAC 



Had the earth chanced to be poised in space with its 

 axis exactly at right angles to its plane of revolution, 

 many computations of the astronomer would be greatly 

 simplified. Again, were the planetary course circular 



[37] 



