402 ARCTIC AND ANTARCTIC REGIONS 



the Western Hemisphere, the former, also called the Old World, comprising 

 Europe, Asia, Africa and Australia, the latter. North and South America. 

 The boundary between the two is quite arbitrary, and a more natural 

 division of the earth is into the North and Southern Hemisphere, the divid- 

 ing line being the equator. 



THE EQUATOR. 



The Equator is the great circle of our globe every point of which is 90° 

 from the poles. All places which are on it have invariably equal days and 

 nights. From this circle is reckoned the latitude of places both north and 

 south. There is also a corresponding celestial equator in the plane of the 

 terrestrial, an imaginary great circle in the heavens the plane of which is 

 perpendicular to the axis of the earth. It is everywhere 90° distant from 

 the celestial poles, which coincide with the extremities of the earth's axis, 

 supposed to be produced to meet the heavens. During the apparent yearly 

 course the sun is twice in the celestial, and vertically over the terrestrial 

 equator, at the beginning of spring and of autumn. Then the day and night 

 are equal all over the earth, whence the name equinox. The magnetic equator 

 is a line which pretty nearly coincides with the geographical equator, and at 

 every point of which the vertical component of the earth's magnetic attraction 

 is zero; that is to say, a dipping needle carried along the magnetic equator 

 remains horizontal. It is hence also called the aclinic line. 



MERIDIANS. 



Greenwich is within a few miles of London, England, and a great astro- 

 nomical observatory is located there. Time in all parts of the world is meas- 

 ured according to meridian east or west of Greenwich. There are in all 180 

 meridians east and 180 meridians west of Greenwich, total 360. It is plain 

 then that the meridians begin to number in both directions from Greenwich. 



The Meridian of Greenwich extends half way around the world from 

 the North Pole to the South Pole. Beyond the poles, however, on the oppo- 

 site side of the world from that covered by the Meridian of Greenwich, it is 

 the i8oth meridian, also extending from pole to pole; the Meridian of Green- 

 wich and the i8oth meridian being the exact antipodes of each other. 



Since the earth's rotation around the sun makes the sun pass 15 meridians 

 each hour, if you will divide the total number of meridians, 360, by 15, you 

 have 24, the number of hours in a day. Roughly speaking, the i8oth meridian 



