206 



PHYSIOGRAPHY 



feet sphere, its various diameters and circumferences are not ex- 

 actly equal. Its longest diameter is 7,926.5 miles, and its shortest 

 nearly 27 miles less (7,899.7) miles. 



The area of the earth's surface is nearly 197,000,000 square 

 miles. The earth is between five and six times as heavy as an equal 

 volume of water would be. 



Motions 



The earth has two principal motions: (1) it rotates on its short- 

 est diameter, which is called its axis, and (2) it revolves around the 

 sun. The axis is an imaginary line, and its ends are the poles. 

 The circumference midway between the poles is the equator. 



Rotation. Rotation gives us day and night, for one side of 

 the earth and then the other is turned toward the sun during each 

 rotation. The time of rotation, 24 hours, determines the length 

 of a day (day and night). 



Revolution. The earth revolves about the sun in a little more 

 than 365 days and the period of rotation fixes the length of the 

 year. The path of the earth around the sun is its orbit. The 



Perihelion 



Aphelioa 



Fig. 201. The orbit of the earth is an ellipse, with the sun in one of the foci. 



orbit of the earth is not a circle, but a curve called an ellipse (Fig. 

 199), and the distance of the earth from the sun varies from time 

 to time. When the earth is nearest the sun, the distance between 

 them is about 3,000,000 miles less than when they are farthest apart. 

 The earth is nearest (about 91,500,000 miles) the sun in the early 

 part of the winter of the northern hemisphere (about January 1st), 

 and farthest (about 94,500,000 miles) from it early in the summer. 



