396 CLIMATE 



the people from advancing in civilization and taking their place 

 with peoples of temperate climates. 



Climate modifies and determines our national and personal 

 habits and characteristics. The most important factors in 

 climate are temperature, moisture, and winds. In order to 

 understand why different parts of the earth have different 

 temperatures, we must know something of the earth and its 

 movements. 



The earth and its movements. The earth is similar to a 

 sphere, but has flattened ends ; its shorter diameter is the axis 

 and the ends are the poles. The earth itself is a dark, cold 

 body, but it revolves around the sun and receives light and heat 

 from it. But the earth not only revolves around the sun, it 

 also rotates on its own axis every twenty-four hours. As 

 the earth turns on its axis it presents to the sun first one half 

 of its surface and then the other half; and the side of the earth 

 illuminated by the sun has daylight, while the side turned 

 away from the sun has darkness, or night. 



While the earth rotates on its own axis in twenty-four 

 hours, it requires 365 days, or one year, to complete its path, 

 or orbit, around the sun. It does not move in the plane of 

 its orbit in an erect position, but inclined to it at an angle of 

 2 3"2"> with its axis pointed unswervingly to the north star. 

 The daily rotation of the earth on its axis, its yearly revolu- 

 tion around the sun, and its inclination to the orbit, are re- 

 sponsible for day and night, long and short days, winter, 

 summer, spring, and fall in temperate regions, prolonged low 

 temperature in the arctics, and prolonged high temperature in 

 the tropics. 



Long and short days. The sun's rays illuminate one half 

 of the earth's surface at all times. The half on which they shine 

 has day ; the other half has night. The margin or edge of the 

 illuminated half is called the circle of illumination, and varies 



