THE ARRIVAL OF MAN IN EUROPE 



summer than in winter is that in summer the 

 days are longer than the nights, so that the sur- 

 face of the earth receives more heat in the day- 

 time than it can lose by radiation during the 

 night ; while in winter the case is exactly the 

 reverse. Another circumstance tends to make 

 the earth warmer at one time than another, 

 namely, the fact that the earth's orbit is not 

 quite circular, but slightly elliptical or eccentric, 

 so that at one season of the year the earth is 

 nearer to or farther from the sun than at another 

 season. At present the northern hemisphere is 

 nearest the sun in winter and farthest from it in 

 summer, but the difference is only about 3,000,- 

 ooo miles. It must also be remembered that 

 when the earth is near perihelion it moves faster 

 than when it is near aphelion, so that the sea- 

 son when it is nearer the sun is always a little 

 shorter than the season when it is farther from 

 the sun. Thus in our northern hemisphere at 

 present the winter half of the year, or the inter- 

 val from the autumnal to the vernal equinox, is 

 nearly eight days shorter than the summer half 

 of the year. Thus the difference in length be- 

 tween our summer and winter seasons, and the 

 difference between our distances from the sun 

 at the two extremes of the year, are not great 

 differences, but the advantage, such as it is, is 

 on the side of summer. 



But these relations between the earth and the 

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