The Earth and its Early History 



nearest to the Sun it is said to be in perihelion, and, 

 when farthest away, in aphelion. 



Now the nearer the Earth is to the Sun the faster it 

 moves in its orbit. At the present time the winter in 

 the Northern Hemisphere occurs when the Earth is in 

 perihelion, and that of the Southern Hemisphere when it 

 is in aphelion ; and since the Earth moves faster when 

 in perihelion, the Northern winter is shorter than the 

 Southern, and therefore the Arctic winter is not so 

 severe as the Antarctic. 



Owing to another movement of the Earth, that 

 called precession, into the details of which we need not 

 enter, the positions of the seasons steadily alter in such 

 a manner that in about 1 2,000 years the positions will 

 be reversed, and the Arctic winter will be the longer 

 and more severe. 



Attempts have been made to explain certain changes 

 of climate which have taken place from time to time 

 during the past history of the Earth by this means, but 

 on the whole it seems inadequate. Of this we shall 

 speak again in a later chapter. 



Several of the other planets are larger than the 

 Earth. Jupiter, for example, is 1 1 times the diameter 

 of the Earth, and Saturn 9 times. This latter planet, 

 as has been already stated, is surrounded by a system 

 of luminous rings which were formerly thought to consist 

 of hot gas, but have now been proved to be multitudes 

 of tiny satellites each moving round the planet in its 

 own orbit. These rings of supposed gas doubtless 

 gave to the French astronomer Laplace his first ideas 

 for his great Nebular Hypothesis, by which he sought 

 to account for the birth of the Sun and planets. This 



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