62 Physical Geography as a Popular Study. 



The size of the earth, its average density, the velocities of 

 its diurnal rotation, and of its annual course, the size and 

 weight of its satellite — these and similar facts belonging to 

 astronomy, supply considerations which lie at the root of phy- 

 sical geography. We are continually, and, on the whole, 

 equally affected by one class of astronomical facts, such as the 

 recurrence of night and day, summer and winter, lunar action 

 on the tides, and many others ; while another class of astro- 

 nomical facts relate to changes very slowly effected, and only 

 producing important results after the lapse of long periods of 

 time. We, or the people of any given generation, are of 

 course most immediately influenced by the quickly-recurring 

 changes ; but, looking to the laws of succession and continuity, 

 manifested throughout the operations of nature, it will be seen 

 that our physical structure, our methods of life, and even our 

 habits of thought, are, to a certain and large extent, the results 

 of previous conditions of our planet, dating back even to its 

 origin in the remote abysses of the past. 



A very important consideration, well adduced by Professor 

 Ansted, arises out of the astronomical fact that the eccentricity 

 of the earth's orbit is the subject of secular changes. " When 

 the eccentricity is greatest, the greatest distance of the sun 

 from the earth may amount to 102,256,873 miles, and its 

 nearest distance will then only be 87,503,039 miles, showing 

 the very considerable difference of 14,753,834 miles, or more 

 than one- seventh of the larger semi- diameter. This is a very 

 important fact ; for, as the amount of heat received from the 

 sun varies as the square of the distance, it follows that the 

 quantity of heat received in the former position, compared with 

 that in the latter, will be as 19 to 26." 



It will take twenty-four thousand years for the minimum of 

 eccentricity to be reached, so that the change does not im- 

 mediately concern the present inhabitants of the globe ; but in 

 past periods it may have materially affected the creatures then 

 living upon it, and likewise produced modifications of form 

 and climate, the influence of which has descended to our 

 own times. 



At present the earth comes nearest to the sun in winter ; 

 but " if, when the eccentricity is greatest, the case is reversed, 

 the land of the northern hemisphere would be warmed only to 

 the very minimum extent in winter, and heated to a maximum 

 in summer, the difference would then equal one-fifth of the 

 whole. There would then be far more extreme climates on the 

 earth than there are now." In his " Outlines of Astronomy," 

 Sir John Herschel, commenting on speculations of this nature, 

 observes that about four thousand years before the Christian 

 era, the place of the perihelion must have coincided with the 



