96 G. F. Becker — Certain Astronomical 



zero eccentricity." It is mechanically possible for the eccen- 

 tricity to become zero,f and it is never a large quantity. The 

 latitude of the tropical circles also fluctuates within somewhat 

 narrow limits, and of course the polar circles fluctuate corre- 

 spondingly. There appear to be no other changes in the orbit 

 which can affect the accumulation of ice, and all of these have 

 been considered before now. 



Dr. James Croll, as is well known, attributed the glacial 

 epoch to the more or less indirect action of the difference in 

 the length of the seasons, some 35 days, which occurs when 

 the eccentricity is high.:}; Sir Robert Ball dwells upon the 

 difference in the amount of heat received in the two seasons 

 by an entire hemisphere, and he regards the low rate at which 

 the winter hemisphere receives sunshine when the winter has 

 its greatest length as an explanation of the ice age. This 

 astronomer computed that the proportion of heat received dur- 

 ing warm season (irrespective of its length), is expressed by 

 \ + sine/^r, where e is the latitude of the tropical circles, and at 

 present this fraction is expressed numerically by 0627. Thus 

 at the period of greatest eccentricity three eighths of the entire 

 heat of the year may be spread over a winter some two hun- 

 dred days in length. § 



Sir Robert quotes a passage from Sir John Herschell from 

 which it appears that this famous astronomer, at least momen- 

 tarily, assumed that each hemisphere would receive the same 

 amount of heat in each of the two great seasons, so that the 

 difference of climate would depend solely on the length of the 

 season.]| If Herschell was under this impression, the mistake 

 was a temporary one ; for a page or two before the passage in 

 which the erroneous statement is found he says : — " Now the 

 temperature of any part of the earth depends mainly on its 

 exposure to the sun's rays. . . . Whenever then the sun re- 

 mains more than twelve hours above the horizon of any place, 

 and less beneath, the general temperature of that place will be 

 above the average ; when the reverse below. As the earth, 

 then, moves from A to B, the days growing longer, and the 

 nights shorter in the northern hemisphere, the temperature of 

 every part of that hemisphere increases and we pass from 

 spring to summer." 



This is of course the usual explanation of the seasons to be 

 found in all astronomies and physical geographies. If it were 

 true that a hemisphere received the same amount of heat in 



* See the note appended to this paper on the calculation of sunshine per unit 

 area. 



\ Of . J. N. Stockwell, Smiths. Cont. to Knowledge, vol. xviii, 1873. 



% Climate and Time. 



§ Cause of the Ice Age, 1892. 



|| Outlines of Astronomy, § 368 (c) in 9th ed. 1867. 



