ASTRONOMIC CAUSES OF CLIMATIC CHANGES. 



nig :i period of 13,000 years. But there is also a still 

 slower change in the direction of the axis major of the 

 Earth s orbit ; from which it results that the alternation AVO 

 have described is completed in about 21,000 years. That 

 is to say, if at a given time the Earth is nearest to the sun 

 at our mid-summer, and furthest from the sun at our mid&quot; 

 winter: then, in 10,500 years afterwards, it will be furthest 

 from the sun at our mid-summer, and nearest at our mid- 

 Avinter. 



NOAV the difference bctAveen the distances from the sun 

 at the tAvo extremes of this alternation, amounts to one- 

 thirtieth ; and hence, the difference between the quantities 

 of heat received from the sun on a summer s day under 

 these opposite conditions amounts to one-fifteenth. Esti 

 mating this, not Avith reference to the zero of our thermome 

 ters, but Avitli reference to the temperature of the celestial 

 spaces, Sir John Ilcrschel calculates &quot;23 Fahrenheit aa 

 the least variation of temperature under such circumstances 

 Avhich can reasonably be attributed to the actual A - ariation 

 of the sun s distance.&quot; Thus, then, each hemisphere has 

 at a certain epoch, a short summer of extreme heat, fol 

 lowed by a long and very cold Avinter. Through the slow 

 change in the direction of the Earth s axis, these extremes 

 arc gradually mitigated. And at the end of 10,500 years, 

 there is reached the opposite state a long and moderate 

 summer, Avith a short and mild winter. At present, in con 

 sequence of the predominance of sea in the southern hem 

 isphere, the extremes to which its astronomical conditions 

 subject it, arc much ameliorated ; while the great propor 

 tion of land in the northern hemisphere, tends to exagge 

 rate such contrast as now exists in it betAvecn winter and 

 summer : Avhence it results that the climates of the two 

 hemispheres are not Avidely unlike. But 10,000 years hence, 

 the northern hemisphere Avill undergo annual variations of 

 temperature far more marked than now. 



