INTRODUCTORY CHAPTER . 
35 
recession of the earth from the sun, while in autumn 
the decline is in part counterbalanced by the earth 
approaching the sun. This would help to raise the 
autumn temperature. If the northern summer oc¬ 
curred when the earth was nearest the sun, the cir¬ 
cumpolar area, it is believed, would be heated more 
strongly than it is now. The intensity of the heat 
might be still further increased by another modifica¬ 
tion of conditions. The present eccentricity of the 
earth’s orbit is slight compared to what it has been 
in past times. At present the earth is 3,000,000 
miles less than the mean distance nearer the sun 
in aphelion than in perihelion; but at certain periods 
it has been 14,500,000 miles less than the mean 
nearer the sun in aphelion than in perihelion. In 
other words the earth when nearest the sun is now 
about 88,400,000 miles from the sun; but during 
past epochs it has approached to within about 
77,000,000 miles. The intensity of the heat is in 
inverse proportion to the square of the distance: 
so that if the Arctic Pole were exposed to this 
greater heat during summer, the temperature, other 
conditions remaining the same, would be proportion¬ 
ately increased. There is yet a third modifying cause 
which would tend to intensify the heat. The obli¬ 
quity of the ecliptic is now 23° 28'; but astronomers 
