204 Frank Carney 



between the perihelion and aphelion positions is about 14,000,000 

 miles; and the heat received at these two positions varies as 19 

 to 26. At the present time, the winter of the northern hemisphere 

 occurs w^hen the earth is nearer the sun; in 10,500 years the earth 

 will be nearer the sun in our early summer. If winter in the nor- 

 thern hemisphere should come at the aphelion position, we would 

 be about eight and a half million miles farther from the sun than 

 now, and in consequence would receive one-fifth less heat in win- 

 ter and one-fifth more in summer. If aphelion winter of the nor- 

 thern hemisphere should coincide with the maximum eccentricity 

 of the earth's orbit, the winter would be 36 days longer than the 

 summer. 



The above are some of the important facts that led Mr. Croll 

 to associate our glacial period wdth astronomical conditions. 

 This explanation is ample to account for low temperatures, 

 that would insure precipitation, if any fell, in the form of 

 snow. If we knew approximately the time that has elapsed since 

 the Pleistocene ice sheet commenced to develop, we could tell 

 whether winter in the northern hemisphere happened at the time 

 of the earth's maximum eccentricity of orbit. But there does not 

 appear to be an agreement among students of astronomy, in re- 

 ference to the variation of the earth's eccentricity during the past, 

 We do not yet possess data of the behavior of other planets of our 

 system, extending over enough time to make it certain that varia- 

 tion in their position might not vitiate our present conclusions of 

 the past changes in the earth's eccentricity of orbit. The other 

 planets probably account for the earth's orbit not being circular, 

 and, as their alignment changes, the earth's orbit must also 

 change. In any event, Croll's explanation makes a strong appeal 

 to some students of the subject. 



Variation in the content of the earth's atmosphere. Our present 

 atmosphere is well understood. We know the gases it contains 

 and the proportion in which they exist. A study of the earth's 

 strata of the past geologic periods, and the life which existed when 

 these rocks were being deposited, points to very appreciable 

 variations in climate. During the Miocene period, animals of 

 tropical waters lived in high latitudes. During the Pennsyl- 

 vanian period, it is customary to hypothecate a mild and moist 

 climate ; this conclusion arises from a study of the flora that existed . 

 It is quite universally agreed that climatic changes have taken 

 place in the earth's past. 



