THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



Art. XIII. — On Certain Astronomical Conditions favorable 

 to Glaeiation ; by Geo. F. Becker. 



The influence of local terrestrial conditions on glaeiation is 

 manifest to observers, and few geologists will entertain the 

 idea that cosmical conditions alone can have determined the 

 glacial epoch. Yet variations in the elements of the earth's 

 orbit have certainly influenced climate, and they must have 

 influenced it more favorably to glaeiation at some periods than 

 at others. The nature and extent of this influence have been 

 much discussed ; but it seems to me that further light can be 

 thrown upon the subject by considering in detail the distribu- 

 tion of solar energy with reference to latitude, and the rate at 

 which this energy is received during the two great seasons 

 separated by the equinoxes. 



The elements of the earth's orbit undergo slow variations, 

 and three of these variations affect climate. The time at 

 which the earth is in perihelion affects the length of the two 

 great seasons. If perihelion coincides with either equinox, 

 the seasons are of equal length. If perihelion coincides with 

 either solstice, the seasons differ in length as much as they can 

 for a given eccentricity of the orbit. The whole time which 

 intervenes between the occurrence of seasons of equal length 

 and that of seasons of the most diverse length is five or six 

 thousand years, being the time required for the precession of 

 the equinoxes to amount to a right angle. The eccentricity of 

 the earth's orbit determines the possible difference between the 

 seasons, and it slightly affects the mean distance of the earth 

 from the sun, so that at a period of high eccentricity the 

 world receives a little more heat than it does at a period of 



Am. Jour. Sci. — Third Series, Vol. XLVIII, No. 284 — August, 1894. 



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