A much more elaborate theory was advanced by Dr. Croll, and is
still entertained by many. The path of the earth round the sun is
not circular, but elliptical, and there are times when the
gravitational pull of the other planets increases the
eccentricity of the orbit. It was assumed that there are periods
of great length, separated from each other by still longer
periods, when this eccentricity of the orbit is greatly
exaggerated. The effect would be to prolong the winter and
shorten the summer of each hemisphere in turn. The total amount
of heat received would not alter, but there would be a long
winter with less heat per hour, and a short summer with more
heat. The short summer would not suffice to melt the enormous
winter accumulations of ice and snow, and an ice-age would
result. To this theory, again, it is objected that we do not find
the regular succession of ice-ages in the story of the earth
which the theory demands, and that there is no evidence of an
alternation of the ice between the northern and southern
hemispheres.
More recent writers have appealed to the sun itself, and supposed
that some prolonged veiling of its photosphere greatly reduced
the amount of heat emitted by it.
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