June, 1892. 
THE ICE AGES, PAST AND FUTURE. 
125 
our day is consequently longer than our night ; and in 
winter the opposite takes place, and the north pole is inclined 
from the sun, so that the night is longer than the day. At 
the half way points, namely the spring and autumn equinoxes, 
the day and night are of equal length ; and, if the axis of the 
earth were not inclined but vertical, the same would be the 
case throughout the whole year, and no variation of seasons 
would take place. 
The seasons in the southern hemisphere are the opposite 
of those in the northern, the summer being there at the 
time of our winter ; and the result of the reversal in the 
direction of the earth’s axis every 10,500 years, would simply 
reverse these conditions, causing our summer to be at 
Christmas and our winter to be in the middle of the year, 
without any change affecting severity of climate or leading to 
the occurrence of an Ice Age ; but there is another cause in 
action that bears upon this, and the result of the two causes 
acting combined makes a very important difference. 
The earth’s annual orbit round the sun is not a concentric 
circle, but an ellipse that closely resembles an eccentric circle, 
that is a circle in which the sun is out of the centre, so that at 
one part of the year the earth is nearer to the sun than at the 
opposite part of the year. Now, it so happens that at the 
present time the period of our winter occurs when the earth 
is in that part of its orbit that is nearest to the sun (or at 
the perihelion) ; and the result is that our winter is milder 
than that of the southern hemisphere, which occurs when 
the earth is farthest from the sun (or at the aphelion). 
The amount of the eccentricity of the earth’s orbit is at 
the present time very small, and only l-60th part of the 
distance of the earth from the sun, so that the reversal of 
the conditions of winter and summer would only cause our 
winter to be somewhat colder than at present, and to be 
similar to the winter of the southern hemisphere at the 
present time, and would not approach the conditions of an 
Ice Age. 
We now come to the most important of the two causes 
that bring about an Ice Age—this is, a change in the amount 
of the eccentricity of the earth’s orbit, which is constantly 
going on, but with extreme' slowness ; and it is calculated 
that at one period, 200,000 years ago, the eccentricity, instead 
of being only l-60tli part as at present, was as great as 
1-18tli part of the earth’s distance from the sun ; and that 
the earth was farther from the sun by that amount at one 
part of its orbit, and as much nearer at the opposite part. 
This disturbance in the earth’s orbit is caused mainly by 
the attraction of the two planets, Jupiter and Venus. Venus 
