24 
Change then, we see, in all the circumstances which govern the 
climate of the earth ; therefore change inthat too. ‘The phenomena 
which are most intimately connected with the climatic conditions, 
and specially with the production of an Ice Age are:—(a) The 
variations in shape of the earth’s orbit, and with that the varying 
relative distances of the sun in summer and winter. (b) The 
precession of the Equinoxes together with changes in the direction 
of the earth’s axis. 
The excentricity of the orbit at present is comparatively slight, 
and is decreasing so that it is becoming more and more circular and 
will (so we are told), be as near to acircle as possible in the year a.D. 
25,780, after which date it will contract again in its minor axis, and 
become more elliptical. The major axis alters in length to such a 
very minute extent that it need not be taken into account in con- 
nection with our subject. It necessarily follows that an alteration 
in the ellipse itself produces an alteration in the relative position 
of the foci, at one of which, as we have seen the sun is placed. 
Such alteration of the shape of the orbit is produced by the influence 
of the other planets, chiefly by Venus and Jupiter. Note then that 
if the ellipticity increases, the two foci are farther apart from each 
other i.e. nearer to the earth’s orbit, and consequently the sun will 
be much nearer to us when we are in perihelion than it is at present 
and much farther off at the the time of aphelion. Since the intensity 
of the sun’s heat and light received by us are proportional, not to 
the simple distance, but to the square of the distance, such variation 
will have a considerable effect on the climate of the earth. The 
difference in distance in summer and winter is nearly 3,000,000 
miles, but it may amount to as much as 14,000,000 and then the 
differential climatic effect would be greatest of all. If, as at present 
the summer happened when the earth were in aphelion it would be 
a much cooler summer than we now enjoy, and the winter would 
be very much warmer. But if, on the contrary, our winter occurred 
in aphelion, there would bea much severer winter and a much hotter 
summer. And, at the same period, the winter would be much 
longer (it might possibly be 33 days longer than summer), and we 
should have in the combination the requisite conditions for a glacial 
period. Under what conditions, we may ask could there be the great- 
est difference in the lengths of summer and wirter? This leads us to 
consider the second variation, viz., the Precession of the Equinoxes. 
The Equinoxes, i.e., the times of equal day and night all over the 
world, occur twenty minutes earlier every year, and hence they travel 
round the whole orbit, and occur successively in every part of it. 
On the slide we see them approximately in their present position. 
By and bye the spring Equinox will be in aphelion, the autumnal 
in perihelion ; the orbit will then be equaily divided, and summer 
