126 
THE ICE AGES, PAST AND FUTURE. 
June, 1892. 
is the nearest to ns of the planets, and nearly as large as the 
earth ; but Jupiter, although so much farther off, being at 
fifteen times the distance of Venus, is of the most importance 
on account of being the largest planet in the solar system, 
and 340 times the weight of the earth, and the attraction of 
Jupiter is twice as great as that of Venus. The other planets 
have all some effect in disturbance of the earth’s orbit; but 
their effect is a comparatively small amount in the calculation ; 
Mars, which is intermediate between Jupiter and the earth, 
being only one-eighth part of the weight of the earth ; and 
Saturn, the next beyond Jupiter, although 250 times the 
weight of the earth, is at double, the distance of Jupiter, and 
the force of attraction diminishes in proportion to the square 
of the distance. 
The earth travels round its orbit in a year, and Jupiter 
takes twelve years to go round, so that the earth passes 
Jupiter in its course about every thirteen months, and those 
are the times when Jupiter becomes such a brilliant object in 
the heavens ; and on each of those occasions the earth gets 
pulled a little further away from the sun. 
Venus travels round in its orbit nearly twice for each 
revolution of the earth, the time of its revolution being 7£ 
months; and in the race round the sun Venus passes the 
earth about every twenty months, and at those times the 
earth gets pulled a little nearer towards the sun, excepting 
when Jupiter happens to be passing at the same time, 
and then the attraction of Jupiter being twice as great as that 
of Venus, the result is a pull of the earth away from the sun 
as the balance of the two forces. Taking the force of 
attraction of Jupiter as represented by 100 when nearest 
to us, that of Venus is about 50 when nearest, but only about 
1 when at the farthest position on the other side of the orbit, 
and the attraction of Jupiter is about 40 when at its farthest 
position. The greatest disturbing effect upon the earth’s 
position in pulling it away from the sun is when Jupiter is at 
the nearest and Venus at the farthest position, Jupiter being 
outside the earth’s orbit and Venus inside, and the result is a 
pull outwards amounting to the difference of 100 and 1, or 99 . 
The greatest effect in the opposite direction, pulling the earth 
inwards towards the sun, is when Jupiter is at the farthest 
position and Venus at the nearest, and the result is the sum 
of 40 and 50, or 90 . The pull outwards , when both planets are 
in their nearest positions, is the difference of 100 and 50, 
or 50 ; and when they are in their farthest positions the pull 
inwards is the sum of 40 and 1, or 41 . 
The changes in the eccentricity of the earth’s orbit are the 
result of the earth being pulled outwards at some periods and 
