the case of Venus it is only .007 of the whole, and in no instance
is it more than .2, viz., that of Mercury. This makes the sun appear
twice as large, bright, and hot as seen and felt on Mercury at its
perihelion than at its aphelion. The earth is 3,236,000 miles nearer
to the sun in our winter than summer. Hence the summer in the
southern hemisphere is more intolerable than in the northern. But
this eccentricity is steadily diminishing at a uniform rate, by
reason of the perturbing influence of the other planets. In the case
of some other planets it is steadily increasing, and, if it were to
go on a sufficient time, might cause frightful extremes of
temperature; but Lalande has shown that there are limits at which it
is said, "Thus far shalt thou go, and no farther." Then a
compensative diminution will follow.

Conceive a large globe, to represent the sun, floating in a round
pond. The axis will be inclined 7-1/2° to the surface of the water,
one side of the equator be 7-1/2° below the surface, and the other
side the same distance above. Let the half-submerged earth sail
around the sun in an appropriate orbit. The surface of the water
will be the plane of the orbit, and the water that reaches out
to the shore, where the stars would be set, will be the plane of
the ecliptic. It is the plane of the earth's orbit extended to
the stars.

The orbits of all the planets do not lie in the same plane, but
are differently inclined to the plane of the ecliptic, or the plane
of the earth's orbit. Going out from the sun's equator, so as to
see all the orbits of the planets on the edge, we should see them
inclined to that of the earth, as in Fig. 40.