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 deg. to the surface of the water,
one side of the equator be 7-1/2 deg. 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.

[Illustration: Fig. 40.--Inclination of the Planes of Orbits.]

If the earth, and Saturn, and Pallas were lying in [Page 107] the
same direction from the sun, and the outer bodies were to start in a
direct line for the sun, they would not collide with the earth on
their way; but Saturn would pass 4,000,000 and Pallas 50,000,000
miles over our heads. From this same cause we do not see Venus and
Mercury make a transit across the disk of the sun at every
revolution.

[Illustration: Fig. 41.--Inclination of Orbits of Venus and Earth.
Nodal Line, D B.]

Fig. 41 shows a view of the orbits of the earth and Venus seen
not from the edge but from a position somewhat above. The point E,
where Venus crosses the plane of the earth's orbit, is called the
ascending node. If the earth were at B when Venus is at E, Venus
would be seen on the disk of the sun, making a transit. The same
would be true if the earth were at D, and Venus at the descending
node F.

This general view of the flying spheres is full of interest. [Page
108] While quivering themselves with thunderous noises, all is
silent about them; earthquakes may be struggling on their surfaces,
but there is no hint of contention in the quiet of space. They are
too distant from one another to exchange signals, except, perhaps,