[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,
the fleet of asteroids that sail the azure between Mars and Jupiter.
Some of these come near together, continuing to fill each other's
sky for days with brightness, then one gradually draws ahead. They
have all phases for each other--crescent, half, full, and gibbous.