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Page 31
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.
[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.
These hundreds of bodies fill the realm where they are with
inexhaustible variety. Beyond are vast spaces--cold, dark, void of
matter, but full of power. Occasionally a little spark of light
looms up rapidly into a world so huge that a thousand of our earths
could not occupy its vast bulk. It swings its four or eight moons
with perfect skill and infinite strength; but they go by and leave
the silence unbroken, the darkness unlighted for years.
Nevertheless, every part of space is full of power. Nowhere in its
wide orbit can a world find a place; at no time in its eons of
flight can it find an instant when the sun does not hold it in
safety and life.
_The Outlook from the Earth._
If we come in from our wanderings in space and take an outlook from
the earth, we shall observe certain movements, easily interpreted
now that we know the system, but nearly inexplicable to men who
naturally supposed that the earth was the largest, most stable,
and central body in the universe.
We see, first of all, sun, moon, and stars rise in the east, mount
the heavens, and set in the west. As I [Page 109] revolve in my
pivoted study-chair, and see all sides of the room--library, maps,
photographs, telescope, and windows--I have no suspicion that it is
the room that whirls; but looking out of a car-window in a depot at
another car, one cannot tell which is moving, whether it be his car
or the other. In regard to the world, we have come to feel its
whirl. We have noticed the pyramids of Egypt lifted to hide the sun;
the mountains of Hymettus hurled down, so as to disclose the moon
that was behind them to the watchers on the Acropolis; and the
mighty mountains of Moab removed to reveal the stars of the east.
Train the telescope on any star; it must be moved frequently, or the
world will roll the instrument away from the object. Suspend a
cannon-ball by a fine wire at the equator; set it vibrating north
and south, and it swings all day in precisely the same direction.
But suspend it directly over the north pole, and set it swinging
toward Washington; in six hours after it is swinging toward Rome, in
Italy; in twelve hours, toward Siam, in Asia; in nineteen hours,
toward the Sandwich Islands; and in twenty-four, toward Washington
again, not because it has changed the plane of its vibration, but
because the earth has whirled beneath it, and the torsion of the
wire has not been sufficient to compel the plane of the original
direction to change with the turning of the earth. The law of
inertia keeps it moving in the same direction. The same experimental
proof of revolution is shown in a proportional degree at any point
between the pole and the equator.
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