(Drawn approximately to scale)
The isolation of the Solar System is very great. On the above scale the
_nearest_ star (at a distance of 25 trillions of miles) would be over
_one half mile_ away. The hours, days, and years are the measures of
time as we use them; that is: Jupiter's "Day" (one rotation of the
planet) is made in ten of _our hours_; Mercury's "Year" (one revolution
of the planet around the Sun) is eighty-eight of _our days_. Mercury's
"Day" and "Year" are the same. This planet turns always the same side to
the Sun.]
[Illustration: THE COMPARATIVE SIZES OF THE SUN AND THE PLANETS (Drawn
approximately to scale)
On this scale the Sun would be 17-1/2 inches in diameter; it is far
greater than all the planets put together. Jupiter, in turn, is greater
than all the other planets put together.]
Except when the winds rise to a high speed, we seem to live in a very
tranquil world. At night, when the glare of the sun passes out of our
atmosphere, the stars and planets seem to move across the heavens with a
stately and solemn slowness. It was one of the first discoveries of
modern astronomy that this movement is only apparent. The apparent
creeping of the stars across the heavens at night is accounted for by
the fact that the earth turns upon its axis once in every twenty-four
hours. When we remember the size of the earth we see that this implies a
prodigious speed.
In addition to this the earth revolves round the sun at a speed of more
than a thousand miles a minute. Its path round the sun, year in year
out, measures about 580,000,000 miles. The earth is held closely to this
path by the gravitational pull of the sun, which has a mass 333,432
times that of the earth. If at any moment the sun ceased to exert this
pull the earth would instantly fly off into space straight in the
direction in which it was moving at the time, that is to say, at a
tangent. This tendency to fly off at a tangent is continuous. It is the
balance between it and the sun's pull which keeps the earth to her
almost circular orbit. In the same way the seven other planets are held
to their orbits.
Circling round the earth, in the same way as the earth circles round the
sun, is our moon. Sometimes the moon passes directly between us and the
sun, and cuts off the light from us. We then have a total or partial
eclipse of the sun. At other times the earth passes directly between the
sun and the moon, and causes an eclipse of the moon. The great ball of
the earth naturally trails a mighty shadow across space, and the moon is
"eclipsed" when it passes into this.
The other seven planets, five of which have moons of their own, circle
round the sun as the earth does. The sun's mass is immensely larger than
that of all the planets put together, and all of them would be drawn
into it and perish if they did not travel rapidly round it in gigantic
orbits. So the eight planets, spinning round on their axes, follow their
fixed paths round the sun. The planets are secondary bodies, but they
are most important, because they are the only globes in which there can
be life, as we know life.
If we could be transported in some magical way to an immense distance in
space above the sun, we should see our Solar System as it is drawn in
the accompanying diagram (Fig. 1), except that the planets would be mere
specks, faintly visible in the light which they receive from the sun.
(This diagram is drawn approximately to scale.) If we moved still
farther away, trillions of miles away, the planets would fade entirely
out of view, and the sun would shrink into a point of fire, a star. And
here you begin to realize the nature of the universe. _The sun is a
star. The stars are suns._ Our sun looks big simply because of its
comparative nearness to us. The universe is a stupendous collection of
millions of stars or suns, many of which may have planetary families
like ours.
§ 2