THE LEADING MEMBERS OF OUR FAMILY--SECOND GROUP.
Leaving behind us the busy zone of planetoids, hurrying round and round
the sun in company, we cross a wide gap, and come upon a very different
sight.
The distance from the sun which we have now reached is little less than
four hundred and fifty millions of miles, or about five times as much
as the earth’s distance; and the sun in the heavens shows a diameter
only one-fifth of that which we are accustomed to see. Slowly--yet not
slowly--floating onwards through space, in his far-off orbit, we find
the magnificent planet Jupiter.
Is eight miles each second slow progress? Compared with the wild whirl
of little Mercury, or even compared with the rate of our own earth’s
advance, we may count it so; but certainly not, compared with our
notions of speed upon earth. Eight miles each second is five hundred
times as fast as the swiftest express-train ever made by man. No mean
pace that for so enormous a body. For Jupiter is the very largest of
all the members of the Solar System except the sun himself--quite
the eldest brother of the family. His diameter is about eighty-five
thousand miles, his axis being nearly eleven times as long as that of
earth. Though in proportion to his great bulk not nearly so heavy as
our earth, yet his bulk is so vast that more than twelve hundred earths
would be needed to make one Jupiter.
It must not, however, be forgotten that there is a certain amount of
uncertainty about these measurements of Jupiter. He seems to be so
covered with a dense atmosphere and heavy clouds that it is quite
impossible for us to learn the exact size of the solid body within.
Jupiter does not travel alone. Borne onwards with him, and circling
steadily around him, are five moons; the smallest with a probable
diameter of about one hundred miles; one about the same size as our
own moon, and the others all larger. The nearest of the five, though
distant from the center of the planet over one hundred and twelve
thousand miles, revolves about him in twelve hours; the second, though
farther from Jupiter than our moon from the earth, speeds round him
in less than two of our days. The most distant, though over a million
miles away, takes scarcely seventeen days to accomplish its long
journey. Jupiter and his moons make a little system by themselves--a
family circle within a family circle.
Like the smaller planets, Jupiter spins upon his axis; and he does this
so rapidly that, notwithstanding his great size, his day lasts only ten
hours, instead of twenty-four hours like ours. But if Jupiter’s day
is short, his year is not. Nearly twelve of our years pass by before
Jupiter has traveled once completely round the sun. So a native of
earth who had just reached his thirty-seventh year, would, on Jupiter,
be only three years old.
Passing onward from Jupiter, ever farther and farther from the sun,
we leave behind us another vast and empty space--empty as we count
emptiness, though it may be that there is in reality no such thing as
emptiness throughout the length and breadth of the universe. The width
of the gap which divides the pathway of Jupiter from the pathway of his
giant brother-planet Saturn is nearly five times as much as the width
of the gap separating the earth from the sun. The distance of Saturn
from the sun is not much less than double the distance of Jupiter.
With this great space in our rear, we come upon another large and
radiant planet, the center, like Jupiter, of another little system;
though it can only be called “little” in comparison with the much
greater Solar System of which it forms a part.
Saturn’s diameter is less than that of Jupiter, but the two come near
enough to be naturally ranked together. Nearly seven hundred earths
would be needed to make one globe as large as Saturn. But here again
the dense and cloudy envelope makes us very uncertain about the
planet’s actual size. Saturn is like Jupiter in being made of lighter
materials than our earth; and also in his rapid whirl upon his axis,
the length of his “day” being only ten and a half of our hours.
From Jupiter’s speed of eight miles each second, we come down in the
case of Saturn to only five miles each second. And Jupiter’s long
annual journey looks almost short, seen beside Saturn’s long journey
of thirty earthly years. A man aged sixty, according to our fashion of
reckoning time, would on Saturn have just kept his second birthday.
The system or family of Saturn is yet more wonderful than that of
Jupiter. Not five only but eight moons travel ceaselessly round Saturn,
each in its own orbit; and in addition to the eight moons, he has
revolving round him three magnificent rings. These rings, as well as
the moons, shine, not by their own brilliancy, for they have none, but
by borrowed sunlight. The farthest of the moons wanders in his lonely
pathway about two millions of miles away from Saturn. The largest of
them is believed to be about the same size as the planet Mars. Of the
three rings circling round Saturn, almost exactly over his equator, the
inside one is dusky, purplish, and transparent; the one outside or over
that is very brilliant; and the third, outside the second, is rather
grayish in hue.
Another vast gap--more enormous than the last. It is a wearisome
journey. From the orbit of Jupiter to the orbit of Saturn at their
nearest points, was five times as much as from the sun to the earth.
But from the orbit of Saturn to the orbit of Uranus, the next member of
the sun’s family, we have double even that great space to cross.
Still, obedient to the pulling of the sun’s attractive power, Uranus
wanders onward in his wide pathway round the sun, at the rate of four
miles a second. Eighty-four of our years make one year of Uranus.
This planet has four moons, and thus forms a third smaller system
within the Solar System; but he may have other satellites also, as yet
undiscovered. In size he is seventy-four times as large as our earth.
One more mighty chasm of nine hundred millions of miles, for the same
distance which separates the pathway of Saturn from the pathway of
Uranus, separates also the pathway of Uranus from the pathway of
Neptune. Cold, and dark, and dreary indeed seems to us the orbit on
which this banished member of our family circle creeps round the sun,
in the course of one hundred and sixty-five years, at the sluggish rate
of three miles a second.
On the planet Saturn, the quantity of light and heat received from
the sun is not much more than a hundredth part of that which we are
accustomed to receive on earth. But by the time we reach Neptune, the
great sun has faded and shrunk in the distance until to our eyes he
looks only like an exceedingly brilliant and dazzling star.
We know little of this far-off brother, Neptune, except that he is
rather larger than Uranus, being one hundred and five times as big as
the earth; that he has at least one moon; and also that, like Uranus,
he is made of materials lighter than those of earth, but heavier than
those of Jupiter or Saturn.
After all, it is no easy matter to gain clear ideas as to sizes and
distances from mere statements of “so many miles in diameter,” and “so
many millions of miles away.” A “million miles” carries to the mind a
very dim notion of the actual reality.
Now if we can in imagination bring down all the members of the Solar
System to a small size, keeping always the same proportions, we may
find it a help. “Keeping the same proportions” means that all must be
lessened alike, all must be altered in the same degree. Whatever the
supposed size of the earth may be, Venus must be still about the same
size as the earth, Saturn seven hundred times as large, and so on.
Also, whatever the distance of the earth from the sun, in miles, or
yards, or inches, Mercury must still be one-third as far, Jupiter still
five times as far, and thus with the rest.
First, as to size alone. Suppose the earth is represented by a small
globe, exactly three inches in diameter. It will be a very small globe.
Not only men and houses, but mountains, valleys, seas, will all have to
be reduced to so minute a size, as to be quite invisible to the naked
eye.
Fairly to picture the other members of the Solar System, in due
proportion, you will have them as follows:
Mercury and Mars will be balls smaller than the earth, and Venus nearly
the same size of the earth. Uranus and Neptune will be each somewhere
about a foot in diameter. Saturn will be twenty-eight inches and
Jupiter thirty-two inches in diameter. The sun will be a huge dazzling
globe, _twenty-six feet_ in diameter. No wonder he weighs seven hundred
and fifty times as much as all his planets put together.
Next let us picture the system more exactly on another and smaller
scale. First, think of the sun as a brilliant globe, about nine feet,
or three yards, in diameter, floating in space.
About one hundred yards from the sun travels a tiny ball, not half
an inch in diameter, passing slowly round the sun--slowly, because
as sizes and distances are lessened, speed must in due proportion be
lessened also. This is Mercury. About two hundred yards from the sun
travels another tiny ball, one inch in diameter. This is Venus. Nearly
a quarter of a mile from the sun travels a third tiny ball, one inch
again in diameter; and at a distance of two feet and a half from it a
still smaller ball, one quarter of an inch in diameter, journeys round
it and with it. These are the earth and the moon. About half as far
again as the last-named ball, travels another, over half an inch in
diameter. This is Mars.
Then comes a wide blank space, followed by a large number of minute
objects, no bigger than grains of powder, floating round the sun in
company. These are the Asteroids. Another wide blank space succeeds the
outermost of them.
About one mile distant from the sun journeys a globe, ten inches in
diameter. Round him, as he journeys, there travel five smaller balls,
the largest of which is about the third of an inch in diameter. Their
distances from the bigger globe vary from one and a third to twelve and
a half feet. These are Jupiter and his moons.
Nearly two miles distant from the sun journeys another globe, about
eight and a half inches in diameter. Eight tiny balls and three
delicate rings circle round him as he moves. These are Saturn and his
belongings. About four miles distant from the sun journeys another
globe, four inches in diameter, with four tiny balls accompanying
him--Uranus and his moons. Lastly, at a distance of six miles from
the sun, one more globe, not much larger than the last, with one tiny
companion, pursues his far-off pathway.
These proportions as to size and distance will serve to give a clear
idea of the Solar System.