LESSON VII.
A FRAGMENT OF THE MILKY WAY.
“And now the still stars make all heaven sightly.One, in the low west, like the sky ablaze;The Swan, that with her shining Cross floats nightly,And Bears that slowly walk along their ways.There is the golden Lyre, and there the crown of fire:Thank God for nights so fair to these bright days.”
“And now the still stars make all heaven sightly.One, in the low west, like the sky ablaze;The Swan, that with her shining Cross floats nightly,And Bears that slowly walk along their ways.There is the golden Lyre, and there the crown of fire:Thank God for nights so fair to these bright days.”
“And now the still stars make all heaven sightly.One, in the low west, like the sky ablaze;The Swan, that with her shining Cross floats nightly,And Bears that slowly walk along their ways.There is the golden Lyre, and there the crown of fire:Thank God for nights so fair to these bright days.”
“And now the still stars make all heaven sightly.
One, in the low west, like the sky ablaze;
The Swan, that with her shining Cross floats nightly,
And Bears that slowly walk along their ways.
There is the golden Lyre, and there the crown of fire:
Thank God for nights so fair to these bright days.”
—Lowell,Summer Nightfall.
Looking up into the heavens we cannot fail to notice a broad, shining band that crosses the sky. We are early told that this is the Milky Way, receiving its name from its pale, silvery light and its path-like track across the firmament. All the heavenly bodies are arranged in clusters or nebulæ; no one of them is isolated. The Milky Way is one of these nebulæ. Its shape is that of a wide, irregular ribbon deeply notched at one end. The reason of the white and shining appearance of the entire nebula, is that it is densely crowded with stars, and is so immensely distant that the spaces between the stars are lost to view. About one hundred years ago Herschel endeavored to count the stars in the Milky Way. He found eighteen millions of stars in the central belt alone.
How long is this nebula? Light starting from one limit of its longest diameter, and travelling one hundred and eighty-six thousand miles a second, would travel fifteen thousand years before it reached the last shining star in the Milky Way! Such distances as this are beyond our powerto imagine. The Milky Way is a gigantic cluster of suns, and if each sun has its planetary system about it, each system rides like a shining archipelago in these seas of space. Earth and all other planets, being dark in themselves, do not count in these burning distances. So then our sun is but a star, and the Milky Way is made up of millions of such stars. More than this, our sun is one of the many stars that compose this wonderful band of light. We have now found our place in the heavens—we are a portion of the Milky Way.
HUNG IN SPACE.
HUNG IN SPACE.
All the large stars which grace our most brilliant nights belong to this samegreat cluster. We are ourselves lying near its centre. Among all these suns our own appears to us to be the largest and most magnificent. This is because it is nearest to us, and it is to us the source of life, light, and heat. There may be other suns of tenfold splendor, and there may be other nebulæ far larger and more crowded with suns than the Milky Way.
There are other nebulæ which have received different names, from the shape of their outlines, as globular, annular, spiral, double, multiple; the Crab, the ship Argo,—which is shaped like a comet,—the Magellanic clouds, and so on. These lie like splendid islands rocking in distant space, so far away, that if we travelled with the inconceivable speed of light, we might be millions of years in reaching them.
Many of the greatest human minds have, during many ages, been occupied in studying our solar system, investigating its laws, measuring its distances. The discoveries so made have been applied to other systems, and in this way, by long and slow but sure degrees, the science of astronomy has been built up.
From ancient times it has been the custom to divide the stars into classes according to their apparent size. This apparent size depends chiefly upon their nearness to us, and evidently, as the few only are near, and the many are far off, stars of the first magnitude are few in number, and as they diminish in apparent size the number embraced in the successive classes increases. Thus we find but eighteen stars of the first magnitude; but of the third there are one hundred and seventy, of the fourth five hundred, and thus on.
When we look upward in a starry winter night we fancy we can see millions of stars. In truth with the unaided eye we can see only six thousand at most, and it needs very strong eyes to see more than four thousand. But when we turn a telescope upon the sky we begin to count stars by millions.
All the stars are in apparent motion; they have their rising and their setting as does the sun; the constellations appear above the horizon, and move swiftly down the sky. Only one star is, as viewed from our world, immovable, remaining forever fixed in its place; we call it the North Star, or the Pole Star, because to us of the Northern hemisphere it seems to be always watching above the Northern Pole. But, while unalterable as regards us, the Pole Star is not released from the general law of motion which rules the stars.
When we begin to trace out constellations, we usually take the Pole Star as our starting-point. The Pole Star is in the tail of the constellation of the Little Bear, and we find it by imagining a straight line drawn from the two upper stars in the square of the Great Dipper. The Great Dipper we shall easily find, as it is composed of seven large, bright stars, four of which form an irregular four-sided figure and from one corner of this square three others extend in a slightly curved line. This constellation, seen from any place north of New Orleans, La., never sets, but slowly turns around the fixed Pole Star. In ancient times the Great Dipper was sometimes called the Chariot, or David’s Chariot, and among English country people it is known as Charles’s Wain, a wain being an old English name for a wagon. TheChinese name the Great Dipper “The God of the North”; the Mohammedans often call it “The Hand of God.” It is also called the Great Bear.
Of all the stars the one nearest us is in the southern constellation of the Centaur; but that is so far off that its distance is beyond our comprehension, being over two hundred and eleven thousand times as far distant as the sun. If we could take a ray of light for our steed we must ride three years and six months to reach this sun that is nearest to our sun in the starry hosts.
But is light for a steed all that is wanted for such a trip among the stars? No; for a very little way from our earth we should be in want of an atmosphere, or air to breathe, and also in want of light and heat. Light, heat, and air are probably alike lacking in starry space.
Before we consider the orbs which compose our own solar system, their motions and relationships, which we shall do in another chapter, let us consider for a little the theory of the origin of systems, taking our own for a sample of the rest. Tennyson the poet, puts the story of system-building thus:—
“This world was once a fluid haze of lightTill toward the centre set the starry tides,And eddied into suns that wheeling castThe planets.”
“This world was once a fluid haze of lightTill toward the centre set the starry tides,And eddied into suns that wheeling castThe planets.”
“This world was once a fluid haze of lightTill toward the centre set the starry tides,And eddied into suns that wheeling castThe planets.”
“This world was once a fluid haze of light
Till toward the centre set the starry tides,
And eddied into suns that wheeling cast
The planets.”
We saw in a previous lesson that our world was once a vast ball of fiery vapor. The great astronomer Laplace, after many years of study, published a theory about system-making, called the “Nebular Hypothesis,” which, as simply as we can put it, is this: First, there is a great cloud of glowingvapor, the various particles of which are by the law of gravitation drawn toward a single centre. As the particles press equally to the centre from all sides, it is evident that the first result will be a ball of constantly increasing solidity. This nebulous mass possesses also a motion of rotation, or turning over on its own axis. As the ball grows smaller and more dense, it will spin round faster and faster.
The gaseous matter of the sphere having become fluid or partly fluid, the ball still whirls on, and we must now notice a second motion, called centrifugal or tangental, which has become more apparent as the rotation increases in velocity. While by the force of gravity all atoms seek the centre, by this centrifugal force atoms are driven from the centre. This tangental motion is familiarly seen in the case of mud on a wheel-tire, the mud being flung off from the wheel by a motion created by rotation. So from the spinning globe a ring of matter will be detached and fly off into space.
If this ring were equally hard and thick in all its parts, and exactly poised about the globe from which it sprung, it might keep the ring shape. But in nearly all cases the ring when flung off would be irregular, and would consequently break up. As it broke, the largest fragments, keeping the wheeling motion and made spherical by gravity, would draw into themselves the smaller near fragments. Then, after a while, following the example of the globe from which they spun off, these new globes would cast off rings of matter which would contract and harden into globes, and becometheir satellites or attendants, as they, held by the force of gravity, remain attendants upon the first great globe.
This is the nebular theory of system-building. We see that in it three things are pre-supposed, or taken for granted; first, matter in a state of slowly rotating glowing vapor; second, a law of gravitation; third, a law of tangental motion. In the one hundred years since this hypothesis was first developed by Laplace, no one has ever seen the glowing vapor consolidating to suns. These processes are far too long to have been within any human observation, but long and careful study has enabled man to learn some of the wonderful laws by which the universe is governed.
Let us see if this explanation of Laplace fits anything which we find in our solar system. Has any great globe rings wheeling about it? Yes; the huge planet Saturn has a triple ring. Besides this ring, Saturn has eight moons, as if some other ring had broken up, and its parts had come together into satellites. The planet Jupiter has four satellites, Mars has two, Uranus has four, Neptune one, and our earth has one, called the moon. There are also in our system a number of very small planets, closely grouped, called asteroids. In the nebular hypothesis these asteroids might represent matter cast off at first in rings, breaking and finally condensing into spheres. Here we find in our solar system between the larger planets Mars and Jupiter a splendid zone of minor planets.
Comets are also explained by this theory, as large masses of material, too evenly balanced among the planets to yield to the attraction of any one of them, but owning the attractionof the sun, the centre of gravity of our entire system, and wheeling about it in vast elliptical orbits. Finally it is supposed by some that the general speed of our entire system is slackening. This “slowing up” of the system is supposed to be due to the friction of ether which pervades all space, and also to a great depth of hot and expanded material more thin than air, which extends about the sun, wrapping it in a vast, fine mantle of gas, as our atmosphere surrounds our globe.
But if it is true that the motion of our system is growing slower, it will be many millions of years before the change will be great enough to notice; just as, granting that the sun is losing its heat, it will be many millions of years before it ceases to shine upon and warm our globe.
In fact the lessening of speed in the solar system, and the lessening of sun heat, are as yet only theories and not proved facts. The sun will shine on, and the worlds will whirl around it, through time which no human mind can calculate.
Attraction, the attraction of the sun over the planets, and the planets for each other, has been several times mentioned. What is this attraction? Attraction is a “drawing to.” If a magnet be held near steel filings it attracts the particles of steel, and they adhere to the magnet; this attractive power is magnetic attraction. The attraction or pulling power which bodies have for each other, is called the attraction of gravity or of gravitation.
All bodies have this attraction for each other, but in small bodies we do not observe it. Its force is due to theweight or mass of the bodies, and diminishes as their distance from each other increases. It is by this force, or attraction of gravitation, that whatever is thrown into the air falls towards the earth. If you throw up a ball it moves upward, obeying the force you put into your throw, but soon that force is used up, and the earth pulls the ball back. You jump, and you rise from the earth by the force expended by your legs in the act of jumping, but that force is used up soon, and the earth pulls you down. You jump from a tree or roof, and down you come to the earth, pulled by the force or attraction of gravitation.
This force or attraction, exerted by the great strong sun over our earth and all other planets, keeps them travelling in nearly round paths about the sun. This attraction, exerted by the earth over the moon, keeps the moon travelling in a nearly circular path about the earth. And does not the moon then attract the earth? Oh, yes, all bodies attract each other; but the moon is of much less weight or mass than the earth, and her pull is weak in comparison; she cannot pull the earth out of her orbit. She does pull something however, and what do you suppose she pulls? She pulls the water that is on the earth, heaping it up in the tides! The tide is simply the ocean obeying the attraction of the moon.
It seems a pity that in a few simple lessons on the solar system, we should use any terms that are hard to be understood. But some few such terms we have been obliged to use. “Attraction of gravitation” is one of these which we have tried to make a little plainer. “Inclination of plane to orbit” is another. What does that mean? Can we make it clear?
Let us set a lamp on the table, and call it the sun. Now take an orange and call it the world. Call the stem-place, and the spot opposite, the poles of this world. Next run a knitting-needle through the orange, from pole to pole, to represent the axis on which the earth turns. As we spin our earth-orange around on its axis, let us move our needle around the lamp, in a lemon-shaped path. This shape is called an ellipse. Thus we represent the earth turning around on its axis, or over and over, and at the same time travelling around the sun.
Now it is clear that we can hold our knitting-needle straight out, horizontally, or we can hold it straight up and down, perpendicularly, as it moves around the sun-lamp. But neither of these straight positions would represent the direction of the axis of the earth; we musttipthe needle a little, so as to hold it in a slanting position. Now if we held the needle exactly up and down, or exactly straight out, the sun-lamp would shine on just half of the orange, as divided from pole to pole. But when we tip the needle you see we bring our orange-world in such a position that, as it passes around the sun-lamp, the light falls now more around one pole, now more around the other; this tipped position is “inclination to the plane of orbit.”