LESSON VIII.

PLAN AND PROGRESSION.

“They rise in joy, the starry myriads burning—The shepherd greets them from his mountains free;And from the silvery seaTo them the sailor’s wakeful eye is turning—Unchanged they rise—”

“They rise in joy, the starry myriads burning—

The shepherd greets them from his mountains free;

And from the silvery sea

To them the sailor’s wakeful eye is turning—

Unchanged they rise—”

—Hemans.

We have now found that our earth is one among several planets revolving around a great central star which we name the sun. The sun and his attendant planets and their satellites form what is called the solar system. We have also found that the heavens are filled with such systems, and that these are gathered together in nebulæ or clusters. Prominent among these clusters is the Milky Way, near the centre of which our system is placed.

The bodies which in our system revolve about the sun, are called planets. With respect to the sun and other great stars the planets are small, but considered in themselves they are enormous bodies. They are all dark spheres,[17]having no light in themselves. Is it true that the moon with her silvery radiance and Venus and Mars with their steadfast beams, are dull and dark as burnt-out cinders? This is indeed true; the planets receive their light and heat from the sun, and it is as they reflect back this light that they shine so brilliantly. As an example of such reflection take a new tin plate; it has no light in itself; shut it in a dark room, it gives out nolight; but hold it where a broad ray of sunshine strikes it, and at once the light is reflected back from the tin with such burning splendor that the eyes can scarcely look upon it. So we have observed windows struck by the light of the setting sun, and at once blazing forth like fires.

The planet nearest to the sun is named Mercury. It is distant from the sun about thirty-five millions of miles,[18]and is the smallest of the eight major planets. Mercury is so close to the sun that it is nearly always lost in the sun’s light, and can be seen by the naked eye only occasionally, immediately after a clear sunset, or before a clear sunrise. The telescope shows us that Mercury has phases as the moon has; possesses, as far as known, no satellites, and its path is not so nearly in a circle as that of other planets. To the naked eye it appears as a small star, with a white light faintly tinted with red. Lying so near to the sun it receives much more light and heat than we do. Its day is twenty-four hours long; its year has but eighty-eight days, and each of its seasons is but twenty-two days long. The shortness of the year and of the seasons is the result of the nearness of the planet to the sun, which so shortens its path that it can be traversed in less than three of our months.

The next planet in order of distance from the sun is Venus, the most beautiful of what we popularly call stars. But Venus is a planet, not a star. Venus is visible nearly all the year, and is often the first star to shine forth in the evening or the last one to fade away in the morning. Ithas even been seen shining at noon-day. This dazzling light of Venus is supposed to be caused by a dense layer of cloud, which wraps the planet about, and proves an admirable reflector of the sun’s rays. The day of Venus is about half an hour shorter than our own;[19]its year is two hundred and twenty-four days. The French astronomer, Flammarion, tells us that mountains, much higher than any on our earth, have been measured on the planet Venus. The changes of temperature on Venus are much more sudden than on our earth; but it has no ice at the poles, for its winter does not last long enough for ice to accumulate as it does at the earth’s poles.

As the orbit of Venus is between the earth and the sun, it happens that at certain times Venus lies between the sun and the earth, and so shows us its dark side. Again, when it is to the right or left of the sun, we see only a crescent formed by its quarter,[20]and it is at this phase that its mountains have been measured.

Venus is sixty-six millions of miles from the sun, and when nearest us about twenty-six million miles from the earth, being, with the exception of the moon, our nearest neighbor in the skies.

The third planet from the sun is our earth, ninety-two million miles from the source of its light and heat. Circling around the earth, and moving with it in its orbit, goes the moon. The earth rotates on its axis once in about twenty-four hours, giving us day and night in that period of time,according as, during its revolution, a part of the surface is turned to or from the sun. As the earth rolls over upon itself, it also rolls along its orbit, or sky-path. The path of the earth about the sun is travelled over in three hundred and sixty-five and a quarter days, giving us our change of seasons by the inclination of the earth’s axis to the plane of its orbit.

Next removed beyond the earth from the sun, is the splendid planet Mars, which is easily distinguishable by its red light. It is in some parts of its path only seven millions of miles further from the earth than Venus. Mars has a day of about the same length as ours, and its seasons are of nearly the same intensity; but while a season is three months long with us, on Mars it is very nearly six months, for Mars takes almost two of our years to travel round the sun. Mars has a snow or ice-cap at each pole; it has also an atmosphere, as with a telescope we can observe clouds drifting across its sky, the clouds being formed as clouds here, by the evaporation of water from the surface of the globe, which moisture, being carried up in invisible vapor, is condensed by colder currents of air and forms clouds.

The red light of Mars is supposed to be produced by the color of its surface, and by some it is considered that the vegetation on Mars may be red, as ours is green, and that to an observer on Mars, our planet would shine with a green light, as to us Mars shines in red. The poles of Mars are always white. Next after the moon Mars is the planet best known to us, because sometimes, as our globe and Mars pursue their paths through space, they come upon the sameside of their path at the same time, which shortens the distance between them, and enables close observations to be taken.

Between Mars and the next planet Jupiter there is a very great space, so great that astronomers considered that it could not be empty, but must have in it planets too small for easy observation. They therefore resolved to search that part of the sky constantly and carefully with telescopes. On the first day of this century a small planet was seen in this space, and was named Ceres. This was the first-found asteroid. From their small size these planets are called asteroids or starlets. Others were found, slowly at first, and then more and more; in April, 1891, the 309th was discovered by Dr. Palisa.

Ceres, the first-found asteroid, is so small that one hundred and twenty-five thousand little worlds of its size could be made out of our own world. Fifty little globes like Ceres, set side by side, would form a line as long as the diameter of our world. As the mass of Ceres is so much less than that of our world, the attraction of gravity is less—anything is fifty times as heavy on our world as it would be on Ceres. So if a boy could live on Ceres, and have as much muscular force as he has here, he could play easily with a ball that here weighs a quarter of a ton. A baby on Ceres could tumble about a cannon-ball as easily as a baby here plays with a rubber ball. If a house fell over on a man, he could set it up as easily as here he could set up a fallen peach-basket. Of course it is only a play of fancy that there are men, boys, or babies on Ceres. Perhaps there is no livingthing there. If there are living creatures, all their habits and ways of life must be very different from ours. But if people from our earth could go to Ceres, why would they find such wonderful differences there in weights, and in the effect of an output of strength? We have seen that Ceres is very much smaller than our earth; its mass is greatly less, and just in proportion to the smallness of its mass is the force of the pull it exerts over objects, or the attraction of gravitation; this attraction is dependent upon the mass of the body exercising it, and upon the distance of the body attracted. When the distance between two bodies is great the attraction is weaker, and when a body is small its attracting power is less in proportion. Ceres is such a small planet that its pull over objects is small. Similarly, we might say of the planet Mars that its mass is so small, as compared with that of the earth, that a body which on the earth would weigh one hundred pounds would weigh much less on Mars; and where you could jump up five feet here, you could rise many feet there.

The best athlete who can make a jump here, can rise only a few feet into the air; then down he comes, pulled by the force of gravity, the attraction of the earth. But on Ceres attraction is so small an affair that a man could jump over a dome as high as St. Peter’s or St. Paul’s without creating any excitement. In fact, on Ceres a man would weigh only one-fiftieth part what he weighs here, and the same output of muscular activity as is used here would have a fifty-fold value. Much the same conditions exist on the other asteroids.

When first discovered it was supposed that the asteroids were fragments of some very large planet which had gone topieces. That theory has been disproved, and now the nebular hypothesis of Laplace is generally admitted to account satisfactorily for these little planets.

After passing the asteroids in our system, we come to Jupiter, the largest of the planets. Jupiter often shines more brightly than Venus. It is more than one thousand times larger than the earth, and is seen during most of the year. The year of Jupiter; that is, the time which is required for its journey around the sun, is almost twelve of our years. Day and night are equal, and of about five hours each, and there is probably no change of seasons. But Jupiter seems to be to-day a still highly heated body, as our earth was very many ages ago, and its surface is disturbed by terrific storms.

Jupiter is accompanied by four moons, and its globe is surrounded by several dark belts, which seem variable in number, but apparently belong to its surface. The belts somewhat resemble sun-spots, and indeed the whole planet is considered by astronomers to be much more like the sun than like our earth. The reason that Jupiter does not have changes of season is that the inclination of the planet to its orbit is very slight. When we consider that Jupiter turns on its axis in less than half the time that our earth requires for a rotation, and is also so very much greater in diameter than our earth, we will realize how marvellously rapid its daily motion must be.

Next to Jupiter in place, and next also in size, is Saturn, the sixth planet from the sun. Ancient astronomers supposed that with Saturn our solar system ended. Saturn haseight moons placed at different distances, and is further distinguished by a very beautiful triple ring surrounding the entire planet on its equatorial line. Within this glowing triple ring the planet turns, and the rings themselves are carried round the planet in a circular movement of still greater swiftness. These rings are not always visible, as they, like their planet, shine by the reflection of sunlight, and so are seen only when the sun and earth are both on the same side of the planet.

In 1781 Sir William Herschel, the English astronomer, who had made for himself a very fine telescope, discovered another planet. At first he supposed this was not a planet but a comet. Finally it was received into the family of the planets and named Uranus. Uranus is much smaller than Saturn, but many times larger than the earth. A year on Uranus is eighty-four of our years; that is, it takes Uranus eighty-four years to make its circuit of the sun.

With the discovery of Uranus astronomers began to hope that yet other bodies belonged to our solar system. They saw that the motion of Uranus was often disturbed, as if some other planet near it exercised over it some attracting force. Searching carefully, a planet was finally found, about one hundred times larger than the earth, and so far from the sun that to an observer placed upon it the sun must appear like a large star; even the planet’s day would be no brighter than twilight. This new-found planet was named Neptune, and is thus far the last known planet of our sun system. Its place was determined in 1846. Neptune is the only great planet that has been discovered by a set search in theheavens for it, based on reasoning that such a planet ought to exist. A Frenchman and an Englishman discovered Neptune almost simultaneously. The planet has one moon. Uranus and Neptune are so exceedingly distant from our earth, and from the sun, that the study of them has afforded very little of interest.

FOOTNOTES:[17]Professor Newcomb considers Jupiter partly self-luminous.[18]Sir B. Ball says 36 millions; others 35 millions; Newcomb makes mean distance 40 millions.[19]Sir R. S. Ball. Story of the Heavens, p. 161.[20]These phases can be observed only through a telescope.

[17]Professor Newcomb considers Jupiter partly self-luminous.

[18]Sir B. Ball says 36 millions; others 35 millions; Newcomb makes mean distance 40 millions.

[19]Sir R. S. Ball. Story of the Heavens, p. 161.

[20]These phases can be observed only through a telescope.

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