Читать книгу The Story of the Heavens - Robert S. Ball - Страница 16

Fig. 31.—The Orbits of the Four Interior Planets.

Pause for a moment to think what a velocity of eighteen miles a second really implies. Can we realise a speed so tremendous? Let us compare it with our ordinary types of rapid movement. Look at that express train how it crashes under the bridge, how, in another moment, it is lost to view! Can any velocity be greater than that? Let us try it by figures. The train moves a mile a minute; multiply that velocity by eighteen and it becomes eighteen miles a minute, but we must further multiply it by sixty to make it eighteen miles a second. The velocity of the express train is not even the thousandth part of the velocity of the earth. Let us take another illustration. We stand at the rifle ranges to see a rifle fired at a target 1,000 feet away, and we find that a second or two is sufficient to carry the bullet over that distance. The earth moves nearly one hundred times as fast as the rifle bullet.

Fig. 32.—The Earth's Movement.

Viewed in another way, the stupendous speed of the earth does not seem immoderate. The earth is a mighty globe, so great indeed that even when moving at this speed it takes almost eight minutes to pass over its own diameter. If a steamer required eight minutes to traverse a distance equal to its own length, its pace would be less than a mile an hour. To illustrate this method of considering the subject, we show here a view of the progress made by the earth (Fig. 32). The distance between the centres of these circles is about six times the diameter; and, accordingly, if they be taken to represent the earth, the time required to pass from one position to the other is about forty-eight minutes.

Outside the path of the earth, we come to the orbit of the fourth planet, Mars, which requires 687 days, or nearly two years, to complete its circuit round the sun. With our arrival at Mars we have gained the limit to the inner portion of the solar system.

The four planets we have mentioned form a group in themselves, distinguished by their comparative nearness to the sun. They are all bodies of moderate dimensions. Venus and the Earth are globes of about the same size. Mercury and Mars are both smaller objects which lie, so far as bulk is concerned, between the earth and the moon. The four planets which come nearest to the sun are vastly surpassed in bulk and weight by the giant bodies of our system—the stately group of Jupiter and Saturn, Uranus and Neptune.

Fig. 33.—The Orbits of the Four Giant Planets.

These giant planets enjoy the sun's guidance equally with their weaker brethren. In the diagram on this page (Fig. 33) parts of the orbits of the great outer planets are represented. The sun, as before, presides at the centre, but the inner planets would on this scale be so close to the sun that it is only possible to represent the orbit of Mars. After the orbit of Mars comes a considerable interval, not, however, devoid of planetary activity, and then follow the orbits of Jupiter and Saturn; further still, we have Uranus, a great globe on the verge of unassisted vision; and, lastly, the whole system is bounded by the grand orbit of Neptune—a planet of which we shall have a marvellous story to narrate.

The various circles in Fig. 34 show the apparent sizes of the sun as seen from the different planets. Taking the circle corresponding to the earth to represent the amount of heat and light which the earth derives from the sun then the other circles indicate the heat and the light enjoyed by the corresponding planets. The next outer planet to the earth is Mars, whose share of solar blessings is not so very inferior to that of the earth; but we fail to see how bodies so remote as Jupiter or Saturn can enjoy climates at all comparable with those of the planets which are more favourably situated.

Fig. 34.—Comparative Apparent Size of the Sun as seen from the Various Planets.

Fig. 35 shows a picture of the whole family of planets surrounding the sun—represented on the same scale, so as to exhibit their comparative sizes. Measured by bulk, Jupiter is more than 1,200 times as great as the earth, so that it would take at least 1,200 earths rolled into one to form a globe equal to the globe of Jupiter. Measured by weight, the disparity between the earth and Jupiter, though still enormous, is not quite so great; but this is a matter to be discussed more fully in a later chapter.

Fig. 35.—Comparative Sizes of the Planets.

Even in this preliminary survey of the solar system we must not omit to refer to the planets which attract our attention, not by their bulk, but by their multitude. In the ample zone bounded on the inside by the orbit of Mars and on the outside by the orbit of Jupiter it was thought at one time that no planet revolved. Modern research has shown that this region is tenanted, not by one planet, but by hundreds. The discovery of these planets is a charge which has been undertaken by various diligent astronomers of the present day, while the discussion of their movements affords labour to other men of science. We shall find something to learn from the study of these tiny bodies, and especially from another small planet called Eros, which lies nearer to the earth than the limit above indicated. A chapter will be devoted to these objects.

But we do not propose to enter deeply into the mere statistics of the planetary system at present. Were such our intention, the tables at the end of the volume would show that ample materials are available. Astronomers have taken an inventory of each of the planets. They have measured their distances, the shapes of their orbits and the positions of those orbits, their times of revolution, and, in the case of all the larger planets, their sizes and their weights. Such results are of interest for many purposes. It is, however, the more general features of the science which at present claim our attention.

Let us, in conclusion, note one or two important truths with reference to our planetary system. We have seen that all the planets revolve in nearly circular paths around the sun. We have now to add another fact possessing much significance. Each of the planets pursues its path in the same direction. It thus happens that one such body may overtake another, but it can never happen that two planets pass by each other as do the trains on adjacent lines of railway. We shall subsequently find that the whole welfare of our system, nay, its continuous existence, is dependent upon this remarkable uniformity taken in conjunction with other features of the system.

Such is our solar system; a mighty organised group of planets circulating under the control of the sun, and completely isolated from all external interference. No star, no constellation, has any appreciable influence on our solar system. We constitute a little island group, separated from the nearest stars by the most amazing distances. It may be that as the other stars are suns, so they too may have systems of planets circulating around them; but of this we know nothing. Of the stars we can only say that they appear to us as points of light, and any planets they may possess must for ever remain invisible to us, even if they were many times larger than Jupiter.

We need not repine at this limitation to our possible knowledge, for just as we find in the solar system all that is necessary for our daily bodily wants, so shall we find ample occupation for whatever faculties we may possess in endeavouring to understand those mysteries of the heavens which lie within our reach.