Astronomy for Everybody/Part 1/Chapter 1

Let us enter upon our subject by taking a general view of this universe in which we live, fancying ourselves looking at it from a point without its limits. Far away, indeed, is the point we must choose. To give a conception of the distance, let us measure it by the motion of light. This agent, darting through 186,000 miles in every second, would make the circuit of the earth several times between two ticks of a watch. The standpoint which we choose will probably be well situated if we take it at a distance through which light would travel in 100,000 years. So far as we know, we should at this point find ourselves in utter darkness, a black and starless sky surrounding us on all sides. But, in one direction, we should see a large patch of feeble light spreading over a considerable part of the heavens like a faint cloud or the first glimmer of a dawn. Possibly there might be other such patches in different directions, but of these we know nothing. The one which we have mentioned, and which we call the universe, is that which we are to inspect. We therefore fly toward it—how fast we need not say. To reach it in a month we should have to go a million times as fast as light. As we approach, it continually spreads out over more of the black sky, ​which it at length half covers, the region behind us being still entirely black.

Before reaching this stage we begin to see points of light glimmering here and there in the mass. Continuing our course, these points become more numerous, and seem to move past us and disappear behind us in the distance, while new ones continually come into view in front, as the passengers on a railway train see landscape and houses flit by them. These are stars, which, when we get well in among them, stud the whole heavens as we see them do at night. We might pass through the whole cloud at the enormous speed we have fancied, without seeing anything but stars and, perhaps, a few great nebulous masses of foggy light scattered here and there among them.

But instead of doing this, let us select one particular star and slacken our speed to make a closer inspection of it. This one is rather a small star; but as we approach it, it seems to our eyes to grow brighter. In time it shines like Venus. Then it casts a shadow; then we can read by its light; then it begins to dazzle our eyes. It looks like a little sun. It is the Sun!

Let us get into a position which, compared with the distances we have been travelling, is right alongside of the sun, though, expressed in our ordinary measure, it may be a thousand million miles away. Now, looking down and around us, we see eight star-like points scattered around the sun at different distances. If we watch them long enough we shall see them all in motion around the sun, completing their circuit in times ranging from three ​months to more than 160 years. They move at very different distances; the most distant is seventy times as far as the nearest.

These star-like bodies are the planets. By careful examination we see that they differ from the stars in being opaque bodies, shining only by light borrowed from the sun.

Let us pay one of them a visit. We select the third in order from the sun. Approaching it in a direction which we may call from above, that is to say from a direction at right angles to the line drawn from it to the sun, we see it grow larger and brighter as we get nearer. When we get very near, we see it looking like a half-moon — one hemisphere being in darkness and the other illuminated by the sun's rays. As we approach yet nearer, the illuminated part, always growing larger to our sight, assumes a mottled appearance. Still expanding, this appearance gradually resolves itself into oceans and continents, obscured over perhaps half their surface by clouds. The surface upon which we are looking continually spreads out before us, filling more and more of the sky, until we see it to be a world. We land upon it, and here we are upon the earth.

Thus, a point which was absolutely invisible while we were flying through the celestial spaces, which became a star when we got near the sun, and an opaque globe when yet nearer, now becomes the world on which we live.

This imaginary flight makes known to us a capital fact of astronomy: The great mass of stars which stud the heavens at night are suns. To express the idea in ​another way, the sun is merely one of the stars. Compared with its fellows it is rather a small one, for we know of stars that emit thousands or even tens of thousands of times the light and heat of the sun. Measuring things simply by their intrinsic importance, there is nothing special to distinguish our sun from the hundreds of millions of its companions. Its importance to us and its comparative greatness in our eyes arise simply from the accident of our relation to it.

The great universe of stars which we have described looks to us from the earth just as it looked to us during our imaginary flight through it. The stars which stud our sky are the same stars which we saw on our flight. The great difference between our view of the heavens and the view from a point in the starry distances is the prominent position occupied by the sun and planets. The former is so bright that during the daytime it completely obliterates the stars. If we could cut off the sun's rays from any very wide region, we should see the stars around the sun in the daytime as well as by night. These bodies surround us in all directions as if the earth were placed in the centre of the universe, as was supposed by the ancients.

We may connect what we have just learned about the the universe at large with what we see in the heavens. What we call the heavenly bodies are of two classes. One of these comprises the millions of stars the arrangement and appearance of which we have just described. ​The other comprises a single star, which is for us the most important of all, and the bodies connected with it. This collection of bodies, with the sun in its centre, forms a little colony all by itself, which we call the solar system. The feature of this system which I wish first to impress on the reader's mind is its very small dimensions when compared with the distances between the stars. All around it are spaces which, so far as we yet know, are quite void through enormous distances. If we could fly across the whole breadth of the system, we should not be able to see that we were any nearer the stars in front of us, nor would the constellations look in any way different from what they do from our earth. An astronomer armed with the finest instruments would be able to detect a change only by the most exact observations, and then only in the case of the nearer stars.

A conception of the respective magnitudes and distances of the heavenly bodies, which will help the reader in conceiving of the universe as it is, may be gained by supposing us to look at a little model of it. Let us imagine that, in this model of the universe, the earth on which we dwell is represented by a grain of mustard seed. The moon will then be a particle about one fourth the diameter of the grain, placed at a distance of an inch from the earth. The sun will be represented by a large apple, placed at a distance of forty feet. Other planets, ranging in size from an invisible particle to a pea, must be imagined at distances from the sun varying from ten feet to a quarter of a mile. We must then imagine all these little objects to be slowly moving around the ​sun at their respective distances, in times varying from three months to 160 years. As the mustard seed performs its revolution in the course of a year we must imagine the moon to accompany it, making a revolution around it every month.

On this scale a plan of the whole solar system can be laid down in a field half a mile square. Outside of this field we should find a tract broader than the whole continent of America without a visible object in it unless perhaps comets scattered around its border. Far beyond the limits of the American continent we should find the nearest star, which, like our sun, might be represented by a large apple. At still greater distances, in every direction, would be other stars, but, in the general average, they would be separated from each other as widely as the nearest star is from the sun. A region of the little model as large as the whole earth might contain only two or three stars.

We see from this how, in a flight through the universe, like the one we have imagined, we might overlook such an insignificant little body as our earth, even if we made a careful search for it. We should be like a person flying through the Mississippi Valley, looking for a grain of mustard seed which he knew was hidden somewhere on the American continent. Even the bright shining apple representing the sun might be overlooked unless we happened to pass quite near it.