Part 3

The subject of the planets has in some respects a greater interest even than that of the stars, because we know, after all, more about them. We sometimes have a feeling, though, that we know more of the stars, although the stars are so much farther off. Why we have this feeling it is easy to explain. Knowing them to be so far removed from us, we really approach the stars with a different expectation. The few things that we have learned about them have in themselves such a magnitude that it makes them seem a greater body of knowledge than they truly are. The stars are indeed so far away, and what we know of them has to be expressed in such large terms, that the mind does not demand in that information the minute exactness that it seeks for in the case of nearer objects.

In the case of the stars, we seek mainly to know their distances, the direction of their motions, the speed with which they travel, and their probable connection with each other. The fact that in computing the distance of a single star, many trillions of miles away, the result may be a little less than exact does not keep us from learning what ones are sufficiently near for their distances to be measured at all and what ones are immeasurably remote. Whether they travel at the rate of exactly three or three hundred miles a second, we can learn that some are traveling at somewhat the same rate of speed as our sun travels, and some incredibly faster; that certain groups are going in one direction and certain groups in another; that some are approaching us and some are receding from us. And thus we can classify them and learn the significance of these facts, and, little by little, gain a definite understanding of the construction and meaning of the entire universe. Their very remoteness gives a certain compactness to the information we have about the stars, by making it necessary to generalize more than we would if they were near enough to yield more details; and we are in a way satisfied with this more general sort of knowledge of them.

But the very fact of our knowing so much about the planets extends our curiosity concerning them and makes us feel that we ought to know more. The mind is provoked into more minute speculations about them, and we demand more exactness of information and knowledge of a more specific or intimate sort than would satisfy us in regard to the stars. Atmosphere, habitability, exact size, seasons, and day and night, are the kind of things we most seek to know in reference to the planets. These are such definite things that conclusions concerning them are subject to close criticism, and differences of opinion in regard to them thus sometimes occur which tend to give one a more or less confused notion of what is really known. As a matter of fact, our information about the planets is much fuller than our knowledge of the stars, as we would naturally expect it to be. Much of what we seek to know about the stars has long been common knowledge about the planets.

OUR RELATION TO THE PLANETS

To know about the planets is to know about ourselves. The earth is one of them. Whatever their origin, the earth’s is the same. It and they are formed from the same nebula, controlled by the same central body, subject to the same laws, and destined for the same fate in the end. In this, the stars and the planets are not alike. They all shine upon us with the same sweet friendliness, and commonly we make no difference between them in our feeling for them. But the stars are bright and beautiful acquaintances living far away in their own domain. The planets are members of our own family, bone of our bone and flesh of our flesh, living comparatively near to us, within the domain of our common source of life, the sun.

One evening last autumn I was coming up Broadway, New York, with a friend, when we encountered at Union Square a man with a six-inch telescope directed toward the eastern sky. He was soliciting those who passed to stop and look at Mars and Saturn. Both of these planets were then very bright. They were also fairly near together, and so low in the east that one could scarcely help seeing them. But the people passed back and forth with hardly so much as a glance at the man and his telescope, and for the most part never even raised their eyes to the sky with a passing curiosity to see what it might be that he wanted to show them. My friend and I stopped and took each a view first of Mars and then of Saturn. While we were looking at the planets, a few of the passers-by began to loiter about, half smiling at us for so playing in public, slightly curious to see how we were faring at it, but for the most part apparently indifferent to what we were seeing. We had a fine view of Saturn lightly resting in his nest of rings, and an equally good view of the comical “eye” of Mars.

After we had finished, one or two others, evidently prompted by our example, followed us at the telescope. One or two inquired of us what the stars were that had so interested us, and one, pointing to Mars, wanted to know if it was Venus. As the crowd grew larger a few more ventured to take a look, much as they might venture to take their chance at hitting the bull’s-eye in some shooting-gallery. With the telescope pointed at Saturn, the man droningly chanted: “This planet is 887,000,000 miles from the sun. The ring you see is 170,000 miles in diameter,” and so on. These, to be sure, were the facts--and most marvelous facts, too--but without much meaning to one who knows nothing much about the planets; and the manner of their recital certainly did not make them alluring. I could not myself help feeling that the people there were missing a valuable opportunity, and that it would be only fair to them for some one fairly to cry out: “Come here and look at this planet. It is different from anything else you have ever seen or ever will see. It was at one time a part of the same nebulous mass that we were a part of. It is in the same system of worlds with us. It was formed in the same way that this world was formed. It is in itself the most wonderful thing you ever saw, and it is bound, as we are, to the sun by the ever-drawing tie of gravitation. The very position of our own world in space is more or less influenced by it. If anything should happen to it, it might be a serious matter to us.”

For it is true that we are thus closely bound to the planets. The family tie among us is of far more force and significance than in any ordinary case of common origin. Human family ties wear, as we know, often into the merest threads, or even become no ties at all. But that between the earth and the planets remains apparently as close and strong as ever it was. The law of gravity, under which the earth draws toward its center every atom of matter surrounding it, and thus holds together all the atoms composing it, is not solely terrestrial in its application. It is probably universal. It certainly applies to every part of our little family of worlds. Every particle in the solar system attracts toward it every other particle in that system with a force determined by its mass and its distance. The sun, by reason of its immense size, compels the earth and all the other planets forever to circle around it. But the planets themselves have just as much power of attraction as the sun, atom for atom.

Thus, while the sun controls the motions of all of them, each pulls at the other, and, according to its power, determines how much the path of each shall vary from the course around the sun it otherwise would make. In the case of the smaller planets, this gravitational influence is, of course, very slight, and so subtle that we here on earth are not even conscious of it. But it is, nevertheless, real and continuous. It is greatest between the two largest planets, Jupiter and Saturn; but it was enough in the case of Uranus and Neptune to lead, by its mere manifestation on the earth, to the discovery of Neptune, the farthest planet.

Being thus of the same origin with the planets, having the same life history, being bound to them in space by a tie that is perhaps eternal, how can we fail to have the most intimate interest in their nature and all that concerns them?

But in addition to their close relationship to us there is, to make them of peculiar interest, the fact that, after the sun and the moon, they are for our eyes the most splendid objects in all the brilliant panorama of the sky. Such of them as we can see at all with the naked eye are most of the time much brighter than any first-magnitude star. As they wander from constellation to constellation the soft light of their placid faces gives a beauty and variety to the spectacle that endears them to us, and at the same time enhances by contrast their own charm and that of the glittering, unchanging stars.

There is nothing that gives one such a sense of sweet familiarity with the heavens as a really recognizing acquaintance with the planets. They are not, like the stars, associated with particular seasons. They come sometimes with the gay company of stars that dance their way across the cold winter skies, and sometimes with those that shine during the soft summer nights. Often in the spring and autumn we see some one of them before the sun is fairly down, and, before the light of an ordinary star can yet be seen, hanging in lone brilliancy as the evening star; and often an early riser has the reward of seeing one as a morning star glowing almost in the rays of the rising sun. Thus they are, one and another, with us at all times and seasons, and it accords with the fact of the relation being a family one that we have in their coming and going a sense of frequency and informality which we cannot have in the more regular and seasonal coming and going of the stars.

III

WHAT THE PLANETS ARE, AND WHAT THEY APPEAR TO BE

The planets are dark, opaque bodies which revolve at varying distances and at varying rates of speed in orbits more or less circular around the sun as a center. They have no light of their own, as the stars have, but shine wholly by reflected light received from the sun, which itself is a star. The amount of light they show to us depends upon their size, their distance, and their power of reflecting the light they receive.

In comparison with the stars, the planets are very near to us. Our sun, which is in constitution a star, but very widely separated from any other star in the universe, holds all his family of planets by the tether of gravitation, and so keeps them circling about him in a very small space, as astronomical space is measured. To all of the planets except Mercury, we ourselves are nearer than the sun is. To be sure, this distance between us and the planets, as measured by any terrestrial measure, is not exactly small. It is only by comparison that we can be said to have anything like a cozy relation to them. For merely earthly affairs we use terrestrial measures. In solar affairs we measure by an astronomical unit, which is the sun’s distance from the earth, ninety-three millions of miles. When we say a planet’s distance from the sun is thirty astronomical units, we mean it is thirty times farther than the earth is from the sun.

For matters outside of the solar system, the unit of measure is the number of miles that light travels in a year. The speed of light is a little more than 186,000 miles in a second. This is equal to about six trillions of miles in a year, or about sixty-three thousand times the distance of the sun from the earth, our family measuring-stick. From the nearest star it takes light more than four years to come to us. From the nearest planet light comes in less than three minutes, and from the farthest one it makes the journey in a little more than four hours.

As compared with other heavenly bodies, therefore, the sun and the planets are very near together, occupying a very small space in the immensity of the universe, immeasurably isolated from all the other systems and, so far as we know, immeasurably smaller as a system than most of them.

The whole body of the planets is divided according to size into two classes, the major and the minor planets. When we refer generally to the planets, the major planets only are meant. The minor planets are usually called the asteroids, or planetoids. There are many hundreds of them, and only one--and that barely--can be seen with the naked eye. The other planets are eight in number, including the earth, which is, after all, nothing but one of the smaller of the major planets. They are, in the order of their distances from the sun: Mercury, the nearest, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Of these only five--Mercury, Venus, Mars, Jupiter, and Saturn--can be seen from the earth without optical aid. Occasionally, when Uranus is very favorably situated, a person with an exceptionally good eye, who knows exactly where to look for the planet, can see it. Neptune is about equal to an eighth-magnitude star in brightness, and can never be seen without the aid of a telescope. Mercury, while quite bright enough to be seen, is not often situated favorably for observation. It is very near the sun, and is generally obscured either by the light of the sun when the sun and the planet are above the horizon, or by the haziness of the atmosphere when the sun is below the horizon and the planet a little above it. In regions of considerable altitude with a clear, rare atmosphere, Mercury is more often seen; but never for very long at a time.

The only planets, therefore, that are a part of our evening spectacle in the skies are Venus, Mars, Jupiter, and Saturn. These four happen to be not only the ones we oftenest see, but also the most interesting of all the planets from various points of view. Venus and Mars are the nearest to the earth, and most resemble it, and hence are the most inviting for speculations which have a human interest, such as habitability, the presence of life, and kindred ideas. Jupiter and Saturn are interesting above all the others in their splendor and size, and in their importance as the centers of systems of their own.

As seen by us, the planets are similar to the stars, but with very distinct differences in appearance, which, when once familiar, mark them so unmistakably as planets, and not fixed stars, that we need never get the two confused. The first and easiest distinguishing mark to notice is that they do not twinkle, as the stars do, but shine with a steady light similar to that of the moon. This is an invariable difference between stars and planets, and one needs only to stop and truly look at them in order to detect it. And once it has become familiar, it discloses itself at a glance.

This difference between stars and planets is due almost solely to difference of distance, though the twinkling is caused by our own atmosphere. The stars are too far away to send us anything but a mere point of light, and the unequal density of the waves of air sweeping over this point of light keeps it dancing before our eyes, causing the phenomenon that we call twinkling. But the planets, being nearer to us, show a disc, from every point of which comes a line of light, making the total light of some volume; and these inequalities of the air are too small to interfere with it to any extent. Sometimes, when the atmosphere is particularly unsteady, it happens that the light of a planet is somewhat affected by it when the planet is just rising or setting and is, consequently, near the horizon, and that it then seems to twinkle a little. But this departure from the rule is always slight and of short duration, in the case of the four planets most seen. Mercury, never being seen anywhere except near the horizon, often seems to twinkle; but then he is seldom seen at all, and, when visible, is in other ways so well marked that one cannot fail to recognize him.

So the steady light may justly be said to be invariable, because the unusual conditions are easily detected. When the atmosphere is such as to cause even the planets to blink a little, it has an effect also on the stars. At such a time they will appear to be fairly dancing. This effect is apt to occur on the clear nights of winter, the atmosphere being more unsteady then. Such nights, because of the extreme liveliness and brilliancy that they lend to the stars, are attractive times for amateur observations. For the astronomer, however, they are not so favorable. For the seeing of small details such as he seeks, the steadiest atmosphere is necessary.

Though the planets are near enough to show a disc, they are not sufficiently near to show to the naked eye as sharp an outline as the moon’s. Usually the edge is more or less rayed like that of a fixed star, which adds somewhat to the difficulty of distinguishing them from the stars until their aspect has become familiar to us. The fact that we are looking at a disc is plainly shown when an occultation by the moon occurs. When the moon occults a fixed star, it passes between us and the star. At such times the star disappears behind the edge of the moon instantly, as a mere point naturally would. When a planet is occulted by the moon, it disappears gradually as the moon covers more and more of its disc, thus showing unmistakably the nature of it.

After steadiness of shining, the next most obvious mark of difference between a planet and a star, from our point of view, is the movement of the planets. A star remains always in one place with relation to the other stars, while the planets move about from constellation to constellation, seeming to travel sometimes toward the east and sometimes toward the west.

This difference also is due solely to a difference of distance. The stars as well as the planets are constantly in motion. Most of them, in truth, move at a rate which would make the rate of motion of a planet a mere snail’s pace in comparison. Arcturus, for instance, is supposed to be moving at the rate of two or three hundred miles a second, and there are other fixed stars with an equally rapid motion. The swiftest moving of the planets does not achieve much more than twenty-nine miles a second, while the slowest swings along at a rate of but little more than three miles in the same length of time.

These are the real rates of speed of the stars and planets; but they are not at all what they seem to us. The difference in distance is so great that for centuries and centuries the flying stars have seemed to men to remain in the same place in the skies, and so we call them fixed. The planets, so slow-journeying as they are in comparison, seem to us to be moving among the constellations at rates varying from more than a degree a day in the swiftest to between two and three degrees a year in the slowest.

Hence, if through lack of practice in observation a person is not at once able to distinguish the difference between the stars and the planets in the character of their light--that is, whether they twinkle or shine steadily--he can, by taking a little longer time, at most only a few days, determine whether the object he sees is a star or a planet by noticing whether it has any motion among the other stars. Venus and Mars will show some movement in one evening. Jupiter and Saturn may require a little more time to disclose their motion.