Part 13

It is likely that about all have been discovered that can be seen even with a telescope, for a fairly systematic and thorough search has been made of the heavens for this purpose during the last half-century. This work has resulted in a continually decreasing number of discoveries, until this method of search has finally been practically abandoned. But it not infrequently happens that in photographing the stars a little trail of light is discovered on the plate, showing that some heavenly body with sensible motion has been caught on it. And this usually proves to be a new planetoid. No matter how long a photographic plate is exposed, the fixed stars imprint themselves on it only as points of light. When the impression is a little streak of light instead of a dot, the object is shown to be in motion, and is either a planetoid, a satellite, or a comet. The fixed stars would make a trail also if the photographic apparatus were not regulated by clockwork, so as to follow the star in its apparent daily motion across the skies. The planets and other bodies in the solar system are sufficiently near to have a sensible motion in addition to the motion caused by the rotation of the earth, which is the only motion we have to take into account in dealing with the aspects of the stars.

The first planetoid discovered was called Ceres, the next one Pallas, the third Juno, and the fourth Vesta. This pretty custom of naming them after the gods and goddesses of mythology was continued, with some variations, until perhaps three hundred had been so christened. But the number of them became too prodigious; and when so many began to swarm into view, waiting to be named, the utilitarian method of designating them simply by numbers in the order of their discovery was adopted. The only distinguishing feature of so numbering them is that each number is placed in a little circle. Thus Ceres is ①, Pallas ②, and so on. Those of them that have any special claim to distinction, however, are still referred to by their own names, if they have any, in spite of this most orderly attempt to make them fit for easy reference in a list.

There are so many of the planetoids, and they are so minute, that even after they have been discovered they are frequently lost again. Hence it is sometimes uncertain when they register themselves on the photographic plates whether they are really new to us or have been known before. In such cases they are named temporarily after the letters of the alphabet, and, when the alphabet is exhausted, a second letter is added. Thus A to Z, then AB to AZ, BC to BZ, and so on in a sort of “round.” Sometimes these combinations of letters become the fixed designation of a planetoid, as a nickname sometimes clings to a person. And thus it happens that we sometimes read of one in particular of these little bodies that is conspicuous for the great eccentricity of its orbit, called “WD.” The letters are not its initials, but its nickname. It really has no name other than its number in the list; but it became famous while it was temporarily designated as “WD,” and thus it continues to be called.

The aid of a telescope is necessary in order to see the planetoids, though it is said that Vesta, under very favorable conditions, sometimes comes within the limit of visibility. She is the brightest of them all, though not the largest, and her brilliancy is the subject of much interesting speculation among astronomers, who have not yet been able to account for it. She seems from her excessive brightness to be covered with clouds; and yet it is manifestly impossible that so small a body could have held an atmosphere throughout these long ages, though clouds presuppose an atmosphere. No doubt, in time this mystery of Vesta’s brilliancy will be made plain. Bright as she is in proportion to her size, and even if she sometimes can be seen, one cannot reasonably expect anything very brilliant to our view from a body not much more than a hundred miles in diameter, shining by reflected light, nearly two hundred million miles away.

Ceres, as far as we yet know, is the largest of the planetoids, and may be something more than four hundred miles in diameter. Juno is somewhere near the same size. Pallas is about two hundred miles in diameter, and Vesta about one hundred and eighteen. No doubt, these four were the first to be discovered, because they are the largest and so the easiest to be seen. At any rate, no others yet seen exceed them in size, and some of the more lately discovered are not more than fifteen or twenty miles in diameter. Many of those discovered by photography are doubtless even smaller than these, and are, perhaps, mere meteors in size. The combined mass of all those discovered up to this time is far smaller than that of any of the large planets, or even than that of our moon. Their mass cannot, of course, really be measured, because they are too small to have any perceptible gravitative effect on other bodies, and mass can only be determined by the influence of one body on another. But we do know that their aggregate mass, if it exceeded a certain limit, would show some disturbing effect on Mars; and, since it does not do this, we know that all of them taken together would make an extremely insignificant body.

While the planetoids all revolve around the sun in the same manner and in the same direction as the planets do, yet they are very erratic in their courses, and do not all keep within the narrow limits of the zodiac through which--happily for our convenient observation--the larger bodies travel. The orbits of many of them are extremely elliptical, while some are almost circles; and their inclination to the ecliptic varies from almost nothing to nearly fifty degrees. If one could catch from one side a view of them all together, they would have much the appearance in space of a flock of swallows, the individuals darting this way and that, passing above and below one another in such intricate sweeps and sinuosities that it would be impossible to keep track of them separately. And yet time has brought these apparently tangled orbits into such nice adjustment that the little bodies can continue to cross and recross each other’s paths with no danger of interference from each other. Such collisions as there may have been occurred in the very beginning of their careers. Such of them as came into collision then traveled on together as one body until accommodation was made for all.

One of the most wide-wandering of these tiny bodies has been named Eros, after the little god of love, more commonly known as Cupid. It has a particular interest for us, because of all the heavenly bodies it at times comes nearer to us than any except the moon and an occasional comet. At its nearest it is within fourteen million miles of the earth, which is more than ten million miles nearer than the closest approach of Venus, the nearest of the large planets.

This little body was thus near us in 1894; but we did not then know this, for Eros was not discovered until 1898. After its discovery, however, it was traced back on many photographic plates, and the fact that it had been in our neighborhood was learned. For untold ages it has been making these visits to us every thirty-seven years, and we have known nothing of them. Its next near approach will be in 1931, and it will continue to come thereafter every thirty-seven years. Now that we know about them, these visits are not only pleasant to contemplate, but it is expected that when they occur the planetoid will be of great scientific value to us in helping to determine more surely and accurately the exact distance of the sun.

The planetoids, though so minute and of no value as a spectacle, have been, and still are, very useful little bodies to us in a scientific way. In addition to furnishing an easy means of measuring the distance of the sun, they promise to throw some light on various questions of physics in which the planets, too, are involved. The brilliancy of Vesta, for instance, which has been mentioned, and the unaccountable variability in the brightness of some others of them have yet to be adjusted to known physical laws. Even the extreme eccentricity of some of their orbits, and the large tilt of some of them to the ecliptic, may be suggestive in finally solving certain planetary problems, for these impish little bodies are far from conforming to the regular ways of the planets, and there is, of course, some mechanical reason for their apparent waywardness.

XVIII

CONCLUSION

The great variety of beauty that the planets present to us is sufficient to keep us always interested in them, when once we have acquired an acquaintance with them. Rarely is there an evening when some one of them does not enhance the charm of the splendid spectacle of the sky in which all the heavenly bodies save the sun have a part. Their greater brilliancy often brings them into view before the stars have begun to glow in the evening, and prolongs our sight of them after the rays of the sun have blotted out the light of the stars in the morning. Thus they are always single in their loveliness, and always hold a distinguished place in the midst of the brilliant company of the stars.

Having considered these brilliant bodies individually and in detail, as we have, we ought by this time to be able to identify any one of them that shows itself in the evening sky, and to have a pretty fair notion of the general character and peculiarities of each. But even if one does not much care for detailed information concerning them, or, before seeking that, prefers first to become familiar with their appearance, a quick and sure recognition of them may be had by noting their positions and their very striking individual aspects as set forth in the preceding chapters.

On seeing a bright object in the sky that does not seem to be a familiar star, simply stop and look at it. Does it twinkle? If it does not, it is a planet. If it is more than forty-five degrees from the sun, or if it is seen at a time when the sun has been down more than three hours, then it is neither Mercury nor Venus, and must be either Mars, Jupiter, or Saturn. Is it very bright and pinkish in tone? Then it is Jupiter. Is it very bright and quite red? It is Mars, not far from opposition. Is it not very bright, but small and rosy? Then it is Mars going toward conjunction. Is it yellow in tone and, while large and conspicuous, still not so very brilliant? It is Saturn.

If the planet we seek to name is nearer to the sun than forty-five degrees, but is still well above the horizon, it may be either of these three--Mars, Jupiter, Saturn--or it is Venus. If it is very bright and silvery, it is certainly Venus. If it is very low in the sky and very near the sun, it may be any one of the five visible planets. In such a position Mars will always be very small, and the others always larger than a first-magnitude star; and they may all twinkle a little--Mercury almost as much as a star. Their size will show them all (except Mars) as planets, but it will be somewhat more difficult to tell which is which than it is when they are higher up in the sky. The best thing to do in such circumstances is to look up their positions either in this book or in an almanac. The almanac will serve as a footman to announce them. The book, it is hoped, has so recorded their peculiarities and habits that either their appearance or their place will be sufficient to make them known.

In any event, the problem of identification in this position will not keep one long, for in a situation presenting these greater difficulties the planet will be visible for less than an hour after sundown. Besides, it is not likely at such times to attract one’s involuntary attention, but when under observation in such a situation is usually sought out by those already somewhat informed as to the planet’s habits and appearance, which will betray its identity. It is information of this sort that I have endeavored to give in these pages, and it is hoped that the reading of them will be the beginning of a long and intimate acquaintance with these charming and always interesting individuals.

Individuals the planets inevitably become to any one who learns to know them during the long, quiet nights in the country, or wherever an opportunity is afforded really to contemplate their peculiar traits and features. Like individuals of whatever kind, they impress different persons in different ways. As I have watched them from year to year I have come to have a very distinct impression of Jupiter as slow and majestic, and yet not lacking in joviality; Saturn as friendly, but reserved; Mars as sturdily brisk and busy; Venus as always gracious and smiling; and Mercury as irresponsible and roguish. Others might have an entirely different feeling in regard to them; but an intimate acquaintance with them, which is not wholly scientific, cannot fail to stamp them as in some sort individuals.

And when we consider that these interesting individuals are closely related members of our cosmic family, their ever-changing beauty of aspect, the history of their development and their affairs generally, gain a significance to us that no other heavenly bodies can have. The two groups of planets--the inner and the outer--are like two sets of children in a family: born of the same parent, but under very different circumstances, and in very different surroundings. Mars, the earth, Venus, and Mercury are all, as compared with the outer planets, small and dense, with more or less thin atmospheres and an abundance of heat and light. They all lie comparatively near to the sun, and are composed of the denser material lying near the center of the great nebula, which was the original form of the entire solar system. Probably denser to begin with than the others, they have, on account of their diminutive size, developed more rapidly and are further advanced toward the final state of solidity which we shall all attain in the end. Mercury, the smallest, is already old and seamed and hardened. Mars, the next in size, is well advanced, but still has an atmosphere and some other signs of vitality. Venus, though we know so little about her, has probably a long period of development yet before her; while this warm, nourishing earth, which seems to us the best one of them all, will probably for a still longer time than Venus hold its atmosphere and remain green and flourishing.

On the other side of the vast space which divides the two groups of the sun’s family dwell Jupiter, Saturn, Uranus, and Neptune. They are all tremendous in volume, enveloped in immense atmospheres, far, far from our common source of heat and light, of comparatively slight density, and probably formed from the lighter material composing the outer edges of the parent nebula, and, because of their immense size, still in a very early stage of development. The two groups could scarcely seem more widely different if they belonged to different systems; but the members of each are all closely akin, and each one in its own way, determined by its size and environment, is developing toward the same end.

If there is life on any of these outer planets, it must be of a sort of which we have no conception. Jupiter and Saturn are probably red-hot, and could sustain nothing more cold-blooded than a race of salamanders, though why a race of intelligent salamanders should or should not exist there, is a question that one might make bold to answer according to one’s fancy. Uranus and Neptune are smaller, and perhaps less hot than Jupiter and Saturn; but we really know very little about the state of their domestic affairs, and the little we do know in no way indicates a place of abode for any sort of intelligence conceivable to us. We can, however, conceive of a time in the far-distant ages when these four hot and vaporous planets may have become sufficiently condensed to have a solid crust, and yet have sufficient internal heat to moderate the frigid temperature that would be normal at their distance from the sun, and they might then support life even somewhat resembling and perhaps even more gloriously beautiful than that with which we are familiar.

Of the existence of life somewhat similar to ours on the smaller, near-by planets we may have something nearer a reasonable conception, though we are nowhere near the possession of any real knowledge concerning it. Mercury, we have every reason to think, cannot support life, mainly because of his lack of atmosphere; but also because of his long rotation, which affords no alternations of day and night, but leaves him with one side always burning-hot and the other inconceivably cold. Venus might very well have a climate not utterly unlike ours, and hence be habitable for beings somewhat resembling us, if she has, as she has long been thought to have, a heavier atmosphere than the earth has, and if she has alternations of day and night. But we have seen that, owing to the obscurity of the surface of Venus, our knowledge in regard to these conditions is far from certain, and we have little reason to have even speculative ideas concerning life there. With Mars it is a more open question. We can see that planet, and see it fairly well. It has an atmosphere and changes of seasons, and while it may not afford a climate that would be exactly attractive to us as a place of transmigration, it is not particularly unreasonable to let our fancy play over the rather pleasant speculation concerning the presence there of beings at least understandable by us, even if not wholly congenial.

Whatever each planet affords in the way of life and human interests, all of them must ever be to us the most interesting things in all nature, outside of our own earth, in the two regards already pointed out: first, as the most beautiful objects of vision among all the starry hosts, and, second, as our nearest kindred in this universe of suns and systems of worlds. Together the earth and they circle ceaselessly around and around the sun, following in nicely adjusted orbits that great luminary as it sweeps majestically on through space toward the beautiful Vega, itself a sun, and, so far as we now know, in this close companionship we shall continue until every planet and the sun itself has become cold and dark and lifeless. And then, perhaps, or even before the light of our system is finally extinguished, we may meet another wandering sun, and in the marriage of the two great bodies another system of worlds may be evolved of which we and the planets shall form a part.

SYMBOLS USED IN ALMANACS

☿ = Mercury. ⚫ = New Moon.

♀ = Venus. ☽ = First Quarter.

⊕ = Earth. ⚪ = Full Moon.

♂ = Mars. ☾ = Last Quarter.

♃ = Jupiter. ☉ = Sun.

♄ = Saturn. ☌ = Conjunction with the sun; or, in the case ♅ or ⛢ = Uranus. of two planets or a planet and the moon, ♆ = Neptune. near together.

☍ = Opposition.

□ = Quadrature.

Examples:

☌ ♂ ♀ = Mars and Venus near together.

☍ ♃ ☉ = Jupiter in opposition.

☌ ♃ ☉ = Jupiter in conjunction.

☌ ☿ ☉ Inf. = Mercury in inferior conjunction.

☌ ☿ ☉ Sup. = Mercury in superior conjunction.

☌ ♀ ☽ = Venus and Moon near together.

INDEX

Adams, 236–238.

Alcor, star in Great Dipper, 105, 236.

Aldebaran, first-magnitude star, 79–80, 153, 188, 210.

Antares, star in Scorpio, 86, 153, 160, 187, 189, 209, 212.

Aquarius, constellation of the zodiac, 76, 88–89, 91–92, 187, 212–213, 221, 242.

Arcturus, 24, 84; color of, 102.

Ariel, satellite of Uranus, 232–233.

Aries, constellation of the zodiac, 76–78, 90–92, 212, 242.

Asteroids, 244–257.

Bee-hive, 82, 211–212.

Bode’s law, 245–249.

Boötes, star of first magnitude, 102.

Callisto, satellite of Jupiter, 200, 205.

Cancer, constellation of zodiac, 76, 82, 91–92, 188, 211–212.

Capella, star of first magnitude, 191.

Capricornus, one of the twelve constellations of the zodiac, 76, 88–89, 91–92, 187, 212, 229.

Cassiopeia, constellation, 77.

Castor and Pollux, 81, 188, 211, 242–243.

Ceres, first planetoid discovered, 251, 253.

Constellations of the zodiac, 75–92.

Deimos, satellite of Mars, 180–181.

Dione, satellite of Saturn, 222.

Earth, relation to planets, 11–15, 19; nearness to sun, 19; terrestrial planet, 41; movement of, 51; position in regard to Mercury, 120–121; likeness to Venus, 138–140.

Enceladus, satellite of Saturn, 222.

Encke’s comet, 109.

Equinox, derivation of word, 74.

Eros, small planet, 255–256.

Europa, satellite of Jupiter, 200–201.

Flagstaff, Arizona, observatory of, 175–176.

Fomalhaut, 187, 209, 213.

Galileo, 136.

Ganymede, satellite of Jupiter, 200–201, 205.

Gemini, constellation of the zodiac, 76, 81–82, 91–92, 188, 210–211, 213.

George III., Uranus first called _Georgium Sidus_ after, 226.

Great Dipper, 73, 77, 84, 96, 104, 105, 186, 236.

Hamal, star in constellation of Aries, 78.

Herschel, discovery of Uranus by, 226–227, 232.

Hyades, the, 79.

Hyperion, satellite of Saturn, 222.

Inferior planets, 40.

Io, satellite of Jupiter, 200, 201.

Japetus, satellite of Saturn, 222.

Juno, planetoid, 251, 253.

Jupiter, color, 5; attraction between Saturn and, 15; distance from sun, 19; size and importance of, 20; movement, 25, 65; satellites, 34, 106, 199–205; long known, 38; superior planet, 41; space between Mars and, 42; influence on comets, 44; gibbous, 66; distance from ecliptic, 72; near Antares, 86; in Scorpio, 127; size and velocity, 183–185; place in sky, 186–190; distance, light, and heat, 190–193; seasons and atmosphere, 193–195; surface features, 195–199; symbol, 205; compared to Saturn, 213–214, 215–218; nearness of asteroids to, 244; how to recognize, 259–264.

Laplace, nebulæ hypothesis of, 28, 30.

Leo, constellation of zodiac, 76, 82–83, 91–92, 188, 211–212, 221.

Leverrier, discovery of Neptune by, 236–238.

Libra, constellation of zodiac, 76, 85, 91–92, 188, 212.

Little Dipper of the Pleiades, 79.

Lyre, constellation of the, 54.

Major planets, 19.

Mars, “eye” of, 12; distance from sun, 19; nearness to earth, 20; movement of, 25, 65; long known, 38; superior planet, 41; space between Jupiter and, 42; speed, 51; gibbous, 66; distance from ecliptic, 72; color, 80, 86, 259; position in regard to Antares, 87; density, 110; nearness to Venus, 128; variety in brightness, 151–152; how and where to identify, 152–162, 259–265; size, atmosphere, and temperature, 162–165; distance and brilliancy, 166–170; seasons, 170–171; surface aspect, 172–179; satellites, 180–181; symbol of, 182; nearness of asteroids to, 244; Bode’s law and, 245–246, 248–249; smallness, 260.

Mercury, 18; nearest planet, 19; unfavorable situation for observation, 20; easily recognized, 22; age of, 34; dense matter of, 37; long known, 38; inferior planet, 40; terrestrial planet, 41; irregularities of, 44–45; number of revolutions, 47; orbit, 48; apparent motions, 57–58; transits, 61; distance from ecliptic, 72–73; color, 80, 86; in Scorpio, 87; elusiveness of, 93–95; how to find, 96–100, 259; distance and brightness of, 101–105; size, 106–110; relation to sun, 111–118; transits, 119–121; lack of atmosphere, 144, 146; resemblance to Mars, 153; Bode’s law and, 245.

Milky Way, 87, 88, 89.

Mimas, satellite of Saturn, 222.

Minor planets, 19.

Mizar, star in Great Dipper, 105, 236.

Moon, 23; once called planet, 39; distance from ecliptic, 73.

Moulton, Professor, 178.