The Ways of the Planets

It is believed that Saturn’s rings were never a part of the planet, but are mere particles of cosmic materials which happened to be left over, and which he has gathered up by his force of gravity and compelled to revolve about him.

Saturn, more fortunate than Jupiter, has escaped the unimaginative naming of his moons by number, though one would think that, having such a numerous offspring, a shortage in names would be more likely to occur in his than in any other planet family. They all have names more or less connected with the great god whose name the planet bears, and are, in order of their distance from Saturn: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Japetus, Phœbe, and Themis. The largest and brightest of them all is Titan. It is larger than our moon, which is one of the large moons in the solar system, or than Mercury, and is not much smaller than Mars. It is more than three-quarters as large as all the other moons of Saturn put together. Naturally, it was the first to be discovered, and was under observation as long ago as 1655. Rhea and Japetus are next in size, and were discovered in 1671–72; Dione and Tethys were both discovered in 1684, and Enceladus and Mimas in 1789.

Until 1848 seven moons were all that were known to belong to Saturn. In that year little Hyperion, whose diameter, it is thought, can hardly exceed two hundred miles, came into our view. A little more than fifty years later (in 1898) Phœbe made her bright mark on a photographic plate at Harvard, and was caught. By tracing her from one plate to another her orbit was computed, her probable size determined, and practically all that is known about her was found out before she was seen, which was not until 1904. She is not much larger than a good-sized mountain, but is a unique and interesting little satellite that, far outside of the paths of any of the other moons, circles in an eccentric orbit around Saturn in an opposite direction from the rest of the satellites, and thus gives rise to many interesting astronomical speculations. Themis, also a tiny body, was discovered in the same way in 1906, and is thought to be the smallest body in the solar system. Titan is the only one of this group of satellites whose true disc we can see even with a telescope. Only one other (Rhea) can be seen in transit across the planet. The others are not much more than bright points of light, while Phœbe and Themis are almost at the limit of visibility.

On account of their great distance from the sun Saturn’s moons are, of course, not very bright, and all of them put together do not give one-tenth as much light to Saturn as we receive from our moon. But, such as they are, they may some day be very useful to Saturn as a means of illumination. Receiving as he does a hundred times less light from the sun than we do, he may be some day much in need of the light reflected from all his rings and moons.

SEASONS

The seasons on Saturn are somewhat like ours in the succession of spring, summer, autumn, and winter; but the inclination of its axis to its orbit being twenty-seven degrees instead of twenty-three and a half, as ours is, each season is much more accentuated than ours. The sun climbs higher during the northern summer, and sinks correspondingly lower during the winter. But in length Saturn’s seasons are very different from ours. Like his year, they are about twenty-nine and one-half times as long as ours. Each one is more than seven years long. Even the agreeable seasons might grow monotonous to one in that time; but to be spinning through the rapidly alternating days and nights of Saturn during seven long years of winter is a situation that one does not care to contemplate. It is with world personalities as with human personalities: however much we may admire their superior grandeur, when we consider details we would not change places with them.

The symbol of Saturn is an ancient scythe (♄), which gets its appropriateness from the fact that the deity of that name was the special protector of agriculture.

XV

URANUS

Venus, Mars, Jupiter, and Saturn, brilliant beauties that they are, have always been distinguished features of the heavenly view. The records of Mercury do not go back so far as those of these more easily seen planets, yet there is no reason to think that he has not been always known, though less widely, perhaps, than the four planets more frequently in view. To Uranus belongs the distinction of being the first planet that was discovered—a distinction that one cannot help but feel was too long delayed, for it did not come until 1781. For ages and ages his lovely pale beams had been shining down upon us from his little disc, no fainter in brilliancy than many a sixth-magnitude star (a degree of brightness which we think is within the limit of good vision, even in these days), and no human being had been conscious that this bright body was only another member of the solar family, circling with the rest of us around our parent, the sun, and having nothing in common with the far-off stars among which we had numbered him. Nineteen times he had been charted as a fixed star before his identity was suspected, and after he became known to us as a planet he was, by means of these charts, traced back for one hundred and thirty years, and much information was thus gained concerning his orbit and movements.

Uranus was not, however, discovered through observation of his movement among the stars. A view of his actual disc was caught by the musician and astronomer, Herschel, as he gleaned with his telescope in that part of the sky where the planet lay, one hundred and seventy-one years after the invention of that aid to vision. It was at first thought that a comet had been discovered, but later investigation showed a much more important member of the solar system, and the discovery of a new planet was announced.

George III. was then King of England, and the loyal Herschel called the planet Georgium Sidus in honor of that monarch. Fortunately, the world-wide interest in this newly discovered body saved it from so local an appellation, and it finally came to be called after Uranus, the father of Saturn, a name somewhat more in keeping with its place among the planets. In England, however, a very commendable loyalty to Herschel has resulted in the planet’s sometimes being called Herschel, after its discoverer, and we see this name often in English books on astronomy, especially the older books; but Uranus is now the generally accepted name.

The symbol of the planet as it appears in all almanacs—at least in all English almanacs—is a capital H with a planet swinging from the cross-bar in the letter, thus ♅. And to this extent the discovery of the planet by Herschel is commemorated. In American almanacs the symbol is contracted into this figure ♅.

It is a matter for regret that Uranus does not come more easily within our view; for he is a very beautiful planet, pale green in color, and unlike any of the others in his aspect. There are, however, very few persons nowadays who can see him without the aid of at least a small glass, and to most of us he must ever remain a body with which we can have no personal acquaintance. None the less he must have an interest to us such as attaches always to anything so closely related to us, and sharing with us a common origin and a common destiny. To those who have unusually keen vision—or a small telescope—there will be much pleasure in viewing the planet. But even to those who have not these facilities for seeing, it ought to be interesting to know in what region of the skies this far-off member of our family dwells, what his wanderings are, and something of his personality and habits.

It requires a few days more than eighty-four years for Uranus to make one revolution around the sun, so that he moves even more slowly than Saturn from one constellation to another; and if we could only see him more easily, he would be scarcely more difficult to keep track of than a fixed star. He remains in each constellation somewhere near seven years and his change of place in the skies amounts in one year to but little more than four degrees, which is less than the distance between the pointers.

Since Uranus was discovered he has made one circuit of the skies, which he finished in 1865, and he is now (1912) more than half-way around on another. His position now is in Capricornus, nearly twenty degrees east of the “milk dipper” in Sagittarius, and for the next quarter of a century he can be seen by any who have eyes, or a glass, to accomplish this during the summer evenings. Each year he will be about seven degrees farther east. He is, however, still pretty far south of the equator, and not so easily seen as he will be when he reaches that part of the ecliptic which runs somewhat higher in the skies. Even an opera-glass will bring Uranus into the view of many persons. His path deviates very little from the line of the ecliptic—never quite so much as half a degree. The knowledge of this makes it less difficult to find him.

The synodic period of Uranus is about three hundred and sixty-nine days, so that an opposition occurs about four or five days later each year. He was in opposition this year (1912) on July 24th. In 1913 an opposition will take place on July 29th, and in 1914 on August 2d, and oppositions will occur about four days later each year thereafter.

Uranus is twice as far from the sun as Saturn is, and nineteen times as far as the earth. Its mean distance from the sun is 1,784,732,000 miles, and at this distance more than two hours and a half would be required for light to travel from the sun to the planet. Viewed from the planet, the sun would appear only about two and a half times larger than Jupiter appears to us, and the earth would be a very small telescopic body, if, indeed, it would be visible at all. Even at this great distance from the sun, and with the sun showing so small as it does, the planet would still have more than a thousand times as much light as we get from our moon, and so in this respect might be fairly comfortably provided for even for eyes constructed like those of human beings. The heat the sun’s radiant energy furnishes to Uranus is, from our point of view, almost a negligible quantity. If there were no other source of supply, the normal temperature of the planet would be more than three hundred degrees below zero, Fahrenheit. There is no reason to think, however, that this is the temperature that prevails on Uranus. As far as we can tell, it has a dense and extensive atmosphere, and probably very considerable internal heat.

Uranus is smaller than either Jupiter or Saturn; but it is much larger than Mars, Venus, Mercury and the earth combined. Its diameter is nearly thirty-three thousand miles. Its volume is sixty-five times as great as that of the earth; but its mass is only about fourteen times the mass of the earth, which shows it to be a very much expanded body. It is slightly more dense than water, but only about two-tenths as dense as the earth. Its force of gravity is small for so large a body—only about nine-tenths that of the earth.

There is every indication that the planet is not a solid body at all, and that it is, perhaps, largely vapor. We undoubtedly cannot see the surface of it; but through the telescope it faintly shows the same belted appearance that we see on Jupiter and on Saturn, though it is difficult to see the belted region, which is near the equator, because the axis of the planet is so inclined to its orbit that much of the time the poles are pointed almost toward us. The spectroscope indicates something of the same materials in its atmosphere that the other large and faraway planets have, and there is no reason to doubt that the planet is in a much earlier stage of development than any of the terrestrial planets.

We really know nothing certainly about the rotation of Uranus; but there seems to be some indication that, like Jupiter and Saturn, it revolves swiftly—in perhaps ten or twelve hours, and hence has a very short day and night. The great inclination of its axis must make its seasons so abnormal, from our point of view, that it is difficult to understand what they are. Moreover, the planet is, at this stage of its development, so far from being a habitable body, for beings such as we know anything about, that the subject of its seasons seems not very important or interesting.

It seems but fitting that this vapory, pale green planet should have satellites with the fairy names of Ariel, Umbriel, Titania, and Oberon. One can forgive a good many utilitarian feats in nomenclature for the sake of these charmingly appropriate names for the satellites of Uranus. Titania and Oberon were discovered in 1787 by Herschel, the discoverer of the planet. They are not very much farther from Uranus than our moon is from us, and are easily seen with a telescope. Titania, the nearer to Uranus and the larger, is probably about one thousand miles in diameter; and Oberon is not very much smaller. In 1852 Umbriel and Ariel were discovered. They are both smaller and nearer to Uranus than either of the two first discovered, and are seen with considerable difficulty, because of their proximity to the larger and brighter body of the planet. There is not, however, very much difference between any of the four in real brightness.

XVI

NEPTUNE

It is rather curious to what extent we have a feeling of kinship with Neptune, notwithstanding he dwells forever in far-off space where we cannot expect even to have a glimpse of him without the aid of a telescope. Uranus, the other very distant planet, is so nearly within the limit of ordinary vision that we have always a hope that, by some lucky chance of situation or atmosphere, we may some day be able to see him face to face, and know for ourselves what manner of planet this is which, though a member of our own cosmic family, remains always just beyond easy exchange of glances with us; and so we in a measure keep a lookout for him that gives us a sense of his reality.

With Neptune there can be no feeling of this sort to keep us with a lively interest in him, and yet he is hardly less real to us than Uranus, and we have a more intimate sense of nearness to him than we have for any fixed star. Far away as he is, the distance between us is short compared with the many trillions of miles farther that we must go to reach the nearest star, and in thinking of him we always have a sense of this. Then, however aloof he may keep himself from this cozy little bunch of planets near the sun, of which the earth is one, he is still of the same parentage with us, and his life history is part of our family history, so that we can never feel indifferent to what concerns him.

Close as Neptune is to us in kinship and distance, as astronomical distances go, we never knew of his existence until sixty-six years ago. He is to us almost a recent arrival in the solar domain, but we know that he has been here as long as we have; and whether he was detached before we were from the great nebula which gave birth to us all, or at about the same time, we know that for long ages before there were eyes on the earth to see him he was, as he still is, circling slowly and majestically around our common center of control.

The discovery of Neptune in 1846 created truly a sensation in astronomical circles. And, unlike most sensational happenings, it fully justified the extreme interest it aroused. The computation that led to it was a mathematical triumph, and the final result was a most splendidly convincing proof of the theory of gravitation. For the place of this hitherto unknown planet was found by means of computations based on the fact that at certain times Uranus went a little out of his way, thus showing some disturbing body outside of his orbit pulling him slightly from the course he would otherwise take. The deviation was not much—only about one and three-fourths of a minute, which is equal to about one-seventeenth of the apparent diameter of the moon, or one-sixth of the distance between Mizar and Alcor, situated at the bend of the handle of the Big Dipper, two stars that it is difficult for some eyes to separate.7 But this slight irregularity of Uranus was enough to set at least two able men at work in an effort to locate the disturbing cause. These two men were Adams, of England, and Leverrier, of France.

The result of Adams’s work was announced to the Astronomer Royal in England in the autumn of 1845; but the actual search for the planet in the place predicted was delayed until the following summer. In the mean time Leverrier had completed his work and had communicated with astronomers in Berlin, directing them where to look for the planet. The facilities for that sort of work were then better in Berlin than in England; and within half an hour after the search was begun, on the night of September 23, 1846, the new planet was discovered a little more than half a degree from the exact position Leverrier had found for it. It was first recognized as having a sensible disc, and within a day its motion was apparent. No wonder the astronomical world was thrilled by this achievement!

Although the planet was actually discovered by following the directions of Leverrier, it was found that it might have been seen months before if the English astronomers had shown more promptness in using the computations of Adams; and there has always been a disposition among astronomers, both in France and in England, to give both men credit for their extraordinary achievement, though, naturally, there is somewhat more stress laid upon the work of each in his own country. The newly discovered body was at first named for its discoverer, Leverrier, but a sense of justice to Adams prevailed to such an extent that in the end a less commemorative name was chosen, and the planet was called after Neptune, the son of Saturn and the brother of Jupiter—a name more fitting, on the whole, for a member of this planet family, whose other members all bear the names of some of the ancient deities. The trident (♆), Neptune’s three-pronged spear, is the symbol of the planet.

The mean distance of Neptune from the sun is more than two and a half billion miles (2,790,000,000), and his orbit is so nearly circular that the variation between his perihelion and aphelion distance is only about fifty million miles. His orbit is, in fact, less eccentric than that of any other planet except Venus. His immense distance from the sun, of course, deprives him of any great amount of heat or light from that source as compared with the other planets. The sun would appear to an observer on Neptune a little smaller than Venus appears to us. But so great is the intensity of its radiance that even as so diminutive a sun as that it would give to Neptune more than six hundred times as much light as our full moon gives to us. This, however, would be as much as nine hundred times less light than we get from the sun. Such light as the planet receives from the sun reaches it after a journey of a little more than four hours.

Of the heat the planet has, either inherent or acquired from the sun, we do not know much. The normal temperature at that distance from the sun would be more than three hundred and sixty degrees below zero, Fahrenheit, and there is not much to indicate in what state the planet is with reference to its own heat. Investigations thus far made do not show it to be so intensely hot as Jupiter and Saturn undoubtedly are; but with its heavily vapor-laden atmosphere it could not have the frigidity normal to a black, unprotected body at its distance from the sun.

Neptune is thought to have an immense atmosphere, and, like the other outer planets, one of a composition not wholly familiar to us. Consequently we do not know as yet just what this atmosphere does for the planet. It has a fairly good reflecting power, though the planet, on the whole, is darker in color than Jupiter or Saturn. Its color is of that bluish cast which sometimes suggests a leaden appearance. The color, as well as the fact that Neptune is denser than any of the other outer planets, indicates that it may be in a more advanced stage of development than at least Jupiter and Saturn are, and perhaps than Uranus is.

That Neptune has made greater progress toward solidity (though it is still very far from that state) than the other outer planets is suggested also by its size; for, as we have seen, the smaller planets develop more rapidly than the larger ones. The diameter of Neptune is a little less than thirty thousand (29,827) miles. The planet is somewhat smaller, therefore, than Uranus, and much smaller than Jupiter or Saturn. But as compared with the earth, the largest of the inner planets, it is a vastly greater body. Its mass is seventeen times more than that of the earth; its surface is as much as sixteen times more extensive than the earth’s; and its volume is more than eighty times greater than the volume of the earth.

Of the time of Neptune’s rotation on its axis very little is known. That little, however, indicates a slower rotation than the other planets seem to have, and the alternations of day and night on Neptune are, therefore, probably less swift than on Jupiter and on Saturn. The planet is too far away for us to see its surface markings with any distinctness, but there are indirect processes by which we can get approximate information concerning the facts about rotation. One of these processes is by observation of the motions of the satellites. Of these useful bodies Neptune, fortunately, has one—a very excellent moon about the size of our own. It has some eccentricities, such as revolving about the planet in the opposite direction from that which the more conventional satellites follow, and having an orbit a good deal inclined to the plane of the equator of the parent body. But it is a very interesting moon to astronomers, and will no doubt in time help to make clear some things in the history of Neptune which are now not quite understood.

Being so far from the sun, Neptune moves, of course, very slowly in comparison with the nearer planets, though his speed is at the rate of three and a half miles a second, which, after all, does not denote any high degree of sluggishness. His change of position in the sky amounts to a little more than two degrees a year; so that in an ordinary lifetime he does not make any very great progress along the zodiac.

When Neptune was discovered he had just left the constellation Capricornus, and in the sixty-six years that his movements have been followed he has passed through Aquarius, Pisces, Aries, Taurus, and is now (1912) in Gemini, very near Castor and Pollux. The time required for his circuit around the sun is nearly one hundred and sixty-five (164.6) years, so that he remains for about thirteen years in each constellation. He will complete one sidereal period, dating from the time of his discovery, in the year 2011.

The apparent motion of Neptune is direct a little more than six months in the year, and retrograde a little more than five months, so that it seems to present the old mental arithmetic problem of the climber that fell back so much every time after he had climbed a certain number of feet. But the falling back in the case of Neptune is an illusion, as we know. He keeps straight on in his journey, as we may see if we watch him from year to year, and his change of position is so slight during any year that the change of direction is hardly noticeable.

Neptune is as bright as an eighth-magnitude star, and it is possible to see him with a good field-glass. The difficulty is in distinguishing him from a star, for his disc does not show except through a telescope. If one has such a glass, however, it will be worth while to direct it toward that part of the ecliptic just under Castor and Pollux any time within the next two or three years, and a sight of this yet strange brother planet may be the reward. He will be in opposition on January 14, 1913, and thereafter about two days later each year.

XVII

THE LITTLE PLANETS, OR THE ASTEROIDS

The asteroids, or minor planets, are situated almost wholly in the vast space between Mars and Jupiter. Their orbits are very irregular, both as to shape and situation; but, so far as is known, only two of them pass beyond the orbit of Jupiter, and only one has been discovered which at any point in its journey around the sun comes nearer than the orbit of Mars.

The minor planets are called by astronomers almost indifferently asteroids or planetoids. “Asteroids” is probably the name by which they are most popularly known. But because they are in fact simply little bodies that revolve about the sun as the planets do, “planetoids” seems to be more truly descriptive of them, and it is the word I have chosen to use here.

It was early noted that, except in one instance, the planets seemed to show in their distance from the sun something like a mathematical progression. Struck by this appearance, an astronomer named Bode worked it out into a formula, known ever since as Bode’s law, though the idea seems to have originated with another astronomer. One almost always sees it mentioned in any work dealing with this phase of planetary history, and it is especially interesting because of the part it played in the discovery of the planetoids. It was as follows: Beginning with nothing for Mercury, add three for Venus, twice three, or six, for the earth, twelve for Mars, and continue thus to double the number for each planet out to and including Saturn. Then to each one of the numbers so obtained add four, and the numbers resulting will very nearly represent the relative distances of the planets from the sun. Thus:

The exception was that at the fifth number, 28, there was no planet to correspond, and Jupiter was nearly twice as far away from Mars as it should have been to conform to the law, thus leaving room for another planet to occupy the allotted position and fill out this very beautiful progression.

About nine years after this law was set forth Uranus was discovered circling out in space far beyond Saturn, and was found to conform to the law in a most satisfactory manner, its distance being approximately twice that of Saturn. With such close accord between the actual distances and the prescribed distances of the planets from the sun, and with the one exception leaving almost exactly the space allotted by Bode’s law for another planet, astronomers naturally had a very strong feeling that there must be another planet between Mars and Jupiter. They accordingly set to work to prove this, if possible, and to find what had become of this lost member of the planet family, if it ever existed.

As a result of this work, on January 1, 1801, the first planetoid was discovered, and in rapid succession many like it were found, until now many hundreds are known to astronomers. Their discovery seemed at first almost a certain confirmation of Bode’s law, and the fact that where one large planet should have been found there proved to be such a swarm of small ones could be accounted for in no other way than to suppose that something had happened in the making of the planet. At any rate, the promulgation of Bode’s law was the direct cause of the search for the missing planet which led to the discovery of the planetoids. And this is the only reason why Bode’s law has continued to be mentioned in the history of the planets. For it was no real law, it had no scientific foundation, and its conformity to the facts of the relative distances of the planets was only one of those very interesting and singular coincidences that startle one for the moment into thinking that there is some scientific significance in them. Another example of such a coincidence is in the fact that the mass of any given planet exceeds the total mass of all the planets of any less mass than itself.

In less than half a century after the discovery of the first planetoid, Neptune was discovered at a distance not at all corresponding to that indicated by Bode’s law. It was not nearly far enough away, and yet, strangely enough, it was by taking Bode’s law into consideration that the position was indicated which finally led to the discovery of the planet. So while Bode’s law has been found to be no law at all, it is, nevertheless, entitled to some mention because of its having thus stimulated research that has had such important results.

No really satisfactory and final explanation of the present state of the planetoids has ever been given. At one time it was suggested that another planet had originally existed in the space between Mars and Jupiter, and through some catastrophe had been shattered into the small bodies that now occupy that space. But this has been shown to be impossible.

It is now thought probable that in the original nebula the matter forming the planetoids might have been prevented from condensing into a planet by the powerful gravitative influence of Jupiter. This influence, however, was not sufficiently strong to bring them entirely under his control. Even yet he pulls some of them five or six degrees out of the path they otherwise would take when they venture within the limits of his domain; but he does not capture them, so they have been left to circle around the sun as mere fragments of bodies, with no force to combine and make a world, no mass to hold an atmosphere, and with nothing to prevent them from quickly condensing and from radiating all their heat into space. They are, in the main, just cold, dark, lifeless rocks and lumps of matter whirling through space in a maze of interlacing orbits, some of them almost as far from the sun as Jupiter and some almost as near as Mars—one, indeed, a little nearer than Mars at certain times—but most of them swarming more thickly about half-way between Mars and Jupiter, not far from the place that Bode’s law assigned to a planet.

After the first planetoid was discovered and had been observed for a few weeks, it was lost and had to be rediscovered by means of mathematical computation of its orbit. Where this computation showed that it ought to be, there it was found, on the very last day of the same year, 1801. Early the next year another body of the same sort was discovered, two years later another was found, and still three years later a fourth came into view. These four were the only ones known in this branch of the solar family for nearly forty years thereafter.

In 1845 another period of discovery commenced, and has ever since continued, until there are now between six and seven hundred of these little bodies that have disclosed their right to be known as members of the sun’s family. It is probable that there may be still many more of them, since a new one comes to light every now and then on a photographic plate, and there is no indication of any limit to the number that may thus appear.

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