JUPITER, SATURN, URANUS, NEPTUNE

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LECTURE IV. JUPITER, SATURN, URANUS, NEPTUNE.

Jupiter, Saturn, Uranus, Neptune—Jupiter—The Satellites of Jupiter—Saturn—The Nature of the Rings—William Herschel—The Discovery of Uranus—The Satellites of Uranus—The Discovery of Neptune.

Our lecture to-day ought to make us take a very humble view of the size of our earth. Mercury, Venus, and Mars may be regarded as the earth’s peers, though we are slightly larger than Venus, and a good deal larger than Mercury or Mars; but all these four globes are insignificant in comparison with the gigantic planets which lie in the outer parts of our system. These great bodies do not enjoy the benefits of the sun to the same extent that we are permitted to do; they are so far off that the sun’s rays become greatly enfeebled before they can traverse the distance; but the gloom of their situation seems to matter but little, for it is highly improbable that any of these bodies could be inhabited.

A view of parts of the paths of these four great planets is shown in Fig. 65. The innermost is Jupiter, which completes a circuit in about twelve years; then comes Saturn, revolving in an orbit so great that twenty-nine years and a half are required before the complete journey is finished. Still further outside is Uranus, which has a longer journey than Saturn, and213 moves so much more slowly that a man would have to live to the ripe old age of eighty-four if a complete revolution of Uranus was to be accomplished during his lifetime. At the boundary of our system revolves the planet Neptune, and though it is a mighty globe, yet we cannot see it without a telescope. It is invisible to the naked eye for two reasons: first of all, because it is so far from the sun that the light which illuminates it is excessively feeble; and, secondly, because it is so far from us that whatever brilliancy it has is largely reduced.

Fig. 65.—The Orbits of the Four Giant Planets.

JUPITER.

Of all these bodies Jupiter is by far the greatest; he is, indeed, greater than all the other planets rolled into one. The relative insignificance of the earth when compared with Jupiter is well illustrated by the fact that if we took 1200 globes each as big as our earth, and made them into a single globe, it would only be as large as the greatest of the planets. A view of the comparative sizes of the earth and Jupiter is shown in Fig. 66.

Fig. 66.—Jupiter and the Earth compared.

Fig. 67 shows a picture of Jupiter as seen through the telescope. First, you will notice that the outline of the planet’s shape is not circular, for it is plain that the vertical diameter in this picture is shorter than the horizontal one; in fact, Jupiter is flattened at the Poles and bulges out at the equator, so that a section through the Poles is an ellipse. Jupiter is turning round rapidly on his axis, and this will account for the protuberance.215 We find that the planet has assumed almost the same form as if it were actually a liquid. This we can illustrate by a globe of oil which is poised in a mixture of spirits of wine and water so carefully adjusted that the oil has no tendency to rise or fall. As we make the globe of oil rotate, which we can easily do by passing a spindle through it, we see that it bulges out in the form that Jupiter as well as other planets have taken.

Fig. 67.—The Clouds of Jupiter.

On the picture of the planets you will see shaded bands. These are constantly changing their aspect, and for a double reason. In the first place, they change because Jupiter is rotating so quickly that in five hours the whole side of the planet which is towards us has216 been carried out of sight. In another five hours the original side of the globe will be back again, for the entire rotation occupies about 10 hours, or, more precisely, 9 hours 55 minutes 21 seconds.

But these bands are themselves not permanent objects. They have no more permanence than the clouds over our own sky. Sometimes Jupiter’s clouds are more strongly marked than on other occasions. Sometimes, indeed, they are hardly to be seen at all. It is from this we learn that those markings which we see when we look at the great planet are merely the masses of cloud which surround and obscure whatever may constitute his interior.

There is a circumstance which demonstrates that Jupiter must be an object exceedingly different from the earth, though both bodies agree in so far as having clouds are concerned. What would you think when I tell you that we were able to weigh Jupiter by the aid of his little moons, of which I shall afterwards speak? These little bodies inform us that Jupiter is about 300 times as heavy as our earth, and we have no doubt about this, for it has been confirmed in other ways. But we have found by actual measurement that Jupiter is 1200 times as big as the earth, and therefore, if he were constituted like the earth, he ought to be 1200 times as heavy. This is, I think, quite plain; for if two cakes were made of the same material, and one contained twice the bulk of the other, then it would certainly be twice as heavy. If there be two balls of iron, one twice the bulk of the other, then, of course, one has twice the weight of the other. But if a ball217 of lead have twice the bulk of a ball of iron, then the leaden ball would be more than twice as heavy as the iron, because lead is the heavier material. In the same way, the weights of the earth and Jupiter are not what we might expect from their relative sizes. If the two bodies were made of the same materials and in the same state, then Jupiter would be certainly four times as heavy as we find him to be. We are, therefore, led to the belief that Jupiter is not a solid body, at least in its outer portions. The masses of cloud which surround the planet seem to be immensely thick, and as clouds are, of course, light bodies in comparison with their bulk, they have the effect of largely increasing the apparent size of Jupiter, while adding very little to his weight. There is thus a great deal of mere inflation about this planet, by which he looks much bigger than his actual materials would warrant if he were constituted like the earth.

These facts suggest an interesting question. Why has Jupiter such an immense atmosphere, if we may so call it? The clouds we are so familiar with down here on the earth are produced by the heat of the sun, which beats down upon the wide surface of the ocean, evaporates the water, and raises the vapor up to where it forms the clouds. Heat, therefore, is necessary for the formation of cloud; and with clouds so dense and so massive as those on Jupiter, more heat would apparently be necessary than is required for the moderate clouds on this earth. Whence is Jupiter to get this heat? Have we not seen that the great planet is far more distant from the sun than we are? In fact, the intensity218 of the sun’s heat on Jupiter is not more than the twenty-fifth part of what we derive from the same source. We can hardly believe that the sun supplied the heat to make those big clouds on the great planet; so we must cast about for an additional source, which can only be inside the planet itself. So far as his internal heat is concerned, Jupiter seems to be in much the same condition now as our earth was once, ages ago, before its surface had cooled down to the present temperature. As Jupiter is so much larger than the earth, he has been slower in parting with his heat. The planet seems not yet to have had time to cool sufficiently to enable water to remain on his surface. Thus the internal heat of the planet supplies an explanation of his clouds. We may also remark that as the present condition of Jupiter illustrates the early condition of our earth, so the present condition of the earth foreshadows the future reserved for Jupiter when he shall have had time to cool down, and when the waters that now exist in the form of vapor shall be condensed into oceans on his surface.

THE SATELLITES OF JUPITER.

Every owner of a telescope delights to turn it on the planet Jupiter, both for the spectacle the globe itself affords him, and for a view of the wonderful system of moons by which the giant planet is attended. Fortunately the four satellites of Jupiter lie within reach of even the most modest telescope, and their incessant changes relative to Jupiter and each other give them a219 never-ending interest for the astronomer. Compared with the torpid performance of our moon, which requires a month to complete a circuit around the earth, Jupiter’s moons are wonderfully brisk and lively. Nor are they small bodies like the satellites of Mars, for the second of Jupiter’s satellites is quite as big as our moon, and the other three are very much larger. It is, however, true that his satellites appear insignificant when compared with Jupiter’s own enormous bulk.

The innermost of these little bodies flies right round in a period of one day and eighteen or nineteen hours, while the outermost of them takes a little more than a fortnight—that is, rather more than half the time that our moon demands for a complete revolution. Jupiter’s satellites are too far off for us to see much with respect to their structure or appearance even with mighty telescopes. It is, of course, their great distance from us that makes them look insignificant. They would, however, be bright enough to be seen like small stars were it not that, being so close to Jupiter, his overpowering brightness renders such faint objects in his vicinity invisible.

It was by means of the satellites of Jupiter that one of the most beautiful scientific discoveries was made. As a satellite revolves round the giant planet it often happens that the little body enters into the shadow of the great planet. No sunlight will then fall upon the satellite, and as it has no light of its own, it disappears from sight until it has passed through the shadow and again receives sunlight on the other side. We can watch these eclipses with our telescopes, and there can220 be no more interesting employment for a small telescope. The movements of these bodies are now known so thoroughly that the occurrence of the eclipses can be predicted. The almanacs will tell when the satellite is calculated to disappear, and when it ought again to return to visibility. When astronomers first began to make these computations a couple of hundred years ago, the little satellites gave a great deal of trouble. They would not keep their time. Sometimes they were a quarter of an hour too soon, and sometimes a quarter of an hour too late. At last, however, the reason for these irregularities was discovered, and a wonderful reason it was.

Suppose there were a number of cannons all over Hyde Park, and that these cannons were fired at the same moment by electricity. Though the sounds would all be produced simultaneously, yet, no matter where you stood, you would not hear them altogether; the noise from the cannons close at hand would reach your ears first, and the more distant reports would come in subsequently. You can calculate the distance of a flash of lightning if you allow a mile for every five seconds that elapse between the time you saw the flash and the time you heard the peal of thunder which followed it. The light and the noise were produced simultaneously, but the sound takes five seconds to pass over every mile, while the light, in comparison to sound, may be said to move instantaneously. That sound travelled with a limited velocity was always obvious, but never until the discrepancies arose about Jupiter’s satellites was it learned that light also takes time to travel. It is true221 that light travels much more quickly than sound—indeed, about a million times as fast. Light goes so quickly, that it would rush more than seven times around the earth in a single second. So far as terrestrial distances are concerned, the velocity of light is such that the time required for a journey is inappreciable. The distances, however, between one celestial body and another are so enormous, that even a ray of light, moving as quickly as it alone can move, will occupy a measurable time on the way. Our moon is comparatively so near us, that light takes little more than a second to cover that short distance. Eight minutes are, however, required for light to travel from the sun to the earth; in fact, the sunbeams that now come into our eyes left the sun eight minutes ago. If the sun were to be suddenly extinguished, it would still seem to shine as brightly as ever in the eyes of the inhabitants of this earth for eight minutes longer. As Jupiter is five times as far from the sun as we are, it follows that the light from the sun to Jupiter will spend forty minutes on the journey, and the light from Jupiter to the earth will take a somewhat similar time. When we look at Jupiter and his moons, we do not see him as he is now, we see him as he was more than half an hour ago, but the interval will vary somewhat according to our different distances from the planet. Sometimes the light from Jupiter will reach us in as little as thirty-two minutes, while sometimes it will take as much as forty-eight—that is, the light sometimes requires for its journey a quarter of an hour more than is sufficient at other times.

222We can therefore understand that irregularity of Jupiter’s satellites which puzzled the early astronomers. An eclipse sometimes appeared a quarter of an hour before it was expected; because the earth was then as near as it could be to Jupiter, while the calculations had been made from observations when Jupiter was at his greatest distance. It was these eclipses of the satellites which first suggested the possibility that light must have a measurable speed. When this was taken into account, then the occasional delay of the eclipses was found to be satisfactorily explained. Confirmation flowed in from other sources, and thus the discovery of the velocity of light was completely established.

Professor Barnard, when studying Jupiter in 1892 with the splendid refractor at the Lick Observatory, saw a very small point of light nearer to the planet than the nearest of the four satellites already known. Further examination showed that this little object was indeed another satellite. Thus Jupiter has a fifth moon in addition to the four which have been known so long. This little body is so small and faint that it can only be discerned under the most favorable conditions by the most powerful telescopes.

SATURN.

Next outside Jupiter, on the confines of the ancient planetary system, revolves another grand planet, called Saturn. His distance from us is sometimes nearly a thousand millions of miles, and he requires more than a quarter of a century for the completion of each revolution.223 Sometimes people do not pronounce the names of the planets quite correctly. I have heard of a gardener who has a taste for astronomy, and sometimes begins to talk about the planets Juniper and Citron. Probably you will know what he meant to say. The ancients had discovered Saturn to be a planet, for though he looked like a star, yet his movement through the constellations could not escape their notice when attention was paid to the heavens.

Fig. 68.—Saturn and the Earth compared.

In the matter of size Saturn is only surpassed by Jupiter among the planets. He is about 600 times as large as the earth; the small object, E, shown in Fig. 68, represents our earth in its true comparative size to the ringed planet; but Saturn is so far off, that even at his best he is never so bright as Venus, or Mars, or Jupiter become when they are favorably situated. On the globe of Saturn we can sometimes see a few224 bands, but they are faint compared with those on Jupiter. There is, however, no doubt that what we see upon Saturn is a dense mass of cloud. Indeed, he can have comparatively little solid matter inside, for this planet does not weigh so much as a ball of water the same size would do. Saturn, like Jupiter, must be highly heated in his interior.

The ring, or rather series of rings, by which the planet is surrounded are also shown in Fig. 68: these appendages are not fastened to the globe of Saturn by any material bonds; they are poised in space, without any support, while the globe or planet proper is placed symmetrically in the interior.

I have made a model which shows Saturn with his rings, but it is necessary for me to fasten the rings by little pieces of wire to the globe, for there is no mechanical means by which the rings of the model could be poised without support, as they are around the planet. If we throw the beam of the electric lamp on the little planet, we see the shadow which the planet casts on its ring. Similar shadows can be observed in the actual Saturn of the sky, and this is a proof that the planet does not shine by its own light, but by the light of the sun which falls upon it. Here again we illustrate the wide difference between a planet and a star, for were our sun to be put out, Saturn and all the other planets in the sky would vanish from sight, while the stars would, of course, twinkle on as before. There are three rings round Saturn; they all lie in the same plane, and they are so thin, that when turned edgewise towards us the whole system almost disappears, except in very225 powerful telescopes. The outer and the inner bright rings are divided by a dark line, which can be traced entirely round. At the inner edge of the inner ring begins that strange structure called the crape ring, which extends halfway towards the globe of the planet. The most remarkable point about the crape ring is its semi-transparency, for we can sometimes see the globe of the planet through this strange curtain. The crape ring can only be observed with a powerful telescope. The other two rings are within the power of very moderate instruments.

THE NATURE OF THE RINGS.

For the explanation of the nature of Saturn’s rings we are indebted to the calculations of mathematicians. You might have thought, perhaps, that nothing would be simpler than to suppose the rings were stiff plates made from solid material. But the question cannot be thus settled. We know that the ring could not bear the strain of the planet’s attraction upon it if it were a solid body. I may illustrate the argument by familiar facts about bridges. Where the span is but a small one, as, for instance, when a road has to cross a railway, a canal, or a river, the arch is, of course, the proper kind of structure. There is, for example, a specially beautiful arch over the river Dee at Chester. But if the bridge be longer than this, masonry arches are not suitable. Where a considerable span has to be crossed, as at the Menai Straits, or a gigantic one, as at the Firth of Forth, then arches have to be abandoned, and226 iron bridges of a totally different construction have to be employed. Arches cannot be used beyond a limited span, because the strain upon the materials becomes too great for their powers of resistance to withstand. Each of the stones in an arch is squeezed by intense pressure, and there is a limit beyond which even the stoutest stones cannot be relied upon. As soon, therefore, as the span of the arch is so great that the stones it contains are squeezed as far as is compatible with safety, then the limit of size for that form of arch has been reached.

Suppose that you stood on Saturn at his equator, and looked up at the mighty ring which would stretch edgewise across your sky. It would rise up from the horizon on one side, and, passing over your head, would slope down to the horizon on the other. You would, in fact, be under an arch of which the span was about 100,000 miles. Owing to the attraction of Saturn, every part of that structure would be pulled forcibly towards his surface, and thus the materials of the arch, if it were a solid body, would be compressed with terrific force.

It does not really signify that the arch I am now speaking of is half of a ring the other half of which is below the globe of the planet. That is only a difference with respect to the support of two ends of the arch, and does not affect the question as to the pressure upon its materials; nor does the fact that the ring is revolving remove the difficulty, though it undoubtedly lessens it. We know no solid substance which could endure the pressure. Even the toughest steel that ever was227 made would bend up like dough under such conditions. We cannot, therefore, account for Saturn’s ring by supposing it to be a solid, for no solid would be strong enough.

Do you not remember the old fable of the oak tree and the pliant reed—how when the storm was about to arise the oak laughed at the poor reed, and said it would never be able to withstand the blasts? But matters did not so turn out. The mighty oak, which would not yield to the storm, was blown down, while the slender reed bent to the wind and suffered no injury. This gives us a hint as to the true constitution of Saturn’s ring; it is not a solid body, trying to resist by mere strength; it is rather to be explained as an excessively pliant structure. Indeed, I ought not to call it a structure at all; it is rather a multitude of small bodies not in the least attached together. I do not know what the size of these bodies may be. For anything we can tell, they may be no larger than the pebbles you find on a gravel walk.

Let us see how we could encircle our earth with rings like those which surround Saturn. I shall ask you to be provided with a sufficiently large number of pebbles, and you must also imagine that I have the means of ascending high up into space, halfway from here to the moon. Suppose I went up there and simply dropped the pebble, of course, it would tumble straight down to the earth again. If, however, I threw it out with proper speed and in the proper direction, I could start it off like a little moon, and it would go on round and round our earth in a circle. I mention a pebble,228 but really it is little matter what the size of the object may be—it may be as small as a grain of shot or as big as a cannon-ball. Now take another pebble. Cast it also in a somewhat similar path, taking care, however, that the planes of the two orbits shall be the same. Each of these little bodies shall pursue its journey without interference from the other. Then proceed in the same way with a third, a fourth, with thousands and millions and billions of pebbles, until at last the small bodies will become so numerous that they almost fill a large part of the plane with a continuous shoal. Each little object, guided entirely by the earth’s attraction, will pursue its path with undeviating regularity. Its neighbors will not interfere with it, nor will it interfere with them. Let us circumscribe the limits of our flat shoal of moonlets. We first take away all those that lie outside a certain large circle; then we shall clear away sufficient to make a vacant space between the outer ring and the inner ring, and thus the two conspicuous rings have been made; at the inside of the inner ring we shall take out numbers of pebbles here and there, so as to make this part much less dense than the outer portions, and thus produce a semi-transparent crape ring; then we shall clear away those that come too close to the planet, and form a neat inner boundary.

Could we then view our handiwork from the standpoint of another planet, what appearance would our earth present? The several pebbles, though individually so small, would yet, by their countless numbers, reflect the sun’s light so as to produce the appearance229 of a continuous sheet. Thus we should find a large bright outer ring surrounding the earth, separated by a dark interval from the inner ring, and at the margin of the inner ring the pebbles would be so much more sparsely distributed that we should be able to see through them to some extent. That beautiful system of rings which Saturn displays is undoubtedly of a similar character to the hypothetical system which I have endeavored to describe. No other explanation will account for the facts, especially for the semi-transparency of the crape ring. The separate bodies from which Saturn’s rings are constituted seem, however, so small that we are not able to see them individually. There are some other fine lines running round the rings beside the great division, and these can also be explained by the theory I have stated.

Saturn has other claims on our attention besides those of its rings. It has an elaborate retinue of satellites—no fewer, indeed, than nine; but some of them are very faint objects, and not by any means so interesting as the system by which Jupiter is attended. The ninth of these was discovered quite recently by Professor W. H. Pickering, of Harvard College Observatory. This little moon, for which the name “Phoebe” has been suggested, is further from the planet than any of the others. It is a minute object shining as a star of the 15th or 16th magnitude, and moves around the planet in a period of about sixteen months.

Saturn was the last and outermost of the planets with which the ancients were acquainted. Its path lay on the frontiers of the then known solar system,230 and the magnificence of the planet itself, with its attendant luminaries and its marvellous rings, rendered it worthy indeed of a position so dignified. These five planets—namely, Mercury, Venus, Mars, Jupiter, and Saturn—made up with the sun and the moon the seven “planets” of the ancients. They were supposed to complete the solar system, and, furthermore, the existence of other members was thought to be impossible. In modern times it has been discovered that there are yet two more planets. I do not now refer to those little bodies which run about in scores between Mars and Jupiter. I mean two grand first-class planets, far bigger than our earth. One of them is Uranus, which revolves far outside Saturn, and the other is Neptune, which is much further still, and whose mighty orbit includes the whole planetary system in its circuit. To complete its journey round the sun not less than 165 years is required.

WILLIAM HERSCHEL.

I have to begin the account of this discovery by telling you a little story. In the middle of the last century there lived at Hanover a teacher of music whose name was Isaac Herschel. He had a family of ten children, and he did the best for them that his scanty means would permit. Of his children William was the fourth, and he inherited his father’s talents for music, as did most of his brothers and sisters. He was a bright, clever boy at school, and he made such good progress in his music that by the time he was fourteen years old231 he was able to play in the military band of the Hanoverian Guards. War broke out between France and England, and as Hanover was then under the English crown, the French invaded it, and a battle was fought in which the poor Hanoverian Guards suffered very terribly. Young Herschel spent the night after the battle in a ditch, and he came to the conclusion that he did not like fighting, though he was only a member of the band, and he resolved to change his profession. That was not so easy to do just then, for even a bandsman cannot leave the service in war time at his own free will. William Herschel, however, showed all through his life that he was not the man to be baffled by difficulties. I do not know whether he asked for leave, but at all events he took it. He deserted, in fact, and his friends succeeded in sending him away to England.

He was nineteen years old when he commenced to look for a career over here, and certainly he found his prospects in the musical profession very discouraging. Herschel was, however, very industrious; and at last he succeeded in getting appointed as organist of the Octagon Chapel at Bath. He gradually became famous for his musical skill, and had numbers of pupils. He used also to conduct concerts and oratorios, and was well known in this way over the West of England. Busy as Herschel was with his profession, he still retained his love of reading and study. Every moment he could spare from his duties he devoted to his books. It was natural that a musician should specially desire to study the theory of music, and to understand it properly you232 should know Euclid and algebra, and, indeed, higher branches of mathematics as well. Herschel did not know these things at first; he had not the means of learning them when he was a boy, so he worked very hard after he became a man. And he studied with such success that he made fair progress in mathematics, and then it appeared to him that it would be interesting to learn something about astronomy. After he had begun to read about the stars, he thought he would like to see them, and so he borrowed a telescope. It was only a little instrument, but it delighted him so much that he said he must have one for himself. So he wrote to London to make inquiries.

Telescopes were much dearer in those days than they are now, and Herschel could not give the price that the opticians demanded. Here again his invincible determination came to his aid. What was there to prevent him from making a telescope? he asked himself; and forthwith he began the attempt. You will think it strange, perhaps, that a music-teacher who had no special training as a mechanic should at once commence so delicate and difficult a task; but it is not really so hard to make a telescope as might be imagined. The amateur cannot make such a pretty-looking instrument as he is able to buy at the shops—the tubes will not be so beautifully polished and the finish will be such as a trained workman would be ashamed of—but the essential part of a telescope is comparatively easy to make; at least, I should say of a reflecting telescope, which is the kind Herschel attempted to make, and succeeded in making. You must know that there are two kinds of233 telescopes. The commoner one with which you are more familiar is called the refracting telescope, and it has glass lenses. It was an instrument constructed on this principle that we spoke of in a former lecture (p. 97). The reflecting telescope depends for its power upon a bright mirror at the lower end, and when using this instrument you look at the reflection of the stars in the mirror. It was a reflector like this that Herschel began to construct, and he engaged in the task with enthusiasm. His sister Caroline had come to live with him, and she used to help him at his work. So much in earnest was he that he used to rush into his workshop directly he came home from a concert, and without taking off his best clothes he would plunge into the grinding and polishing of his mirrors. His sister tried to keep the house as tidy as possible, but Herschel put up a carpenter’s shop in the drawing-room, and turning-lathes in the best bedroom. At last he succeeded. He made a mirror of the right shape, and found that it exhibited the stars properly. It was not a looking-glass in the ordinary sense, with glass on one side and quicksilver on the other. The mirror that Herschel constructed was entirely of metal. It consisted of a mixture of two parts of copper with one of tin.

Fig. 69.—The Mirror.

The copper has first to be melted in a furnace, for the metal must be above a red heat before it will begin to run. Then the tin has to be carefully added, and the casting of the mirror is effected by pouring the molten metal into a flat mould. Thus the rough mirror is obtained, which in Herschel’s earlier telescopes seems to have been about six or seven inches in diameter, and234 nearly an inch thick. Though copper is such a tough substance, and though tin is also tough, yet when melted together to make speculum metal, as this alloy is called, they produce an exceedingly hard and brittle material. When we remember that we could never break a copper penny piece by throwing it down on the flags, it may seem strange that the “speculum metal” should be so exceedingly brittle. A piece the size of a penny would be more brittle than a bit of glass of the same dimensions, and when the speculum is cast, unless it is cooled very carefully, it will fly into pieces. Herein lay one of the difficulties that Herschel encountered. Speculum metal must be put into an oven as soon as the casting has become solid, and then the heat is gradually allowed to abate. When the speculum has been at last obtained, next follows the labor of giving it the true figure and polish. It is not only more fragile than glass, but it is also quite as hard, and therefore the grinding is a tedious operation. First the surface has to be ground with coarse sand, and then with emery, which is gradually made finer and finer until the true figure has been given (Fig. 69). The mirror is then somewhat basin-shaped, but the depression is very slight. For example, in a mirror six inches across the depression at the centre235 would perhaps be not more than the twentieth of an inch. Small though this depression is, yet it has to be made with exactness. In fact, if it were wrong at any point by so much as the tenth of the thickness of this sheet of paper, the telescope would not perform accurately. The tool that is used in grinding is made of cast iron, and has been turned in a lathe to the right shape. It is divided into squares in the manner shown in Fig. 70. After the grinding comes the polishing, and this is effected with a tool like the grinder in shape. This has to be covered over with little squares of pitch, so that when warmed and put down on the mirror it is soft enough to receive the right shape. Some rouge and water is spread over the mirror, and the polisher is worked backward and forward with the hand until a brilliant surface is obtained.

Fig. 70.—The Grinding Tool.

When the amateur astronomer has completed this part of the task, all the great difficulties about his telescope are conquered. The tube may be made of wood, and, indeed, a square tube will do just as well as a round one. He must also provide for the top of the tube a small mirror, which has to be perfectly flat. The preparation of this requires much care, because it is not so easy as one might suppose to obtain an accurately flat surface. One way of doing this is to get three pieces, and grind each two of them together until every pair will touch all over; then they will certainly all be236 flat. One more part you want, and that is an eyepiece. This presents no difficulty. A single glass lens can be made to answer and your telescope is complete.

THE DISCOVERY OF URANUS.

It was in the year 1774 that Herschel first had a view of the heavens through the telescope he had himself constructed. During the early part of his career he does not seem to have made any important discoveries. He was gradually preparing himself for the great achievement by which his name became famous.

It was on the 13th of March, 1781, that the organist of the Octagon Chapel at Bath turned his telescope on the constellation of the Twins, and began to look at one star after another. You must know that a star merely looks like a little point in a telescope; even the greatest instrument will only make a star look brighter, and will never show it with a perceptible disk. In looking over the stars that night, Herschel’s attention was arrested by one object that did look larger when magnified, and therefore was not a star. The only other objects which would behave in this way were the planets, or possibly a comet. Indeed, at first Herschel imagined that what he saw must be a comet. It could hardly have occurred to him that he was to have such good fortune as to discover a new planet. The five great planets had been known from all antiquity. Was it reasonable to suppose that there could be yet another that had never been perceived? Fortunately, there was a test available. A star remains in the same place237 from night to night and from year to year; while a planet, as we have already had occasion to mention, is a body which is wandering about. The movements of a planet are, however, not at all like those of a comet. To decide on the nature of Herschel’s newly discovered body, it was sufficient to observe the character of its motion. A few nights sufficed to do this. The position of the body was carefully marked relatively to the neighboring stars, and it was soon shown that it was a planet.

Here, then, a great discovery was made. A new planet, now called Uranus, was added to our system. It would be nothing to discover a new star. You might as well talk of discovering a new grain of sand on the seashore. The stars are in untold myriads. They are so far off that they have no relation whatever to our system, which is presided over by the sun. But by the detection of a new planet, revolving far outside Saturn, Herschel showed that a new and most interesting member had to be added to the five old planets which have been known from the earliest records of history.

It may well be imagined that a discovery so startling as this excited astonishment throughout the scientific world. “Who is this Bath organist?” everybody asked. Accounts of him and his discoveries appeared in the papers. His fellow-citizens were not so familiar with the name as we are, happily, now; and the spelling of the unusual name showed many varieties. When George III. heard of Herschel’s great achievement, he directed the astronomer to be summoned to Windsor,238 that the King might receive an account of the wonderful discovery from the lips of the discoverer himself. Herschel of course obeyed, and he brought with him his famous telescope, and also a map of the whole solar system, to show to the King. No doubt he thought that his Majesty had probably not paid much attention to astronomy. Herschel was, therefore, prepared to explain to the King what it would be necessary for him to know before he could fully appreciate the magnitude of the discovery.

You will remember that Herschel while still a boy had deserted from the army, many years previously. It appears that the King had learned this fact in some way, so that when Herschel was ushered into his presence his Majesty said that before the great astronomer could discuss science there was a little matter of business that must be disposed of. The King accordingly handed Herschel a paper, in which he was, I dare say, greatly surprised to find a pardon to the deserter written out by the King himself.

Then Herschel unfolded his wonderful discovery, which the King thoroughly appreciated, and in the evening the telescope was set up in the gardens, and the glories of the heavens were displayed. Herschel made a most favorable impression on his Majesty, and when the King told the ladies of the Castle next day of all that Herschel had shown him, their astronomical ardor was also aroused, and they asked to see through the marvellous tube. Of course Herschel was ready to comply, and the telescope was accordingly carried to the windows of the Queen’s apartments at Windsor,239 which would have commanded a fine view if the clouds had not been in the way, which they unfortunately were. Even for royalty the clouds would not disperse, so what was to be done? Herschel was equal to the occasion. He specially wanted to exhibit Saturn, for it is one of the most beautiful objects in the sky, and will fascinate any intelligent beholder. No astronomers would have been able to see Saturn through the clouds, but Herschel did not disappoint his visitors; he directed the instrument, not to the sky (nothing was there to be seen); he turned it towards a distant garden wall. Now what would you expect to see by looking through a telescope at a garden wall—bricks, perhaps, or ivy? What these ladies saw was a beautiful image of Saturn, his globe in the centre and his rings all complete, forming so true a resemblance to the planet that even an experienced astronomer might have been deceived. In the afternoon Herschel had seen that the clouds were thick, and that there would be little probability of using the telescope properly. Accordingly he cut out a little image of Saturn, illuminated it by lamps, and set it up at a suitable distance on a garden wall.

Herschel’s visit to Windsor was productive of important consequences. The King said it was a pity that so great an astronomer should devote himself to music, and that it would be far better for him to give up that profession and come and live at Windsor. His Majesty promised that he would pay him a salary, and he also undertook to provide the cost of erecting great telescopes. His faithful sister Caroline came with him as his assistant, and also received some bounty from the240 King. From that moment Herschel renounced all his musical business, and devoted himself to his great life-task of observing the heavens.

He built telescopes of proportions far exceeding those that had ever been then thought of. He used to stand at night in the open air from dusk to dawn gazing down the tube of his mighty reflector, watching the stars and other objects in the heavens as they moved past. He would dictate what he saw to Caroline, who sat near him. It was her business to write down his notes and to record the position of the objects which he was describing. Sometimes, she tells us, the cold was so great that the ink used to freeze in her pen when she was at this work. Until he became a very old man, Herschel devoted himself to his astronomical labors. His discoveries are to be counted by thousands, though not one of them was so striking or so important as the detection of the new planet which first brought him fame.

The question of a name for the addition to the sun’s family had, of course, to be settled. Herschel had surely a right to be heard at the christening, and as a compliment to his Majesty he named the stranger the Georgium Sidus. So, indeed, for a brief while, the planet was actually styled. The Continental astronomers, however, would not accept this designation; all the other planets were named after ancient divinities, and it was thought that the King of England would seem oddly associated with Jupiter and Saturn; perhaps also they considered that the British dominions, on which the sun never sets, were already quite large241 enough, without further extension to the celestial regions. Accordingly a consultation was held, the result of which was that George III. was deprived of his planetary honors, and the body was given the name of Uranus, which, by universal consent, it now bears.

The planet Uranus lies just on the verge of visibility with the unaided eye. It can sometimes be glimpsed like a faint star, and, of course, with a telescope it is readily perceived. Many generations of astronomers before Herschel’s time had been observing the heavens, making maps of the stars, and compiling great catalogues in which the places of the stars were accurately put down. It often happened that Uranus came under their notice, but it never occurred to them that what seemed so like a star was really a planet. I have, no doubt, said that Uranus looked unlike a star when Herschel examined it; but then that was because Herschel was a particularly skilful astronomer. To an observer of a more ordinary type Uranus would not present any very remarkable appearance, and would be passed over merely as a small star. In fact, the planet was thus observed not once or twice, but no fewer than seventeen times, before the acute eye of Herschel perceived its true character. On many previous occasions the planet had been noted as a star by astronomers who are in every way entitled to our respect. It required a Herschel, determined to see everything in the very best manner, to grasp the discovery which eluded so many others.

When Uranus was observed on these former occasions242 and mistaken for a star, its place had been carefully put down. These records are at present of the utmost use, because they show the past history of the planet; and they appear all the more valuable when we remember that Uranus requires no less than eighty-four years to accomplish a single revolution around the sun. Thus, since the planet was discovered in 1781, it had completed one revolution by 1865, and is now (1899) about one-third of the way around another. The earlier observations extend backwards almost 200 years, so that altogether we have more or less information about the movements of the planet during the completion of two circuits and a half.

Uranus is a great deal bigger than the earth, as you will see in the view of the comparative sizes of the planets (Fig. 47). It appears to be of a bluish hue, but we cannot tell whether it turns round on its axis, or rather, I should say, we are not able to see whether it turns round on its axis; for we can hardly doubt that it does so.

Notwithstanding that Uranus is at so great a distance from the earth, we have been able to put this planet, no less than the nearer ones, in the weighing scales, and we assert with confidence that Uranus is fifteen times as heavy as our earth. We are indebted to the satellites for this information.

THE SATELLITES OF URANUS.

You must use a very good telescope to see the satellites of Uranus. They are four in number, bearing243 the names of Ariel, Umbriel, Titania, and Oberon. The innermost of these, Ariel, completes a journey round the planet in two days and a half; Oberon, the most distant, requires thirteen days and a half. A planet is always tending to pull its satellite down, and the satellite is kept from falling by the speed with which it revolves. The heavier the planet, the faster must its satellites go round. Thus, to take an illustration from our own moon, we know that, if the earth were to be made four times heavier than it is, the moon would have to spin round twice as fast as it does, in order to remain in the same orbit. The speed with which the satellites of Uranus revolve accordingly affords a measure of the mass of the planet. Were Uranus heavier than he is, his satellites would revolve more quickly than they do; were he lighter, the satellites would take a longer period to go round.

Uranus also seems to be greatly swollen by clouds, in the same manner as are both Jupiter and Saturn; in fact, if our earth was as big as Uranus, it would weigh four or five times as much as Uranus does. Hence we are certain that Uranus must consist of materials less dense on the whole than are those of which our earth is made.

There is another singular circumstance connected with the moons of Uranus. I have told you how every body revolving round another by gravitation will describe an ellipse; but, of course, there are many different kinds of this curve, and some may be nearly circles. There is nothing whatever to prevent a satellite from revolving around its primary in an exact circle if it be244 started properly; that is, in the right direction and with the right speed. All the four satellites of this planet seem to revolve in circles so perfect that we can make an accurate picture of this system with a pair of compasses. It is further to be noticed that the four circles seem to lie exactly in the same plane. The orbits of the other great planets and of their satellites lie in planes inclined at angles of less than 35° to the ecliptic, the plane in which the earth moves. Here again the satellites of Uranus are exceptional. The plane in which they are contained stands up almost squarely to the plane in which the motion of the planet is performed. The moons of Uranus seem to have got a twist, from some accidental circumstance for which we are not able to account.

THE DISCOVERY OF NEPTUNE.

The boundaries of the solar system had been much extended by the discovery of Uranus, but they were destined to receive still further enlargement by the detection of another vast planet, revolving far outside Uranus, the orbit of which forms, according to our present knowledge, the outline of the planetary system.

I have here to describe one of the greatest discoveries that have ever been made. It is not the magnificence of the outermost planet itself that I refer to, though, indeed, it is bigger than Uranus. I am rather thinking of the way in which the discovery was made. I do not mean any disrespect to Herschel when I say that the discovery of Uranus was chiefly a stroke of245 good fortune; but I may be permitted to describe it in this manner by way of emphasizing as strongly as I can how utterly different was the train of ideas which led to the discovery of Neptune. Herschel merely looked at one star after another till suddenly he dropped on the planet, having beforehand not the slightest notion that any such planet was likely to exist. But Neptune was shown to exist before it was ever seen, and, in fact, the man that first saw the planet, and knew it to be a planet, was not the discoverer. This is rather a difficult subject; and it would take you years of hard study to be able to understand the train of reasoning by which Neptune was found. I shall, however, make an attempt to explain this matter sufficiently to give at least some idea of the kind of problem that had to be solved.

You will remember that law of Kepler which tells us that every planet moves round the sun in an ellipse. If the planet be uninterfered with in any way and guided only by the attraction of the sun, it will forever continue to describe precisely the same ellipse without the slightest alteration. It was ascertained that the path which Uranus followed was not always regular. The early observations of the planet, when it was mistaken for a star, have here been of the utmost service. They have indicated the ellipse which Uranus described the last time it went round, and our modern observations have taught us the path which the planet is at present describing. These two ellipses are slightly different, and the consequence is that, supposing we take the observations of Uranus made 100 years ago, and calculate from them where Uranus ought to be now, we246 find that the planet is a little astray. Astronomers are not accustomed to be wrong in such calculations, and when discrepancies arise, the first thing to be done is to see what has caused them. It is certain that the position in which Uranus is found this very night, for example, is not what it would have been had the sun alone been guiding the planet. Perhaps you will think that it is impossible for reliable computations to be made about such matters; but I assure you they can, and the very fact that the motion of Uranus appeared to be irregular made it interesting to try and find out the cause of the disturbance.

I have already explained, when speaking about Mars (p. 187), that there is an attraction between every two bodies, but in the group of planets to which the earth belongs the sun’s attraction is so much stronger than any other force that all the movements are guided mainly by it. Nevertheless it is true that not only does the sun pull our earth and all the other planets as well, but all the planets, including the earth, are pulling one another. In fact, there is an incessant struggle going on in the family party. Fortunately the sun is so much more powerful than any other member, that he keeps them all pretty well in order; and unless you look very carefully you will not see the effects of the little struggles that are going on between every pair of the system. Our earth itself is pulled and swayed to and fro by the actions of its brothers and sisters. It is dragged perhaps a thousand or two thousand miles this way by Jupiter, or it gets a good tug in the other direction by Venus. Mars and Saturn also do their247 little best to force the earth away from its strict path. However, our earth does not suffer much from these irregularities. It pursues its route fairly enough, just as a coach from London to Brighton will get safely to its destination notwithstanding the fact that it has to swerve a little from its path whenever it meets other vehicles on the way, or when the coachman wishes to avoid a piece of the road on which stones have been freshly laid down.

The track followed by Uranus was found to be somewhat irregular, like that of every other planet. Jupiter gave it a pull, and so did Saturn, and at first it was thought that the irregularities which were perceived could be explained by the action of these planets, so big and so well known. Here is a question for calculation; it involves a very long and a very hard piece of work, but it is possible to estimate how far each of the other planets is capable of dragging Uranus from its path. Is it not remarkable that by working out long calculations we should be able to find what one planet hundreds of millions of miles away was able to do to another planet still further off, and not only for to-day or yesterday, but for past time extending over more than a century? If, however, you will listen to me a little longer, I think I shall give you a proof that these sums could be worked out correctly.

When the calculations had been made which showed how much the known planets could disturb Uranus, it was found that there were still some deviations of the planet that remained unexplained. They were not large; they only amounted to showing that the body was just248 a little astray from the spot where the calculations indicated it should be. The rest of astronomy was so perfect, and the law of attraction prevailed so universally, that it was thought the law of attraction must provide some way of explaining the behavior of Uranus. He could not have left his track of his own accord; therefore there must be some agency at work upon him of which we did not know. What could this unknown source of disturbance be? Every such trouble had hitherto been found to be a consequence of the attraction of gravitation; therefore there must be some unknown body pulling at Uranus which no one had ever seen. Where could it be? How was it to be discovered? Such were the questions that were asked, and they were answered in a most satisfactory manner.

First of all, what sort of body could it be that was pulling Uranus? It is obvious that none of the stars would be competent to produce so great an effect; they are all so far off that they have nothing whatever to say to any of the domestic matters in our little solar system, which is simply a group by itself. It would be more reasonable to suppose that there must be yet another planet which nobody had ever recognized, but which affected Uranus so as to account for his truant behavior. To begin to search for this planet with telescopes without some guidance would be futile; in fact, astronomers had been scanning the heavens for planets for nearly fifty years, and though several had been discovered, they all belonged to the zone of little planets, and none of them were big enough to pull Uranus about appreciably. Of course, if all the stars could be blotted249 out of the sky, so that nothing but planets were left, then, by sweeping the telescope over the heavens, every planet that exists might be speedily found. The difficulty is that the planets, which are either small or very distant, look so like the stars that it is impossible to recognize them among the millions of glittering points in the sky. It was, however, hoped that the unknown planet would be large enough to be visible in the telescope, if only we knew exactly where to point it.

Two illustrious astronomers, Adams of Cambridge, and Leverrier of Paris, both separately undertook an astonishing piece of calculation. They tried to find out the position of the unknown planet from the mere fact that it deranged Uranus in a particular way. I dare say many of those who are reading this book have learned simple equations in algebra, and they have worked such questions as to find the length of a pole, half of which is in mud, a quarter in water, and ten feet above the water. Those who know this much can perhaps realize the problem that had to be solved in trying to discover the unknown planet. So difficult a question as this had to be solved in a way that your masters would hardly allow you to use when working your sums in algebra. I do not think they would let you make a series of guesses. Let us try 20 feet, for instance, as the length of the pole; that will make 10 feet in the mud, 5 feet in the water, and 5 feet outside. This will not do; it is not enough; we must try again; and after another guess or two, we see that a pole 40 feet long will exactly answer. We do not use this method of guessing in algebra, because solving the250 simple equation is a much better method. Adams and Leverrier found that to discover the unknown planet was a question so very difficult, that they were obliged to use a sort of guessing, but very intelligent guessing, I need hardly assure you. They proceeded in this way (Fig. 71). They would draw a circle outside the path of Uranus, and then suppose that a planet was revolving in that circle. Its effect upon Uranus would then be calculated, and it would be found whether the observed irregularities could be in this manner accounted for. The first planet they tried was not the right one; then they began again with another, until at last, after many trials and much very hard work, they saw that there might be a planet in a particular path far outside Uranus, such that if this planet were of the right weight and moving with the right speed, then it would pull Uranus exactly in the way that astronomers had observed it to be pulled. They found at last that there251 could be little doubt about the matter; for this unknown body would account for all the facts. Then, indeed, they had solved their equation; they had found the unknown.

Fig. 71.—Orbits of Uranus and Neptune.

The two great astronomers had thus discovered a planet, but as yet it was only a planet on paper. Those who could judge of the subject had no doubt that the planet was really in the sky; but just as you like to prove that you have found the correct answer to your sum, so people were naturally anxious to prove the truth of this wonderful sum that Adams and Leverrier had worked out. This was to be done by actually seeing the planet of which the astronomers had asserted the existence. Leverrier calculated that the new planet in a certain night would be in a particular position on the sky. Accordingly he wrote to Dr. Galle, of the observatory at Berlin, requesting him on the evening in question to point his telescope to the very spot indicated, and there he would see a planet which human eyes had never before beheld. Of course, Dr. Galle was only too delighted to undertake so marvellous a commission. The evening was fine; the telescope was opened; it was directed towards the heavens; and there, in the very spot which the calculations of Leverrier had indicated, shone the beautiful little planet. At Cambridge arrangements had also been made to search for the new member of the solar system, in accordance with Professor Adams’ calculations. There also the planet that had given all this trouble to Uranus was brought to light. At first it looked like a star, as all such planets do; but that it was not a star was speedily proved, by252 the two tests which are sure indications of a planet. First the body was so moving that its position with respect to the adjacent stars was constantly changing. Then, when a strong magnifying power was placed on the telescope, the little object was seen, not to be a mere starlike point, but to expand into the little disk which shows us we are not looking at a distant sun, but at a world like our own.

Was not this truly a great discovery? Have we not shown you how entitled the calculations of astronomers are to our respect, when we find that they actually discovered the existence of a majestic planet before the telescope had revealed it? See also the greatly increased interest that belongs to Herschel’s discovery of Uranus. We can hardly imagine anything that would have given more gratification to this old astronomer than to think that his Uranus should have given rise to a discovery even more splendid than his own. He died, however, more than twenty years before this achievement.

The authorities who decide on such matters christened the new planet Neptune; and this body wanders round on the outskirts of our solar system, requiring for each journey a period of no less than 165 years. The circle thus described has a radius thirty times as great as that of the earth’s track.

Neptune is altogether invisible to the unaided eye, but it is sufficiently bright to have been occasionally recorded as a star. Indeed, nearly fifty years before it was actually discovered to be a planet it had been included by the astronomer Lalande in a list of stars he was observing. A curious circumstance was afterwards253 brought to light. When reference was made to the books in which Lalande’s observations were written, it was found that he had observed this object twice, namely, on May 8 and May 10, 1785. Of course, if the object had indeed been a star its position on the two days would have been the same, but being a planet it had moved. When Lalande, on looking over his papers, saw that the places of this supposed star were different on the two nights, he concluded that he must have made a mistake on the first night, and accordingly treated the object as if the place on the 10th was the right one. Just think how narrowly Lalande missed making a discovery! Unhappily for his renown, he took it for granted that one or both of his observations were erroneous, and so they must have been if the object had been a star. But they were both right; it was the planet which had moved in the interval.

As Neptune is half as far again from the earth as Uranus, we can hardly expect to learn much about the actual nature of the planet. We do know that it has four times the diameter of the earth, so that it exceeds the earth in the same proportion that the earth is larger than the moon.

Like the other great planets, Neptune is also enveloped with copious clouds; in fact, it only weighs one-fifth part as much as it would do if it were made of materials as substantial as are those of the earth. Like our earth, Neptune is attended by one moon, which revolves round the planet in a little more than six days.

The orbit of this great planet marks the boundary254 of our known system of planets. We have seen how the five great planets of antiquity have been increased in these modern days by the addition of two more, Uranus and Neptune, while the discovery of a multitude of small planets has given a further increase to the number of the sun’s family. We have still some other objects in our solar system to describe; some of them are excessively big; these are the comets. Some of them are exceedingly small; they are the shooting stars. We shall talk about comets and shooting stars in our next lecture.

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