Part 13

Up till quite recently all the small planets which had been discovered confined themselves to the space lying between the paths of the major planets Mars and Jupiter. This invariable rule was, however, departed from in the case of one of these bodies which was discovered in August, 1898. This little body, which was known for some time by the provisional appellation of D Q, and which has now been definitely christened Eros, is an exception to this rule. It travels at an average distance from the sun actually less than that of Mars, and at the nearest point can come within 15,000,000 miles of the earth.

We occasionally get information from these little bodies; for in their revolutions through the solar system, they sometimes pick up scraps of useful knowledge, which we can elicit from them by careful examination. For example, one of the most important problems in the whole of astronomy is to determine the sun’s distance. I have already mentioned one of the ways of doing this, which is given by the transit of Venus. Astronomers never like to rely on a single method; we are therefore glad to discover any other means of solving the same problem. This it is which the little planets will sometimes do for us. Juno on one occasion approached very close to the earth, and astronomers in various parts of the globe observed her at the same time. When they compared their observations they measured the sun’s distance. But I am not going to trouble you now with a matter so difficult. Suffice it to say, that for this, as for all similar investigations, the observers were constrained to use the very same principle as that which we illustrated in Fig. 5.

Let me rather close this lecture with the remark that we have here been considering only the lesser members of the great family which circulate round the sun, and that we shall speak in our next lecture of the giant members of our system.

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, and 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.

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. 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. 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.

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 has 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 ball 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 intensity 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 a 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 can 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 true 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.

We 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. 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.

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 few 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 very 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, and 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 was 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.