"Lighten our darkness,"-from his tower, 66 Lighten our darkness," '-scarce were spoken And lo!" at evening-time 'twas light."'1 RICHARD TOMLINS, M.A. OUR SOLAR SYSTEM. CHAPTER III. THE SECONDARY PLANETS. THE COMETS OF SHORT PERIOD. THE Secondary Planets, Moons, or Satellites, must now briefly engage our attention. They are, as has been said, twenty in number, unless we are to exclude two of the Moons which Sir W. Herschel assigned to the Planet Uranus. Of these Secondary Planets one is an attendant upon the Earth, four on Jupiter, eight on Saturn, six (or four) on Uranus, and one on Neptune. No Moon has yet been discovered which we may conclude to have been an attendant upon the destroyed Planet between Mars and Jupiter. With reference also to this Planet I would observe, that I nowhere find any computation of the astronomers as to its probable bulk. It were interesting to know what would be the size of a globe produced by the fusion of the fifty-five Planetoids at present known. I have here a diagram representing the comparative sizes of the Earth and its moon, and also to the same scale of the moons of Jupiter. The Earth is drawn with a diameter of 16 inches, its moon of 4 inches; Jupiter's first moon of 42 inches, his second moon of 44 inches, his third moon of 7 inches, (it is larger than the planet 1 Zech. xiv. 7. Mercury), and his fourth moon of 6 inches. I could not introduce Jupiter himself in this scale, for his diameter would have been above 15 feet. The dimensions of the eight satellites of Saturn are not ascertained; their names are Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus. Of these Titan is stated to be probably but little less than the planet Mars, and if so it is larger than any of the moons I have drawn. All the secondary planets revolve around their principals as these do about the Sun, they have also a rotation upon their several axes. In the case of our own moon the time of rotation is equal to the period of revolution about the earth, viz. 27 days 7 hours and 44 minutes; and it is conjectured that the same is the case with all the satellites, viz. that their revolutions around their principals, and their rotations on their axes, are effected in equal time. One result of this would be that the same face would always be presented to the principal planet. In the case of our own Moon it will be remembered that there was a fierce contest some few years since regarding its rotation. The fierceness it must be owned was mostly on one side, and strange to say, (as an illustration of the maxim that there is no rule without an exception), it was exhibited on the side of the truth. A little reflection will show you that the Moon must rotate upon its axis. For our present purpose we may consider the Earth to be the fixed centre of a circular orbit in which the moon travels. I place this ball upon the table to represent the Earth. This other ball, which is painted half black and half white, I use to represent the moon, and I make it to revolve about the earth. You see that now its white face is towards the earth. I hold the moon-ball so that it shall not rotate. It advances until it has described a quarter circle. What is the result? Of the face turned towards the earth, one half is white, the other black. Again it travels on for a quarter circle. What is to be seen now from the earth? A face entirely black. Once more it travels, and, at the end of the third quarter, a parti-coloured face is again presented to the earth. It advances to its first place, and the white face is presented as at the be ginning. Every part of the moon's surface has been seen from the earth. Now I will make it go round again, but this time keeping the white face always to the earth. I can only do so if I make the moon-ball slowly rotate, and complete its rotation in the same time as its revolution. It is a very simple, but a very sufficient proof of the point disputed; we can only wonder that an intelligent man, one of Her Majesty's Inspectors of Schools, should have been found so long and so resolutely to maintain an astronomical heresy. The interesting topics connected with our Moon, its physical condition, its influence on the tides, weather, and sanitary estate of the Earth; the phenomena of its phases and eclipses, the character of the path which it actually describes in space, would alone suffice to form the subject of a Lecture. I dare not enter upon them now. You will be anxious to get to the fourth branch of our inquiry-the Comets, and so am I: with a few words then on the rings of Saturn I will hasten on to that part of our programme. The true character of Saturn's rings was not ascertained until the year 1659. Before that the appearance of the planet, as seen through the imperfect telescopes of the time, had been taken for a flattened oblong oval, approaching to the form of an elongated rectangle, rounded off at the corners. An improvement of the telescope presented the planet with a great central disk, with two smaller disks, one on each side. Again these smaller disks took the form of handles or ears, like the handles of a vase or jar, and they were accordingly called the ansa of the disk. But in the year which I have named, 1659, Huygens explained the true cause of the phenomenon, and showed that the planet is surrounded by a ring of opaque solid matter. This ring, as subsequent observations have shown, is nearly but not precisely concentric with the planet, and in the plane of its equator. Its thickness is very inconsiderable when compared with its surface. It has also been ascertained that it is divided into two portions, so that we ought rather to say that Saturn is surrounded by two concentric rings. If we view the ring as one, its entire breadth is 29 thousand miles; but considering it as really two rings, the exterior ring is 10 thousand miles broad, the interior ring 17 thousand, and the breadth of the interval 13 thousand miles. The interval between the inner ring and the planet is 19 thousand miles. The thickness of the ring cannot exceed 250 miles. In the diagram the diameter of the planet is represented by 6 inches 7 lines, the first interval by 1 inch 7 lines, the inner ring by 1 inch 5 lines, the second interval by 12 lines, and the outer ring by 10 lines. The thickness of the ring, on this scale, is more than represented by the thickness of the paper. More recent observations have shown that the outer ring is double; some have even said septuple, but this is not confirmed. We may understand at least that Saturn is surrounded by three luminous rings, the planes of which do not exactly agree, and of which the edges are not regularly circular, but notched and dinged; the surface of the rings being, we may conclude, characterised by considerable mountainous undulations. There is besides these rings a fourth, situated between the inmost luminous ring and the planet, which itself is only imperfectly reflective, (presenting a very feeble illumination), and is partially transparent, allowing the body of the planet to be seen through it. All the rings of Saturn have a revolution about their common centre and in their own plane; the period of each ring being, it is conjectured, very nearly the same as the period of a satellite whose distance from the centre of the planet would be equal to that of the middle point of the breadth of each ring. The drawing which I have here represents the general appearance of Saturn and his rings; it is copied from one by Mr. Dawes, of Wateringbury, near Maidstone, taken in November, 1852.1 We come at length to the Comets, the fourth division of the members of our Solar System, and the immediate occasion of my present lecture. Humboldt speaks of them as before said-as being in number "myriads." Kepler, long ago, declared that "there are more comets 1 Lardner's Handbook of Astronomy, Vol. I. Plate xii. in space, than fishes in the ocean." And M. Arago has shown from the common principles of the doctrine of probabilities that the number of comets which have passed through our system cannot be less than 3 millions, but that it is possible that they may amount to twice that number. These statements may seem very surprising to those who only consider how seldom it is that a comet is visible in the heavens to the naked eye; but the truth is that while these strange visitors can seldom be seen without the telescope, astronomers observe several with their instruments, in the course of every year. Thus in the course of the present year (1858) eight comets have been observed, and as many were observed last year. Again, there is no reason to doubt that many comets altogether elude observation by their thinness and paleness, and that others again are beyond the reach of any glass. Some of the periodic comets, when they do return to us, are unfavourably placed for observation; and others yet again are seen only in latitudes where no observers are to be found. To approach at all precisely to a statement of the number of the comets is quite impossible in the present state of cometary knowledge. For our consideration of the subject before us, it will be convenient to divide the comets which have been observed and catalogued into three great classes, according to the orbits or paths in which they move, whether ellipses, parabolas or hyperbolas. I do not wish to overwhelm you with hard words, and these are not hard of explanation, as I shall presently show. I must first state, as briefly as I can, the two chiefest of the great principles which it is the glory of Sir Isaac Newton to have proclaimed, as being those under which the infinite wisdom of the Great Creator has appointed that all the bodies of the universe shall harmoniously move together in their courses. The first of these is that universal law of gravitation, to which I have already briefly alluded, when speaking of the centripetal force which counteracts the centrifugal force of bodies in motion. Stated abstractedly, that law is this: "Every particle of matter in the universe attracts every other particle, with a force di |