equality existed between the mean distances and periods of Ceres and Pallas, as we find below :— Pallas' 262,960,000 miles. orbits of the two two worlds might Here we find the mean distances and periods so nearly equal, that, in case the planes of the planets had chanced to coincide, these travel side by side for a long while, and at a distance from each other only about double the distance separating the earth from her satellite. The distance between Mars and Ceres is no less than 120,000,000 of miles. The distance from Ceres' orbit to that of Jupiter is more than 280,000,000 of miles; and yet, here are two planets which may approach each other to within a distance less than half a million of miles. It is true, the eccentricities of the orbits differ greatly, and the inclinations of their orbital planes are also very great; so that Pallas, by this inclination, is carried far beyond the limits within which the planetary excursions north and south of the ecliptic had been previously confined; yet a time. would come in the countless revolutions of these remarkable worlds, when each would fill, at the same time, points of the common line of intersection of their orbital planes ; and these two points, owing to the revolutions of the perihelion, might, possibly, at some future period, come to coincide. In case these speculations were within the limits of the probable, and if it were permitted to anticipate in the future the possible collision or union of these minute planets, a like train of reasoning, running back into the past, would lead to the conclusion that in case their revolution had been in progress for unnumbered ages, there was a time in the past when these two independent worlds might have occupied the same point in space; and hence the thou they were fragments of some great plan sibly he power of some tremendous internal convulsion, had been burst into many separate fragments. This strange hypothesis was first propounded by Dr. Olbers, and has met with more or less favour from succeeding astronomers, even up to the present day, as we shall see hereafter. The True or false, it soon produced very positive results, for it occasioned a renewal of the research which had been discontinued after the discovery of Ceres, and in a few years two more planets were added to the list of asteroids. search was long continued, and it was not until the end of fifteen years that Olbers and his associates became satisfied that no more discoveries could be expected to reward their diligence. Thus it became a received doctrine, that, in case a large planet had been rent asunder by some internal explosive power, it had been burst into four pieces, and that no other fragments existed sufficiently large to be detected. even by telescopic power. This opinion prevailed up to December, 1845, when the astronomical world was somewhat startled by the announcement of a new asteroid, discovered by Hencke, of Dresden. This event awakened attention to this subject, and a new generation of observers entered the field of research, whose efforts have resulted in revealing a large group of small planets, of which no less than fifty-five have already been discovered, and their orbits computed.* The theory of the disruption of one great planet as the origin of the asteroids has been revived and extensively discussed, but thus far no satisfactory conclusion has been reached. So strangely are the orbits of these bodies related to each other, that, in case they all lay on the same plane, they would in some instances intersect each other, exhibiting relations nowhere else found in the solar system. None of the asteroids are visible to the naked eye, nor are they distinguishable from the stars with the telescope except under the most favourable circumstances. When carefully watched, * Three more of these Asteroids have been lately discovered; making a total of fifty-eight. some of them exhibit rapid changes in the intensity of their light, sometimes suddenly increasing in brightness, and again as rapidly fading out. These changes have been accounted for on the supposition that these worlds are indeed angular fragments; and that, rotating on an axis, they sometimes present large reflective surfaces, and again angular points, from whence but a small amount of light reaches the earth. As the stars of the smaller magnitudes are becoming more extensively and accurately charted, their places being determined with great precision, we may anticipate a large increase in the number of known asteroids during the remainder of the current century, and so forward; for, if so great a multitude has already been revealed almost without effort, and nearly by accident, what must be the result when a systematic scheme of examination shall have been executed, based on an accurate knowledge of the places of all the stars down to the twelfth magnitude? We have just ground for supposing that there are thousands of these little worlds revolving in space. CHAPTER VII. JUPITER, ATTENDED BY FOUR MOONS, THE SIXTH PLANET IN THE ORDER OF DISTANCE FROM THE SUN. Arc of Retrogradation.-Stationary Point.-Distance of the Planet determined.-Periodic Time.-Synodical Revolution gives the Sidereal.— Surface of Jupiter as given by the Telescope.-Period of Rotation.Diameter.-Volume.-Mean Distance.-Amount of Light and Heat.Figure of Jupitor.-Equatorial and Polar Diameters.-Discovery of the Four Moons by Galileo.-Effect on the Copernican Theory.-Jupiter's Nocturnal Heavens. THE SATELLITES OF JUPITER. -How discovered.-Their Magnitude.— Form of their Orbits.-Period of Revolution.-Eclipses.-Transits.Occultations.-Velocity of Light discovered.-Terrestrial Longitude.Rotation of these Moons on an Axis. IN passing from the diminutive asteroids to the magnitude and splendour which distinguish the vast orb which holds the next position in the planetary system, we are the more disposed to adopt the theory that the exceeding disparity now existing in the magnitude of these neighbouring worlds is due to the fact that the asteroids are but a few of the fragments of some object in which they were all once united. We shall hereafter present a speculation on this subject, which seems entitled to consideration. The planet Jupiter is one of the five revolving worlds discovered in the primitive ages. Its revolution among the fixed stars is slow and majestic, comporting well with its vast dimensions, and the dignity conferred by four tributary worlds. Like all the old planets, the ancients had determined with considerable precision the period of revolution of Jupiter, and his relative position among the planetary worlds. The points in his orbit where he becomes stationary, the arc over which he retrogrades, and his period of retrogradation, were all pretty well determined from the early observations. As we recede to greater distances from the sun, the arc of retrogradation diminishes in extent, while the time employed in describing these arcs must by necessity increase. This will become evident if we recall to mind the cause of this apparent retrogradation. When the sun, earth, and planet are all on the same straight line, the earth and planet being on the same side of the sun, then the planet is exactly in opposition. The earth and planet starting from this line, as the earth moves the swifter in its orbit, at the end of, say twenty-four hours, the line joining the earth and planet will take a direction such, that it will meet the first line exterior to the orbit of the planet, as seen below: OEP is the line on which the three bodies are found on the day of opposition. At the end of, say twenty-four hours, the earth arrives at E' in its orbit, the planet at P', and then the planet is seen from the earth in the direction E' P'S', whereas on the day previous it was seen in the direction EPS. Thus it appears to have moved backwards from S to S' among the fixed stars, while in reality it has moved forward in its orbit from P to P'. Admitting the orbits to be circles and the motions to be uniform, it is very easy to locate the places of the earth and planet on successive days after opposition, and joining those places by straight lines, we should soon reach a position in which the lines thus drawn on consecutive days would be parallel. There the planet would appear stationary among the fixed stars, and there its advance would commence, as is manifest from the figure below E E EA in which S is the sun, EE' E" E" the successive places of the earth, PP' P" P" the successive places of the planet. The lines EP and E' P' meet on the side opposite the sun, the lines E' P' and E" P" also meet on the same side; but E"" P'"' and E" P" are parallels, and in P" the planet becomes stationary; and after passing this point, the earth still advancing, the lines joining the earth and planet meet on the side next the earth, and henceforward the motion of the planet, as seen from the earth, must continue to be direct, until the earth comes round again to occupy the conjunctio |