The Story of the Heavens

The Great Size of Jupiter—Comparison of his Diameter with that of the Earth—Dimensions of the Planet and his Orbit—His Rotation—Comparison of his Weight and Bulk with that of the Earth—Relative Lightness of Jupiter—How Explained—Jupiter still probably in a Heated Condition—The Belts on Jupiter—Spots on his Surface—Time of Rotation of different Spots various—Storms on Jupiter—Jupiter not Incandescent—The Satellites—Their Discovery—Telescopic Appearance—Their Orbits—The Eclipses and Occultations—A Satellite in Transit—The Velocity of Light Discovered—How is this Velocity to be Measured Experimentally?—Determination of the Sun's Distance by the Eclipses of Jupiter's Satellites—Jupiter's Satellites demonstrating the Copernican System. In our exploration of the beautiful series of bodies which form the solar system, we have proceeded step by step outwards from the sun. In the pursuit of this method we have now come to the splendid planet Jupiter, which wends its majestic way in a path immediately outside those orbits of the minor planets which we have just been considering. Great, indeed, is the contrast between these tiny globes and the stupendous globe of Jupiter. Had we adopted a somewhat different method of treatment—had we, for instance, discussed the various bodies of our planetary system in the order of their magnitude—then the minor planets would have been the last to be considered, while the leader of the host would be Jupiter. To this position Jupiter is entitled without an approach to rivalry. The next greatest on the list, the beautiful and interesting Saturn, comes a long distance behind. Another great descent in the scale of magnitude has to be made before we reach Uranus and Neptune, while still another step downwards must be made[Pg 246] before we reach that lesser group of planets which includes our earth. So conspicuously does Jupiter tower over the rest, that even if Saturn were to be augmented by all the other globes of our system rolled into one, the united mass would still not equal the great globe of Jupiter. Fig. 56.—The Relative Dimensions of Jupiter and the Earth. The adjoining picture (Fig. 56) shows the relative dimensions of Jupiter and the earth, and it conveys to the eye a more vivid impression of the enormous bulk of Jupiter than we can readily obtain by merely considering the numerical statements by which his bulk is to be accurately estimated. As, however, it will be necessary to place the numerical facts before our readers, we do so at the outset of this chapter. Jupiter revolves in an elliptic orbit around the sun in the focus, at a mean distance of 483,000,000 miles. The path of Jupiter is thus about 5·2 times as great in diameter as the path pursued by the earth. The shape of Jupiter's orbit departs very appreciably from a circle, the greatest distance from the sun being 5·45, while the least distance is about 4·95, the earth's distance from the sun being taken as unity.[Pg 247] In the most favourable circumstances for seeing Jupiter at opposition, it must still be about four times as far from the earth as the earth is from the sun. This great globe will also illustrate the law that the more distant a planet is, the slower is the velocity with which its orbital motion is accomplished. While the earth passes over eighteen miles each second, Jupiter only accomplishes eight miles. Thus for a twofold reason the time occupied by an exterior planet in completing a revolution is greater than the period of the earth. Not only has the outer planet to complete a longer course than the earth, but the speed is less; it thus happens that Jupiter requires 4,332·6 days, or about fifty days less than twelve years, to make a circuit of the heavens. The mean diameter of the great planet is about 87,000 miles. We say the mean diameter, because there is a conspicuous difference in the case of Jupiter between his equatorial and his polar diameters. We have already seen that there is a similar difference in the case of the earth, where we find the polar diameter to be shorter than the equatorial; but the inequality of these two dimensions is very much larger in Jupiter than in the earth. The equatorial diameter of Jupiter is 89,600 miles, while the polar is not more than 84,400 miles. The ellipticity of Jupiter indicated by these figures is sufficiently marked to be obvious without any refined measures. Around the shortest diameter the planet spins with what must be considered an enormous velocity when we reflect on the size of the globe. Each rotation is completed in about 9 hrs. 55 mins. We may naturally contrast the period of rotation of Jupiter with the much slower rotation of our earth in twenty-four hours. The difference becomes much more striking if we consider the relative speeds at which an object on the equator of the earth and on that of Jupiter actually moves. As the diameter of Jupiter is nearly eleven times that of the earth, it will follow that the speed of the equator on Jupiter must be about twenty-seven times as great as that on the earth. It is no doubt to this high velocity of rotation that we must ascribe the extraordinary ellipticity of Jupiter; the rapid[Pg 248] rotation causes a great centrifugal force, and this bulges out the pliant materials of which he seems to be formed. Jupiter is not, so far as we can see, a solid body. This is an important circumstance; and therefore it will be necessary to discuss the matter at some little length, as we here perceive a wide contrast between this great planet and the other planets which have previously occupied our attention. From the measurements already given it is easy to calculate the bulk or the volume of Jupiter. It will be found that this planet is about 1,300 times as large as the earth; in other words, it would take 1,300 globes, each as large as our earth, all rolled into one, to form a single globe as large as Jupiter. If the materials of which Jupiter is composed were of a nature analogous to the materials of the earth, we might expect that the weight of the planet would exceed the weight of the earth in something like the proportion of their volumes. This is the matter now proposed to be brought to trial. Here we may at once be met with the query, as to how we are to find the weight of Jupiter. It is not even an easy matter to weigh the earth on which we stand. How, then, can we weigh a mighty planet vastly larger than the earth, and distant from us by some hundreds of millions of miles? Truly, this is a bold problem. Yet the intellectual resources of man have proved sufficient to achieve this feat of celestial engineering. They are not, it is true, actually able to make the ponderous weighing scales in which the great planet is to be cast, but they are able to divert to this purpose certain natural phenomena which yield the information that is required. Such investigations are based on the principle of universal gravitation. The mass of Jupiter attracts other masses in the solar system. The efficiency of that attraction is more particularly shown on the bodies which are near the planet. In virtue of this attraction certain movements are performed by those bodies. We can observe their character with our telescopes, we can ascertain their amount, and from our measurements we can calculate the mass of the body by which the movements have been produced. This is the sole method which we possess for the investigation of the masses[Pg 249] of the planets; and though it may be difficult in its application—not only from the observations which are required, but also from the intricacy and the profundity of the calculations to which those observations must be submitted—yet, in the case of Jupiter at least, there is no uncertainty about the result. The task is peculiarly simplified in the case of the greatest planet of our system by the beautiful system of moons with which he is attended. These little moons revolve under the guidance of Jupiter, and their movements are not otherwise interfered with so as to prevent their use for our present purpose. It is from the observations of the satellites of Jupiter that we are enabled to measure his attractive power, and thence to calculate the mass of the mighty planet. To those not specially conversant with the principles of mechanics, it may seem difficult to realise the degree of accuracy of which such a method is capable. Yet there can be no doubt that his moons inform us of the mass of Jupiter, and do not leave a margin of inaccuracy so great as one hundredth part of the total amount. If other confirmation be needed, then it is forthcoming in abundance. A minor planet occasionally draws near the orbit of Jupiter and experiences his attraction; the planet is forced to swerve from its path, and the amount of the deviation can be measured. From that measurement the mass of Jupiter can be computed by a calculation, of which it would be impossible to give an account in this place. The mass of Jupiter, as determined by this method, agrees with the mass obtained in a totally different manner from the satellites. Nor have we yet exhausted the resources of astronomy in its bearing on this question. We can discard the planetary system, and invite the assistance of a comet which, flashing through the orbits of the planets, occasionally experiences large and sometimes enormous disturbances. For the present it suffices to remark, that on one or two occasions it has happened that venturous comets have been near enough to Jupiter to be much disturbed by his attraction, and then to proclaim in their altered movements the magnitude of the[Pg 250] mass which has affected them. The satellites of Jupiter, the minor planets, and the comets, all tell the weight of the giant orb; and, as they all concur in the result (at least within extremely narrow limits), we cannot hesitate to conclude that the mass of the greatest planet of our system has been determined with accuracy. The results of these measures must now be stated. They show, of course, that Jupiter is vastly inferior to the sun—that, in fact, it would take about 1,047 Jupiters, all rolled into one, to form a globe equal in weight to the sun. They also show us that it would take 316 globes as heavy as our Earth to counterbalance the weight of Jupiter. No doubt this proves Jupiter to be a body of magnificent proportions; but the remarkable circumstance is not that Jupiter should be 316 times as heavy as the earth, but that he is not a great deal more. Have we not stated that Jupiter is 1,300 times as large as the earth? How then comes it that he is only 316 times as heavy? This points at once to some fundamental contrast between the constitution of Jupiter and of the earth. How are we to account for this difference? We can conceive of two explanations. In the first place, it might be supposed that Jupiter is constituted of materials partly or wholly unknown on the earth. There is, however, an alternative supposition at once more philosophical and more consistent with the evidence. It is true that we know little or nothing of what the elementary substances on Jupiter may be, but one of the great discoveries of modern astronomy has taught us something of the elementary bodies present in other bodies of the universe, and has demonstrated that to a large extent they are identical with the elementary bodies on the earth. If Jupiter be composed of bodies resembling those on the earth, there is one way, and only one, in which we can account for the disparity between his size