Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/878

It has been shown by Leverrier, from the observed secular motion of the perihelion of Mars, that the combined mass of all the asteroids cannot exceed one-fourth of the earth s mass, even if the whole perturbation were ascribed to asteroids, and no account taken of the error in the estimated mass of the earth compared with the sun s. But as the recent nev estimate of the sun s distance assigns a rela tively increased mass to the earth, almost sufficient of itself to account for the observed motion of the perihelion of Mars, it appears to follow that the combined mass of the asteroids must be exceedingly small. Prof. Newcomb, of Washington, after carefully analysing the motions of the asteroids, comes to the conclusion, that, " though there are some peculiarities in the mutual relations between the orbits of these bodies which might favour Olbers s hypo thesis, a much greater number of peculiarities negative the assumption."

Prof. Kirkwood, of Bloomington, Indiana, has shown that, when the mean distances of the minor planets are arranged in order of magnitude, certain gaps are recognised, that is, there are no asteroids having mean distances nearly equal to certain definite values. These values correspond to distances at which asteroids would revolve in periods associated with Jupiter s period by certain simple laws of commensurability. He infers, therefore, that the asteroids had an origin resembling that assigned to them by the nebular hypothesis. He com pares the observed peculiarity to the existence of at least one great gap in the Saturnian ring system, showing that " a satellite revolving within that gap would have a period associated with the periods of Saturn s inner satellites, by similar simple laws of commensurability."

Chapter XIV.—The Planet Jupiter.

Jupiter is the largest planet of the solar system. Indeed lie surpasses the rest so greatly, that the combined mass of all of them togc fl er would barely exceed two-fiftH of his. This superiority is deserving of more consideration than it has commonly received. If we are justified in regarding our moon or the asteroids as belonging to a different order of bodies from the earth, Venus, Mars, and Mercury, because so much less than them in mass, we may not unreasonably regard Jupiter as belonging to a different order, because so much exceeding in mass the planets which travel within the zone of asteroids. The earth exceeds the moon only 81 times in mass, but Jupiter exceeds the earth in mass more than 300 times.

Jupiter travels at a distance from the sun exceeding the earth s 5J- times, or at a mean distance of 475,692,000 miles. The eccentricity of his orbit is considerable, amounting to 048239, so that his greatest and least distances amount respectively to 498,639,000 miles and 452,745,000 miles. When in opposition, and at his mean distance, his distance from the earth amounts to about 361,000,000 miles. His orbit is inclined about 1 18 40" 3 to the ecliptic. His sidereal revolution is completed in 4332-5848 days, or 11 years 314-92 days; whence it is easily calculated that his synodical period, or the interval separating his mean returns to opposition, has a mean value of 398 867 days. His mean diameter is nearly 85,000 miles in length, according to the best modern measurements, though strict accuracy cannot be claimed for this estimate, as the various determinations, even by the most skilful observers, differ considerably inter se. Until recently, the compression of his globe had been estimated at yL, from the observations of W. Struve ; but later measurements give T V as the compression. Hia volume exceeds the earth s 1233 205 times, but his density is only about one- fourth of hers, and his mass does not exceed hers more than 301 times.

Although the actual order of discovery would lead us to Jupiter s speak of the satellites of Jupiter before considering the belts, features of his globe, it will be for several reasons more convenient to describe these features first. Cassini was the first to discover that Jupiter s globe is surrounded by belts apparently belonging to his surface. The features of these belts as described by Cassini are those which still continue to be recognised. Their number is variable ; sometimes only one can be readily discerned, while at others the whole surface is covered by them. They are generally parallel to one another, as in fig. 45, but not always so. Their breadth is likewise variable, one belt having been observed to grow narrow, while another in its neighbourhood has increased in breadth, as if one had partially flowed into the other ; and in this case a part of an oblique belt is commonly recognised between them, as if forming the channel of communication. Some of these peculiarities are illustrated in figs. 46, 47, and 48. Sometimes the belts remain unchanged for months ; at others, new belts have formed in an hour or two. Although his surface appears Rotation. thus variable, yet at times spots have been visible for several weeks, whose motions have served to indicate the law of the planet s rotation.

FIG. 45. Jupiter as seen Nov. 27, 1857, by Dawes.

FIG. 46. Jupiter us seen Nov. c . 18, 1858, by Lassell.

FIG. 47. Jupiter as seen March 12, 1860, by Jacob.

FIG. 48. Jupiter as seen April 9, 1860, by Baxendell.

In 1665 Cassini observed a spot near the large central belt of Jupiter. It moved more quickly when near the middle of the disk than when near the edge, and also appeared narrower when at the edge, showing that it belonged to the surface (that is, to what appears as the

surface) of the planet. This principal or ancient spot, as