Jupiter’s true atmosphere encompasses the disturbed shell of vapors observed telescopically. Its general absorptive action upon light is betrayed by the darkening of the planet’s limb—another point of resemblance to the sun; while its special, or selective, absorption can only be detected with the spectroscope.
The actinic power of Jupiter’s light is very remarkable. It surpasses that of moonlight nine times, and that of Mars twenty-four times. Dr. Lohse further ascertained that the Southern Hemisphere is twice as chemically effective as the Northern. This superiority is doubtless connected with the greater physical agitation of the same region. A series of photographs of Jupiter, taken in 1891 with the great Lick refractor, were the first of any value for purposes of investigation.
Jupiter’s satellites were the first trophies of telescopic observation. They are, indeed, bright enough for naked-eye perception, could they be removed from the disk which obscures them with its excessive splendor; and the first and third have actually been seen, in despite of the glare, by a few persons with phenomenally good eyesight. The mythological titles of the Galilean group—Io, Europa, Ganymede, and Calypso (proceeding from within outward)—have been superseded by prosaic numbers.
The Jovian family presents an animated and attractive spectacle. The smallest of its original members (No. II) is almost exactly the size of our moon; the largest (No. III), with its diameter of 3,550 miles, considerably exceeds the modest proportions of Mercury. Satellite I revolves in 42½ hours at the same average distance from Jupiter’s surface that our moon does from that of the earth. No. II has a period of 3 days 13 hours, and its distance from Jupiter’s centre is 415,000 miles. Both these orbits are sensibly circular; and Nos. III and IV travel in ellipses of very small eccentricity, the one at a mean distance of 664,000, the other at 1,167,000 miles, in periods respectively of 7 days 4 hours, and 16 days 16½ hours. All four revolve strictly in the plane of Jupiter’s equator.
They constitute a system bound together by peculiar dynamical relations, in consequence of which they can never be all either eclipsed or seen aligned at one side of their primary at the same time. They can all, however, be simultaneously hidden behind it, or in its shadow; although this moonless condition is looked out for as a telescopic rarity.
The transits of the satellites across the Jovian disk present many curious appearances, due to complicated and changeable effects of light and shade both upon the planetary background and upon the little circular objects self-compared with it. These, in the ordinary course, show bright while near the dusky limb, then vanish during the central passage, and re-emerge again bright at the opposite side. But instead of duly vanishing, they now and then darken even to the point of becoming indistinguishable from their own shadows, by which they are preceded or followed. This difference of behavior can not be attributed wholly to varieties of lustre in the sections of the disk transited; otherwise it could be predicted. But this has never been attempted; “black transits” come when least expected. The third and fourth satellites are those chiefly subject to these phases; the second has never been known to exhibit them; and they but slightly affect the first. Indeed, all the satellites, except, perhaps, No. II, are striped or spotted; and this leads to seeming deformations in their shape, as well as fluctuations in their brightness, the markings being evidently of atmospheric origin, and hence changeable. Their distinct and accurate perception has been made possible by the excellence of the Lick 36-inch refractor.
Jupiter’s moons seem to resemble him in constitution. The first three possess the same high reflective power. No. II is as bright as the planet’s brightest parts, so that its albedo can not fall short of 0.70. And even No. IV (formerly designated “Calypso” in reference to its frequent obscurations) exactly matches, during its darkest phases, the blue-gray polar hoods of its primary. On an average, too, the satellites seem to be of about the same mean density as Jupiter, No. I being considerably the lightest for its bulk; and their spectra, according to Vogel’s observations in 1873, are composed of solar rays modified in precisely the same way as those reflected by the planet.
The discovery, September 9, 1892, of Jupiter’s “fifth satellite” was one of the keenest astronomical surprises on record. Professor Barnard seized the opportunity, lent by the specially favorable opposition of 1892, to rummage the system for novelties. Keeping the telescopic field dark by means of a metallic bar placed so as to occult the gorgeous planetary round, he sought, night after night, for what might appear. At length, on September 9, he caught the glimmer he wanted, and made sure, September 10, that it truly intimated the presence of a new satellite.
This small body revolves in a period of 11 hours, 57 minutes, 23 seconds at a mean distance of 112,160 miles from Jupiter’s centre, or 67,000 from his bulged equatorial surface. Hence, it should by right be called “No. I” instead of “No. V.” The major axis of the ellipse in which it circulates advances so rapidly, owing to the disturbance caused by Jupiter’s spheroidal figure, as to complete a revolution in five months. The implied eccentricity of its orbit, as M. Tisserand has shown, very slightly exceeds that of the orbit of Venus, yet it has been made obvious by Barnard’s observations of the differences between its east and west elongations. Its orbital velocity of 16½ miles a second far surpasses that of any other satellite in the solar system. Close vicinity to a mass so vast as Jupiter’s demands counterbalancing swiftness. Its period of revolution being, however, longer by one hour than Jupiter’s period of rotation, it so far conducts itself normally as to rise in the east and set in the west. On the other hand, since its progress over the sphere is measured by the difference between the two periods, it spends five Jovian days in journeying from one horizon to the other, running, in the meantime, four times through all its phases. Yet it never appears full. Jupiter’s voluminous shadow cuts off sunlight from it during nearly one-fifth of each circuit.