It will be remembered that Uranus presents the unusual spectacle of a system of satellites travelling nearly at right angles to the plane of the ecliptic. The existence of this anomaly gives a special interest to investigations of his axial movement, which might be presumed, from the analogy of the other planets, to be executed in the same tilted plane. Yet this is far from being certainly the case.
Mr. Buffham in 1870-72 caught traces of bright markings on the Uranian disc, doubtfully suggesting a rotation in about twelve hours in a plane not coincident with that in which his satellites circulate.[1126] Dusky bands resembling those of Jupiter, but very faint, were barely perceptible to Professor Young at Princeton in 1883. Yet, though almost necessarily inferred to be equatorial, they made a considerable angle with the trend of the satellites' orbits.[1127] More distinctly by the brothers Henry, with the aid of their fine refractor, two gray parallel rulings, separated by a brilliant zone, were discerned every clear night at Paris from January to June, 1884.[1128] What were taken to be the polar regions appeared comparatively dusky. The direction of the equatorial rulings (for so we may safely call them) made an angle of 40° with the satellites' line of travel. Similar observations were made at Nice by MM. Perrotin and Thollon, March to June, 1884, a lucid spot near the equator, in addition, indicating rotation in a period of about ten hours.[1129] The discrepancy was, however, considerably reduced by Perrotin's study of the planet in 1889 with the new 30-inch equatoreal.[1130] The dark bands, thus viewed to better advantage than in 1884, appeared to deviate no more than 10° from the satellites' orbit-plane. No definitive results, on the other hand, were derived by Professors Holden, Schaeberle, and Keeler from their observations of Uranus in 1889-90 with the potent instrument on Mount Hamilton. Shadings, it is true, were almost always, though faintly, seen; but they appeared under an anomalous, possibly an illusory aspect. They consisted, not of parallel, but of forked bands.[1131]
Measurements of the little sea-green disc which represents to us the massive bulk of Uranus, by Young, Schiaparelli,[1132] Safarik, H. C. Wilson[1133] and Perrotin, prove it to be quite distinctly bulged. The compression at once caught Barnard's trained eye in 1894,[1134] when he undertook at Lick a micrometrical investigation of the system; and he was surprised to perceive that the major axis of the elliptical surface made an angle of about 28° with the line of travel pursued by the satellites. Nothing more can be learned on this curious subject for some years, since the pole of the planet is just now turned nearly towards the earth; but Barnard's conclusion is unlikely to be seriously modified. He fixed the mean diameter of Uranus at 34,900 miles. But this estimate was materially reduced through Dr. See's elimination of irradiative effects by means of daylight measures, executed at Washington in 1901.[1135]
The visual spectrum of this planet was first examined by Father Secchi in 1869, and later, with more advantages for accuracy, by Huggins, Vogel,[1136] and Keeler.[1137] It is a very remarkable one. In lieu of the reflected Fraunhofer lines, imperceptible perhaps through feebleness of light, six broad bands of original absorption appear, one corresponding to the blue-green ray of hydrogen (F), another to the "red-star line" of Jupiter and Saturn, the rest as yet unidentified. The hydrogen band seems much too strong and diffuse to be the mere echo of a solar line, and might accordingly be held to imply the presence of free hydrogen in the Uranian atmosphere. This, however, would be difficult of reconcilement with Keeler's identification of an absorption-group in the yellow with a telluric waterband.
Notwithstanding its high albedo—0·62, according to Zöllner—proof is wanting that any of the light of Uranus is inherent. Mr. Albert Taylor announced, indeed, in 1889, his detection, with Common's giant reflector, of bright flutings in its spectrum;[1138] but Professor Keeler's examination proved them to be merely contrast effects.[1139] Sir William and Lady Huggins, moreover, obtained about the same time a photograph purely solar in character. The spectrum it represented was crossed by numerous Fraunhofer lines, and by no others. It was, then, presumably composed entirely of reflected light.
Judging from the indications of an almost evanescent spectrum, Neptune, as regards physical condition, is the twin of Uranus, as Saturn of Jupiter. Of the circumstances of his rotation we are as good as completely ignorant. Mr. Maxwell Hall, indeed, noticed at Jamaica, in November and December, 1883, certain rhythmical fluctuations of brightness, suggesting revolution on an axis in slightly less than eight hours;[1140] but Professor Pickering reduces the supposed variability to an amount altogether too small for certain perception, and Dr. G. Müller denies its existence in toto. It is true their observations were not precisely contemporaneous with those of Mr. Hall[1141] who believes the partial obscurations recorded by himself to have been of a passing kind, and to have suddenly ceased after a fortnight of prevalence. Their less conspicuous renewal was visible to him in November, 1884, confirming a rotation period of 7·92 hours.
It was ascertained at first by indirect means that the orbit of Neptune's satellite is inclined about 20° to his equator. Mr. Marth[1142] having drawn attention to the rapid shifting of its plane of motion, M. Tisserand and Professor Newcomb[1143] independently published the conclusion that such shifting necessarily results from Neptune's ellipsoidal shape. The movement is of the kind exemplified—although with inverted relations—in the precession of the equinoxes. The pole of the satellite, owing to the pull of Neptune's equatorial protuberance, describes a circle round the pole of his equator in a retrograde direction, and in a period of over five hundred years. The amount of compression indicated for the primary body is, at the outside, 1/85; whence it can be inferred that Neptune possesses a lower rotatory velocity than the other giant planets. Direct verification of the trend theoretically inferred for the satellite's movement was obtained by Dr. See in 1899. The Washington 26-inch refractor disclosed to him, under exceptionally favourable conditions, a set of equatorial belts on the disc of Neptune, and they took just the direction prescribed by theory. Their objective reality cannot be doubted, although Barnard was unable, either with the Lick or the Yerkes telescope,[1144] to detect any definite markings on this planet. Its diameter was found by him to be 32,900 miles.
The possibility that Neptune may not be the most remote body circling round the sun has been contemplated ever since he has been known to exist. Within the last few years the position at a given epoch of a planet far beyond his orbital verge has been approximately fixed by two separate investigators.