In November, 1873, a fine twenty-six-inch object glass, by Alvan Clark, was mounted at the U. S. Naval Observatory at Washington, and it was soon employed upon the difficult task of solving the problem as to the exact periods of the Uranian satellites. This was very satisfactorily effected, and with distinct and conclusive favor to Mr. Lassell, whose observations were fully corroborated. Only four satellites could be distinguished by the American observers, and their periods, as computed from a valuable series of measures, agreed with those previously derived at Malta. In Appendix I. to the "Washington Observations" for 1873, Prof. Newcomb gave a valuable summary of results--the first obtained, be it noted, with that splendid instrument which soon afterward, in 1877, revealed the satellites of Mars--which included the elements of the satellites of Uranus as follows:
Mean Longitude.
Satellite. Epoch 1871. Radius of Period of
Dec. 31, W.M.T. Orbit. Revolution in days.
I. Ariel........ 21.83° 13.78" 2.52038
II. Umbriel..... 13.52 19.20 4.14418
III. Titania..... 229.93 31.48 7.70590
IV. Oberon...... 154.83 42.10 13.43327
Speaking of the comparative brightness of the satellites, Prof. Newcomb says:
"The greater proximity of the inner satellites to the planet makes it difficult to compare them photometrically with the outer ones, as actual feebleness of light cannot be distinguished from difficulty of seeing arising from the proximity of the planet. However, that Umbriel is intrinsically fainter than Titania is evinced by the fact that, although the least distance of the latter is somewhat less than the greatest distance of the former, there is never any difficulty in seeing it in that position. From their relative aspects in these respective positions I judge Umbriel to be about half as bright as Titania. Ariel must be brighter than Umbriel, because I have never seen the latter unless it was farther from the planet than the former at its maximum distance.... I think I may say with considerable certainty that there is no satellite within 2' of the planet, and outside of Oberon, having one-third the brilliancy of the latter, and therefore that none of Sir William Herschel's supposed outer satellites can have any real existence. The distances of the four known satellites increase in so regular a way that it can hardly be supposed that any others exist between them. Of what may be inside of Ariel it is impossible to speak with certainty, since in the state of atmosphere which prevails during our winter all the satellites named disappear at 10" from the planet."
Prof. Newcomb mentions that no systematic search for new satellites was undertaken because it must have interfered with the fullness and accuracy of the micrometer measures of the old satellites, which constituted the main purpose of the observations. Some faint objects were occasionally glimpsed near the planet, and their relative places determined, but they were never found to accompany Uranus. The fact, therefore, that no additional satellites were discovered is not to be regarded as a strong point in favor of the theory of their non-existence, because the great power and excellence of the telescope was expressly directed to the attainment of other ends; and moreover the season in which the planet came to opposition was distinctly unfavorable for the prosecution of a rigorous search for new satellites. There can, however, be no doubt that the analogies of the planetary systems interior to Uranus plainly suggest that this planet is attended by several satellites which the power of our greatest telescopes has hitherto failed to reveal; and that it is in this direction and that of Neptune we may anticipate further discoveries in future years when the conditions are more auspicious and the work is entered upon with special energy, aided by instruments of even greater capacity than those which have already so far conduced to our knowledge of the heavenly bodies.
Notwithstanding the extreme difficulty with which the Uranian satellites are observed, the two brighter ones, Titania and Oberon, discovered by William Herschel in 1787, have been occasionally detected in telescopes of moderate power, and identified by means of an ephemeris which has shown that the computed positions approximately agree with those observed. During the last few years Mr. Marth has published ephemerides of the satellites of both Saturn and Uranus, and many amateurs have to acknowledge the valuable aid rendered by these tables, which supply a ready means of identifying the satellites, and thus act as an incentive to observers who are induced to pursue such work for the sake of the interesting comparisons to be made afterward. In one exceptional instance the two outer satellites of Uranus appear to have been glimpsed with an object glass of only 43 inches aperture, and the facts are given in detail in the "Monthly Notices of the R.A.S.," April 1876, pp. 294-6. The observations were made in January, February, and March, 1876, by Mr. J.W. Ward, of Belfast; and the positions of the satellites, as he estimated them on several nights, are compared with those computed, the two sets presenting tolerably good agreement. Indeed the corroborations are such as to almost wholly negative any skepticism, though such extraordinary feats should always be received with caution.
In this particular case the chances of being misled are manifold; even Herschel himself fell into error in taking minute stars to be satellites and actually calculating their periods; so that when we remember the difficulties of the question our doubts are not altogether dispelled. Extreme acuteness of vision will, in individual instances, lead to success of abnormal character, and certainly in Mr. Ward's case the remarkable accordances in the observed and calculated positions appear to be conclusive evidence that he was not mistaken.
It will be readily inferred that the great distance and consequent feebleness of Uranus must render any markings upon the disk of the planet beyond the reach of our best telescopes; and indeed this appears to have been a matter of common experience. Though the surface has been often scanned for traces of spots, we seldom find mention that any have been distinguished. Consequently the period of rotation has yet to be determined. It is true that an approximate value was assigned by Mr. T.H. Buffham from observations with a nine-inch reflector in 1870 and 1872. but the materials on which the computation was based were slender and necessarily somewhat uncertain, so that his period of about twelve hours stands greatly in need of confirmation. The bright spots and zones seen on the disk in the years mentioned appear to have entirely eluded other observers, though they are probably phenomena of permanent character and within reach of instruments of moderate size. Mr. Buffham [1] thus describes them:
[Footnote 1: "Monthly Notices K. A. S.," January, 1873.]
"1870, Jan. 25, 11h. to 12h. in clear and tolerably steady air; power 132 showed that the disk was not uniform. With powers 202 and 3.0, two round, bright spots were perceived, not quite crossing the center but a little nearer to the eastern side of the planet, the position angle of a line passing through their centers being about 20º and 200--ellipticity of Uranus seemed obvious, the major axis lying parallel to the line of the spots.