"Jan. 27, 10h. to 10½h.; some fog, and definition not good, but the appearance of the spots was almost exactly the same as on the 25th."
On March 19 glimpses were obtained of a light streak and two spots. On April 1, 4, 6, and 8, a luminous zone was seen on the disk, and in February and March, 1872, when observations were resumed, certain regions were noted brighter than others, and underwent changes indicating the rotation of the planet in a similar direction to that derived from the results obtained in 1870. Mr. Buffham points out that, if this is admitted, then the plane of the planet's equator is not coincident with the plane of the orbits of the satellites. Nor need we be surprised at this departure from the general rule, where such an anomalous inclination exists. In singular confirmation of this is Mr. Lassell's observation of 1862, Jan. 29, where he says: "I received an impression which I am unable to render certain of an equatorial dark belt, and of an ellipticity of form."
Some observations made in 1872-3 with the great six-foot reflector of Lord Rosse may here be briefly referred to. A number of measures, both of position and distance, of Oberon and Titania, were made, [1] and a few of Umbriel and Ariel, but "the shortness of the time available (40 minutes) each night for the observation of the planet with the six-foot instrument, the atmospheric disturbance, so often a source of annoyance in using so large an aperture, and other unfavorable circumstances, tended to affect the value of the observations, and to make the two inner satellites rarely within detection."
[Footnote 1: "Monthly Notices R. A. S.," March, 1875.]
On Feb. 10, 1872, Lord Rosse notes that all four satellites were seen on the same side of the planet. On Jan. 16, 1873, when definition was good, no traces of any markings were seen. Diameter of Uranus = 5.29". Power 414 was usually employed, though at times the inner satellites could be more satisfactorily seen with 625.
It may be mentioned as an interesting point that, some fifty years after the first discovery of Uranus by Herschel, it was accidentally rediscovered by his son, Sir John Herschel, who recognized it by its disk, and had no idea as to the identity of the object until an ephemeris was referred to. Sir John mentions the fact as follows, in a letter to Admiral Smyth, written in 1830, August 8:
"I have just completed two twenty-foot reflectors, and have got some interesting observations of the satellites of Uranus. The first sweep I made with my new mirror I re-discovered this planet by its disk, having blundered upon it by the merest accident for 19 Capricorni."
In commenting upon the centenary of an important scientific discovery we are naturally attracted to inquire what progress has been made in the same field during the comparatively short interval of one hundred years which has elapsed since it occurred. We have called it a short interval, because it cannot be considered otherwise from an astronomical or geological point of view, though, as far as human life is concerned, it can only be regarded as a very lengthy period, including several generations within its limits.
Since Herschel, in 1781, discovered Uranus, astronomy has progressed with great rapidity, so that it would be impossible to enumerate in a brief memoir the many additional discoveries which have resulted from assiduous observation. A century ago only five planets were known (excluding the Earth), now we are acquainted with about two hundred and thirty of these bodies; and one of these, found in 1846, is a large planet whose orbit lies exterior to that of Uranus. In fact, the state of astronomical knowledge a century ago has undergone wonderful changes. It has been rendered far more complete and comprehensive by the diligence of its adherents and by the unwearying energy with which both in theory and practice it has been pursued. A zone of small planets has been discovered between Mars and Jupiter just where the analogies of the planetary distances indicated the probable existence of a large planet. The far-off Neptune was revealed in 1846 by a process of analytical reasoning as unique as it was triumphant, and which proved how well the theory of planetary perturbations was understood. The planet was discovered by calculation, its position in the heavens assigned, and the telescope was then employed merely as the instrument of its detection. The number of satellites which a century ago numbered only ten has now reached twenty, and the discovery in 1877 of two moons accompanying Mars shows that the work is being continued with marked success.
In other departments we also find similar evidence of increasing knowledge. The periodicity of the sun spots, the existence of systems of binary stars, meteor showers, and their affinity with cometary orbits may be mentioned as among the more important, while a host of new comets, chiefly telescopic, have been detected. Large numbers of nebulæ and double stars have been catalogued, and we have evidence every year of the activity with which these several branches are being followed up.