Prof. Hall, with the great Washington refractor, has effected many valuable measures of this system in recent years. He finds the orbits of the five inner satellites are sensibly circular, and that they are situated in the plane of the rings. Hyperion revolves in a very eccentric orbit, and this satellite may approach very near to Titan. He obtained an observation on March 25, 1885, which seems pertinent to the question of variation in the light of the satellites. He says:—“Mimas was remarkably bright, and could not be missed even when the full light of the planet was admitted to the eye. Generally this satellite is a difficult object, and from the ease with which it is occasionally seen one might think it variable; but I think the difference is due to the quality of the image.” There is no doubt that this is the main cause of many assumed changes in celestial objects, and especially in regard to those of a minute and delicate character.
Occultations of Stars.—Stars are rarely observed to be occulted by Saturn. Webb mentions that, in 1707 or 1708, Dr. Clark noticed a star in the interval between the ball and rings; and Dawes once remarked a star of 8·5 mag. disappear behind the outer edge of the exterior ring. It would be extremely interesting to watch a tolerably conspicuous star pass centrally behind the Saturnian system, and to trace it through Cassini’s division and the transparent ring, noting any changes in magnitude or appearance as they occurred.
[CHAPTER XIII.]
URANUS AND NEPTUNE.
Discovery of Uranus.—Mistaken for a Comet.—True character revealed.—Period &c.—Observations.—Belts on Uranus.—Further Observations required.—The Satellites.—Discovery of Neptune.—The planet observed in 1795.—Period &c.—Observations.—Supposed Ring.—Satellite.—A trans-Neptunian Planet.—Planetary Conjunctions.
Discovery.—While Sir W. Herschel was a musician at Bath he formed the design of making a telescopic survey of the heavens. When engaged in this he accidentally effected a discovery of great importance, for on the night of March 13, 1781, an object entered the field of his 6·3-inch reflector which ultimately proved to be a new major planet of our system. The acute eye of Herschel, directly it alighted upon the strange body, recognized it as one of unusual character, for it had a perceptible disk, and could be neither fixed star nor nebula. He afterwards found the object to be in motion, and its appearance being “hazy and ill-defined” with very high powers he was led to regard it as a comet, and communicated his discovery to the Royal Society at its meeting on April 26, 1781. His paper begins as follows:—
“On Tuesday, March 13, 1781, between 10 and 11 in the evening, while I was examining the small stars in the neighbourhood of H Geminorum, I perceived one that appeared visibly larger than the rest. Being struck with its uncommon magnitude, I compared it to H Geminorum and the small star in the quartile between Auriga and Gemini, and finding it so much larger than either of them suspected it to be a comet.... The power I had on when I first saw the comet was 227.”
The supposed “comet” soon came under the observation of others, including Maskelyne the Astronomer Royal, and Messier, the “Comet Ferret” of Paris. The latter, in a letter to Herschel, said:—“Nothing was more difficult than to catch it, and I cannot conceive how you could have hit this star or comet several times, for it was absolutely necessary for me to observe it for several days in succession before I could perceive that it was in motion.”
True character revealed.—As observations began to accumulate it was seen that a parabolic orbit failed to accommodate them. Ultimately the secret was revealed. The only orbit to represent the motion of the new body was found to be an approximately circular one situated far outside the path of Saturn, and the inference became irresistible that the supposed “comet” must in reality be a new primary planet revolving on the outskirts of the solar system. This conclusion was justified by facts of a convincing nature, and its announcement created no small excitement in the scientific world. Every telescope was directed to that part of the firmament which contained the new orb, and its pale blue disk, wrapped in tiny proportions, was viewed again and again with all the delight that so great a novelty could inspire. From the earliest period of ancient history, no discovery of the same kind had been effected. The Chaldæans were acquainted with five major planets, in addition to the Earth, and the number had remained constant until the vigilant eye of Herschel enlarged our knowledge, and Saturn was relieved as the sentinel planet going his rounds on the distant frontiers of our system.
When the elements of the new body had been computed a search was instituted amongst the records of previous observers, and it was found that Herschel’s planet had been seen on many occasions, but it had invariably been mistaken for a fixed star. Flamsteed observed it on six occasions between 1690 and 1715, while Le Monnier saw it on 12 nights in the years from 1750 to 1771, and it seems to have been pure carelessness on the part of the latter which prevented him from anticipating Herschel in one of the greatest discoveries of modern times.