How remarkable a discovery this was, will perhaps be realized, when it is stated that Adams was only 2½° out in assigning the position of the new planet, whilst Le Verrier was even nearer, being barely 1° out.
We know practically nothing respecting the physical appearance of Neptune, owing to its immense distance from us, and for the like reason the Neptunian astronomers, if there are any, will know absolutely nothing about the Earth; indeed, their knowledge of the Solar System will be restricted to Uranus, Saturn, and the Sun. Even the Sun will only have an apparent diameter of about 1′ of arc, and, therefore, will only seem to be a very bright star, yielding light equal in amount, according to Zöllner, to about 700 full moons. There is one satellite belonging to Neptune, and as this has been calculated to exhibit a disc 10° in diameter, a certain amount of light will no doubt be afforded by it especially if, as is not unlikely, Neptune is itself possessed of some inherent luminosity independently of the Sun.
The fact that Neptune seems destitute of visible spots or belts, results in our being ignorant of the period of its axial rotation, though it should be stated that in 1883, Maxwell Hall in Jamaica, observed periodical fluctuations in its light, which he thought implied that the planet rotated on its axis in rather less than 8 hours. Several observers thought 20 or 30 years ago, that they had noticed indications of Neptune being surrounded by a ring like Saturn’s ring, but the evidence as to this is very inconclusive. It is quite certain that none but the very largest telescopes in the world would show any such appendage, and this planet seems to have been neglected of late years, by the possessors of such telescopes. Moreover, if a ring existed it would only open out to its full extent once in every 82 years, being the half of the period of the planet’s revolution round the Sun (just as Saturn’s ring only opens out to the fullest extent every 14½ years), so that, obviously, supposing suspicions of a ring dating back 30 or 40 years were well founded, it might well be that another 30 or 40 years might need to elapse, before astronomers would be in a position to see their suspicions revive.
Neptune revolves round the Sun in 164½ years, at a mean distance of 2791 millions of miles. Its apparent diameter scarcely varies from 2¾″. Its true diameter is about 37,000 miles. No compression of the Poles is perceptible. Its one satellite revolves round Neptune in 5¾ days, and in a retrograde direction, at a mean distance of 223,000 miles, and shines as a star of the 14th magnitude. This is a peculiarity which it only shares with the satellites of Uranus, so far as it regards the planetary members of the Solar System, though there are many retrograde Comets.
The question has often been mooted, whether there exists, and belonging to the Solar System, a planet farther off than Neptune. There does seem some evidence of this, as we shall better understand, when we come to the subject of long-period Comets, though it cannot be said that much progress has yet been made in arriving at a solution of the problem.
Unless there does exist a trans-Neptunian planet, a Neptunian astronomer will know very little about planets, for Uranus and Saturn will alone be visible to him. Both will of course be what we call “inferior planets,” and under the best of circumstances will cut a poor figure in the Neptunian sky.
CHAPTER XIII.
COMETS.
I suppose that it is the experience of all those who happen to be in any sense, however humble, specialists in a certain branch of science, that from time to time, they are beset with questions on the part of their friends respecting those particular matters which it is known that they have specially studied. There is no fault to be found with this thirst for information, always supposing that it is kept within due bounds; but my motive for alluding to it here, is to see whether any well-marked conclusion can be drawn from it, within my own knowledge as regards astronomical facts or events. Now in the case of the science of astronomy (for which in this connection I, for the moment, will venture to speak), there is certainly no one department which so unfailingly, at all times and in all places, seems to evoke such popular sympathy and interest as the department which deals with Comets.
Sun-spots may come and go; bright planets may shine more brightly; the Sun or Moon may be obscured by eclipses; temporary stars may burst forth,—all these things are within the ken of the general public by means of newspapers or almanacs, but it is a comet which evokes more questionings and conversations than all the other matters just referred to put together. When a new and bright comet appears, or even when any comet not very bright gets talked about, the old question is still fresh and verdant—“Is there any danger to the Earth to be apprehended from collision with a Comet?” followed by “What is a Comet?” “What is it made of?” “Has it ever appeared before?” “Will it come back again?” and so on. Questions in this strain have more often than I can tell of been put to me. They seem the stock questions of all who will condescend to replace for five minutes in the day the newest novel or the pending parliamentary election.
It may be taken as a fact (though in no proper sense a rule) that a bright and conspicuous comet comes about once in 10 years, and a very remarkable comet every 30 years. Thus we have had during the present century bright comets in 1811, 1825, 1835, 1843, 1858, 1861, 1874 and 1882, whereof those of 1811, 1843, and 1858 were specially celebrated. Tested then by either standard of words “bright and conspicuous,” or “specially celebrated,” it may be affirmed that a good comet is now due, so let us prepare for it by getting up the subject in advance.