I may as well point out, however, that theories so wild have recently been broached respecting Venus, that far more interesting explanations of the enigma than this optical one may be looked for presently. It has been gravely suggested by Mr. Jos. Brett, the artist, that Venus has a surface of metallic brilliancy, with a vitreous atmosphere,—which can only be understood to signify a glass case. This stupendous theory has had its origin in an observation of considerable interest which astronomers (it is perhaps hardly necessary to say) explain somewhat differently. When Venus has made her entry in part upon the sun's face at the beginning of transit, there is seen all round the portion of her disc which still remains outside the sun an arc of light so brilliant that it records its photographic trace during the instantaneous exposure required in solar photography. It is mathematically demonstrable that this arc of light is precisely what should be seen if Venus has an atmosphere like our earth's. But mathematical demonstration is not sufficient (or perhaps we may say it is too much) for some minds. Therefore, to simplify matters, Venus has been provided with a mirror surface and a glass case. (See preceding essay, on Astronomical Paradoxes, for further details.)
The enigma next to be considered is of a more doubtful character than the myth relating to the satellite of Venus. Astronomers are pretty well agreed that Venus has no moon, but many, including some deservedly eminent, retain full belief in the story of the planet Vulcan.
More than seventeen years ago the astronomical world was startled by the announcement that a new planet had been discovered, under circumstances unlike any which had heretofore attended the discovery of fresh members of the solar system. At that time astronomers had already become accustomed to the discovery, year after year, of several asteroids, which are in reality planets, though small ones. In fact, no less than fifty-six of these bodies were then known, whereof fifty-one had been discovered during the years 1847–1858 inclusive, not one of these years having passed without the detection of an asteroid. But all these planets belonged to one family, and as there was every reason to believe that thousands more travel in the same region of the solar system, the detection of a few more among the number had no longer any special interest for astronomers. The discovery of the first known member of the family had indeed been full of interest, and had worthily inaugurated the present century, on the first day of which it was made. For it had been effected in pursuance of a set scheme, and astronomers had almost given up all hopes of success in that scheme when Piazzi announced his detection of little Ceres. Again the discovery of the next few members of the family had been interesting as revealing the existence of a new order of bodies in the solar system. No one had suspected the possibility that besides the large bodies which travel round the sun, either singly or attended by subordinate families of moons, there might be a ring of many planets. This was what the discovery of Ceres, Pallas, Juno, and Vesta seemed to suggest, unless—still stranger thought—these were but fragments of a mighty planet which had been shattered in long-past ages by some tremendous explosion. Since then, however, this startling theory has been (itself) exploded. Year after year new members of the ring of multitudinous planets are discovered, and that, not as was recently predicted, in numbers gradually decreasing, but so rapidly that more have been discovered during the last ten years than during the preceding twenty.
The discovery of the giant planet Uranus, an orb exceeding our earth twelve and a half times in mass and seventy-four times in volume, was a matter of much greater importance, so far as the dignity of the planetary system was concerned, for it is known that the whole ring of asteroids together does not equal one-tenth part of the earth in mass, while Uranus exceeds many times in volume the entire family of terrestrial planets—Mercury, Venus, the Earth, and Mars. The detection of Uranus, unlike that of Ceres, was effected by accident. Sir W. Herschel was looking for double stars of a particular kind in the constellation Gemini when by good fortune the stranger was observed.
The interest with which astronomers received the announcement of the discovery of Uranus, though great, was not to be compared with that with which they deservedly welcomed the discovery of Neptune, a larger and more massive planet, revolving at a distance one-half greater even than the mighty space which separates Uranus from the sun, a space so great that by comparison with it the range of 184,000,000 of miles, which forms the diameter of our earth's orbit, seems quite insignificant. It was not, however, the vastness of Neptune's mass or volume, or the awful remoteness of the path along which he pursues his gloomy course, which attracted the interest of astronomers, but the strangeness of the circumstances under which the planet had been detected. His influence had been felt for many years before astronomers thought of looking for him, and even when the idea had occurred to one or two, it was considered, and that, too, by an astronomer as deservedly eminent as Sir G. Airy, too chimerical to be reasonably entertained. All the world now knows how Leverrier, the greatest living master of physical astronomy, and Adams, then scarce known outside Cambridge, both conceived the idea of finding the planet, not by the simple method of looking for it with a telescope, but by the mathematical analysis of the planet's disturbing influence upon known members of the solar system. All know, too, that these mathematicians succeeded in their calculations, and that the planet was found in the very region and close to the very point indicated first by Adams, and later, but independently, and (fortunately for him more publicly) by Leverrier.
None of these instances of the discovery of members of the solar system resembled in method or details the discovery announced early in the year 1859. It was not amid the star-depths and in the darkness of night that the new planet was looked for, but in broad day, and on the face of the sun himself. It was not on the outskirts of the solar system that the planet was supposed to be travelling, but within the orbit of Mercury, hitherto regarded as of all planets the nearest to the sun. It was not hoped that any calculation of the perturbations of other planets would show the place of the stranger, though certain changes in the orbit of Mercury seemed clearly enough to indicate the stranger's existence.
Early in 1860 Leverrier had announced that the position of Mercury's path was not precisely in agreement with calculations based on the adopted estimates of the masses of those planets which chiefly disturb the motions of Mercury. The part of the path where Mercury is nearest to the sun, and where, therefore, he travels fastest, had slightly shifted from its calculated place. This part of the path was expected to move, but it had moved more than was expected; and of course Mercury having his region of swiftest motion somewhat differently placed than was anticipated, himself moved somewhat differently.
Leverrier found that to explain this feature of Mercury's motion either the mass of Venus must be regarded as one-tenth greater than had been supposed, or some unknown cause must be regarded as affecting the motion of Mercury. A planet as large as Mercury, about midway between Mercury and the sun, would account for the observed disturbance; but Leverrier rejected the belief that such a planet exists, simply because he could not 'believe that it would be invisible during total eclipses of the sun.' 'All difficulties disappear,' he added, 'if we admit, in place of a single planet, small bodies circulating between Mercury and the sun.' Considering their existence as not at all improbable, he advised astronomers to watch for them.
It was on January 2, 1860, that Leverrier thus wrote. On December 22, 1859, a letter had been addressed by a M. Lescarbault of Orgères to Leverrier, through M. Vallée, hon. inspector-general of roads and bridges, announcing that on March 26, 1859, about four in the afternoon, Lescarbault had seen a round black spot on the face of the sun, and had watched it as it passed across like a planet in transit—not with the slow motion of an ordinary sun-spot. The actual time during which the round spot was visible was one hour, seventeen minutes, nine seconds, the rate of motion being such that, had the spot crossed the middle of the sun's disc, at the same rate, the transit would have lasted more than four hours. The spot thus merely skirted the sun's disc, being at no time more than about one forty-sixth part of the sun's apparent diameter from the edge of the sun. Lescarbault expressed his conviction that on a future day, a black spot, perfectly round and very small, will be seen passing over the sun, and 'this point will very probably be the planet whose path I observed on March 26, 1859.' 'I am persuaded,' he added, 'that this body is the planet, or one of the planets, whose existence in the vicinity of the sun M. Leverrier had made known a few months ago' (referring to the preliminary announcement of results which Leverrier published afterwards more definitely).
Leverrier, when the news of Lescarbault's observation first reached him, was surprised that the observation should not have been announced earlier. He did not consider the delay sufficiently justified by Lescarbault's statement that he wished to see the spot again. He therefore set out for Orgères, accompanied by M. Vallée. 'The predominant feeling in Leverrier's mind,' says Abbé Moigno, 'was the wish to unmask an attempt to impose upon him, as the person more likely than any other astronomer to listen to the allegation that his prophecy had been fulfilled.'