“The influence of this eminent body on the globe we inhabit,” he wrote, continuing the subject in 1801, “is so great, and so widely diffused, that it becomes almost a duty to study the operations which are carried on upon the solar surface.” This duty he fulfilled to perfection. His telescopic readings from the changeful solar disc were of extraordinary precision and comprehensiveness. They show his powers as an observer perhaps at their best. And, since reasoning was with him inseparable from seeing, the appearances he noted took, as if of their own accord, their proper places. The history of spots was completely traced. He recorded their birth by the enlargement of pores; their development and sub-division; established their connexion with faculous matter, piled up beside them like mountain-ranges round an Alpine lake, or flung across their cavities like blazing suspension-bridges; and watched finally their closing-up and effacement, not even omitting the post-mortem examination of the disturbances they left behind.
One of Herschel’s curiously original enterprises was his attempt to ascertain a possible connexion between solar and terrestrial physics. “I am now much inclined to believe,” he stated in 1801, “that openings with great shallows, ridges, nodules, and corrugations, may lead us to expect a copious emission of heat, and, therefore, mild seasons. And that, on the contrary, pores, small indentations, and a poor appearance of the luminous clouds, the absence of ridges and nodules, and of large openings and shallows, will denote a spare emission of heat, and may induce us to expect severe seasons. A constant observation of the sun with this view, and a proper information respecting the general mildness or severity of the seasons in all parts of the world, may bring this theory to perfection, or refute it, if it be not well founded.”
But the available data regarding weather-changes turning out to be exceedingly defective, he had recourse to the celebrated expedient of comparing the state of the sun in past years with the recorded prices of corn. Fully admitting the inadequacy of the criterion, he still thought that the sun being “the ultimate fountain of fertility, the subject may deserve a short investigation, especially as no other method is left for our choice.” He obtained, as the upshot, partial confirmation of the surmise that “some temporary defect of vegetation” ensued upon the subsidence of solar agitation. In plainer language, food-stuffs tended to become dear when sun-spots were few and small. No signs of cyclical change could, however, be made out. The discovery of the “sun-spot period” was left to Schwabe. This admirable preliminary effort to elicit the earth’s response to solar vicissitudes was denounced by Brougham as a “grand absurdity;” and the readers of the second number of the Edinburgh Review were assured that “since the publication of ‘Gulliver’s Travels,’ nothing so ridiculous had ever been offered to the world!”
Herschel did not neglect the planets. His observations of Venus extended from 1777 to 1793. Their principal object was to ascertain the circumstances of the planet’s rotation; but they eluded him; which, considering that they are still quite uncertain, is not surprising. He would probably have communicated nothing on the subject had he not been piqued into premature publication by Schröter’s statement that the mountains of Venus rose to “four, five, or even six times the perpendicular elevation of Chimborazo.” Herschel did not believe in them, and expressed his incredulity in somewhat sarcastic terms. “As to the mountains in Venus,” he wrote, “I may venture to say that no eye which is not considerably better than mine, or assisted by much better instruments, will ever get a sight of them.” He rightly inferred, however, the presence of an extensive atmosphere from the bending of the sun’s rays so as to form much more than a semicircular rim of light to the dark disc of the planet when near inferior conjunction—that is, when approximately in a right line between us and the sun. He fully ascertained, too, the unreality of the Cytherean phantom-satellite. The irritability visible in this paper made a solitary exception to Herschel’s customary geniality. It might have led to a heated controversy but for the excellent temper of Schröter’s reply.
Although we may not be prepared to gainsay Herschel’s dictum that “the analogy between Mars and the earth is perhaps by far the greatest in the whole solar system,” we can hardly hold it to be so probable as he did that “its inhabitants enjoy a situation in many respects similar to ours.” Yet the modern epoch in the physical study of Mars began with his announcement in 1784 that its white polar caps spread and shrank as winter and summer alternated in their respective hemispheres. His conclusion of their being produced by snowy depositions from “a considerable, though moderate, atmosphere,” is not likely to be overthrown. He established, besides, the general permanence of the dark markings, notwithstanding minor alterations due, he supposed, to the variable distribution of clouds and vapours on the planet’s surface.
This vigilant “watcher of the skies” laid before the Royal Society, May 6th, 1802, his “Observations of the two lately discovered Bodies.” These were Ceres and Pallas, which, with Juno and Vesta, picked up shortly afterwards, constituted the vanguard of the planetoid army. Herschel foresaw its arrival. He adopted unhesitatingly Olbers’s theory of their disruptive origin, and calculated that Mercury, the least of the true planets, might be broken up into 35,000 masses no larger than Pallas. An indefinite number of such fragments (about 420 are now known) were accordingly inferred to circulate between the orbits of Mars and Jupiter. He distinguished their peculiarities, and, since they could with propriety be designated neither planets nor comets, he proposed for them the name of “asteroids.” But here again he incurred, to use his own mild phrase, “the illiberal criticism of the Edinburgh Review.” “Dr. Herschel’s passion for coining words and idioms,” Brougham declared, “has often struck us as a weakness wholly unworthy of him. The invention of a name is but a poor achievement in him who has discovered whole worlds.” The reviewer forgot, however, that new things will not always fit into the framework of old terminology. He added the contemptible insinuation that Herschel had devised the word “asteroid” for the express purpose of keeping Piazzi’s and Olbers’s discoveries on a lower level than his own of Uranus.
Herschel made no direct reply to the attack; only pointing out, in December, 1804, how aptly the detection of Juno had come to verify his forecasts. “The specific differences,” he said, “between planets and asteroids appear now, by the addition of a third individual of the latter species, to be more fully established; and that circumstance, in my opinion, has added more to the ornament of our system than the discovery of another planet could have done.”
His endeavours to determine the diameters of these small bodies were ineffectual. Although he at first estimated those of Ceres and Pallas at 162 and 147 miles, he admitted later his inability to decide as to the reality of the minute discs shown by them; and they were first genuinely measured by Professor Barnard with the great Lick refractor in 1894.
The “trade-wind theory” of Jupiter’s belts originated with Herschel; and he took note of the irregular drifting movements of the spots on his surface, and their consequent uselessness for determining the period of his rotation. That of Saturn’s he fixed quite accurately at ten hours sixteen minutes, with a marginal uncertainty of two minutes, the period now accepted being of ten hours fourteen minutes. The possession by this planet of a profound atmosphere was inferred from the changes in its belts, as well as from some curious phenomena attending the disappearance of its satellites. They were commonly seen to “hang on the limb”—that is, to pause during an appreciable interval on the brink of occultation. Mimas, on one occasion, remained thus poised during twenty minutes! For so long it was geometrically concealed, although visible by the effect of refraction. Saturn was an object of constant solicitude at Slough; and it was only with the surpassing instruments mounted there that much could be learned about Galileo’s altissimo pianeta. Herschel supposed, with Laplace, the rings to be solid structures; and he added that the interval of 2,500 miles separating them “must be of considerable service to the planet in reducing the space that is eclipsed by the shadow of the ring.” The “crape ring” was seen, but not recognised. In one of his drawings it figures as a dusky belt crossing the body of the planet.
His satellite discoveries proved exceedingly difficult to verify. The Saturnian pair were lost, after he left them, until his son once more drew them from obscurity. Regarding the outermost member of the system, Japetus, discovered by Cassini in 1671, Herschel noticed, in 1792, a singular circumstance. It was already known to vary in brightness; we receive from it, in fact, four and a-half times more light at certain epochs than at others. The novelty consisted in showing that this variation depended upon the satellite’s situation in its orbit in such a manner as to leave no doubt that, like our moon, it keeps the same face always directed inwards towards its primary. So that Japetus was inferred to turn on its axis in the period of its revolution round Saturn, that is, in seventy-nine and one-third days.