SOLAR AND PLANETARY STUDIES.
One result of Herschel's fame as the founder of stellar astronomy has been that his greatness as an observer and student of the Sun and planets has been largely overlooked. Nevertheless his work in solar and planetary astronomy alone would have gained for him the highest position among the astronomical observers of the day. "Among the celestial bodies," he wrote in 1799, "the Sun is certainly the first which should attract our notice." From an early date he was attracted by solar phenomena, and in the course of his career contributed several papers on the Sun to the Royal Society. In the first of these, "On the Nature and Construction of the Sun and Fixed Stars," read 18th December, 1794, he propounded his hypothesis of the Sun's constitution, to which he adhered throughout his lifetime.
Before Herschel's time, the Sun had been observed by Galileo, Scheiner, Fabricius, Hevelius, Cassini and others. Sun-spots had been observed for over a century and a half and many important details had been detected, but concerning their nature controversy had raged and uncertainty prevailed. In 1774, just at the beginning of Herschel's career as an observer, the well-known theory of sun-spots was propounded by Alexander Wilson, Professor of Astronomy in the University of Glasgow. A series of observations convinced him that the spots were depressions beneath the general surface of the Sun; not mountains, as many observers had supposed, but cavities in the glowing surface through which the darker interior was visible. "Is it not reasonable," he asked, "to think that the great and stupendous body of the Sun is made up of two kinds of matter, very different in their qualities: that by far the greater part is solid and dark and that this immense and dark globe is encompassed with a thin covering of that resplendent substance from which the Sun would seem to derive the whole of his revivifying heat and energy?" Herschel's earliest observations were confirmatory of Wilson's conclusions. In 1783 he closely followed a large spot, noticing that "it was plainly depressed below the surface of the Sun: and that it had very broad shelving sides ". He was soon led to adopt Wilson's view of the solar constitution, which he developed in his first paper on the Sun in 1794.
His observations of the great spot of 1774 led him to conclude that he viewed "the real solid body of the Sun itself, of which we rarely see more than its shining atmosphere." This interpretation of sun-spot observations was the foundation-stone of his theoretical edifice. He concluded, in agreement with, but independently of, Wilson, that the solar globe was dark and solid, surrounded by a glowing atmosphere composed of various "elastic fluids that are more or less lucid and transparent". This lucid fluid, named by Schröter the "photosphere," Herschel believed to be generated in the Sun's atmosphere. An analogy, he said, might be drawn from the generation of clouds in the terrestrial atmosphere. In his paper of 1801, Herschel concluded that there are "two different regions of solar clouds, like those upon our globe. In that case their light is only the uniform reflection of the surrounding superior self-luminous region."
"The solid body of the Sun beneath these clouds," Herschel said, "appears to be nothing else than a very eminent, large and lucid planet, evidently the first, or in strictness of speaking, the only primary one of our systems; all others being truly secondary to it." This solid body, he believed to be protected from the great heat of the elastic fluid of its own atmosphere by dense planetary clouds; it was diversified by mountains and valleys, and was in every way analagous to the Earth and the other planets. "We need not hesitate to admit," he said in 1794, "that the Sun is richly stored with inhabitants." In 1801, after further close study, he claimed that all his former arguments had been confirmed.
Herschel's theory met with general acceptance for many years. It harmonised with the trend of later eighteenth century thought, which—possibly as a result of the reaction from narrow theological views of the Earth's supreme place in nature—shrank from the idea of empty worlds. It was not until the invention of the spectroscope that the theory was universally abandoned. Untenable though it was, it was the first serious attempt to co-ordinate the isolated facts ascertained concerning phenomena of the solar disc.
The fallacious nature of his theory should not blind us to Herschel's great work as an observer of solar phenomena. In his paper of 1794 he noted the existence of "elevated bright places " which, after Hevelius, he named faculae. "I see these faculae extended . . . over about one-sixth part of the Sun. . . . Towards the north and south I see no faculae; there is all over the Sun a great unevenness in the surface, which has the appearance of a mixture of small points of an unequal light; but they are evidently an unevenness or roughness of high and low parts." Very few details of the solar surface escaped his persistent scrutiny. His paper of 1801 described the process of spot-formation with a wonderful accuracy of detail. Curiously enough, Herschel although he devoted attention to the prevalence or absence of spots failed to notice the sun-spot period. Probably he would have done so, had his attention to the Sun been more exclusive. Herschel's solar work was not purely telescopic. His investigations on light and heat occupied a great deal of his time at the beginning of the century; and he contributed to the Royal Society on these subjects four papers in rapid succession. He was led to the inquiry by his search for the most suitable dark glasses for solar observation, in the course of which he found that some materials were opaque to light and others to heat. His papers have been called "the first exposition worth mentioning of the principles of radiant heat". In this exposition he showed that radiant heat obeyed the laws of reflection, refraction and dispersion. His investigation of the infra-red heat rays led to one of his greatest discoveries—that of the invisible portions of the solar spectrum.
Early in his career, Herschel paid considerable attention to the Moon. His second paper in 1780 dealt with his measures of the height of the lunar mountains. In 1783 a sensation was caused in scientific circles by the news that Herschel had seen lunar volcanoes in violent eruption. In a letter to Magellan, a Portuguese amateur astronomer, he stated that on 4th May, he "perceived in the dark part of the Moon a luminous spot. It had the appearance of a red star of about the fourth magnitude." In 1787 he communicated a paper to the Royal Society, in which he announced the appearance of other three volcanoes, and in which he promised the Society an account of the eruption of 4th May, 1783. This account was never forthcoming. The leading French astronomers were inclined to the view that the appearances were actually due to earth-shine. It is possible that Lalande, who visited Slough in 1788, may have converted Herschel to this view. At all events, nothing further was published concerning the supposed volcanoes.
Herschel believed the Moon to be both habitable and inhabited. Yet he laid it down in 1794 "that we perceive no large seas in the Moon, that its atmosphere (the existence of which has even been doubted by many) is extremely rare and unfit for the purposes of animal life; that its climates, its seasons, and the length of its days totally differ from ours; that without dense clouds (which the Moon has not) there can be no rain; perhaps no rivers, no lakes". His belief in the habitability of our satellite arose from the view that its inhabitants "are fitted to their conditions as well as we on this globe are to ours".
Of the planets, Mercury alone was neglected by Herschel; he studied it only when in transit across the solar disc. His observations on Venus—"an object," he said," that has long engaged my particular attention"—were commenced in April, 1777, and were continued for sixteen years. Herschel had four objects in view—to measure the rotation period of the planet, to ascertain the presence or absence of an atmosphere, to determine accurately the planet's diameter, and to give "attention to the construction of the planet with regard to permanent appearances". He satisfied himself that Venus did rotate, but the diurnal motion, he said, "on account of the density of the atmosphere of this planet, has still eluded my constant attention, as far as concerns its period and direction". The spots which Herschel discovered on the planet's surface were faint and ill-defined. His observations on Venus were in direct contradiction to those of Schröter. The German astronomer had not only estimated the planet's rotation period at 23½ hours, but had announced the existence of mountains on Venus, whose height exceeded five or six times the perpendicular elevation of Chimborazo. "As to the mountains on Venus," said Herschel, "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." Herschel's negative conclusions have been on the whole confirmed by later astronomers.
Herschel may be safely called the founder of Martian astronomy. In 1777 he commenced observations on Mars, and he early satisfied himself that "the constant and determined shape" of the spots "as well as remarkable colour, show them to be permanent and fastened to the body of the planet". The spots which chiefly attracted his attention were the white polar caps. On 17th April, 1777, he noted, "There are two remarkable bright spots on Mars". These spots had been noticed by Maraldi early in the century, but Herschel was the first to investigate their nature and to chronicle their periodical variation in size. "I may well be permitted to surmise," he wrote in 1784, "... that the bright polar spots are owing to the vivid reflection of light from frozen regions; and that the reduction of these spots is to be ascribed to their being exposed to the Sun. In the year 1781, the south polar spot was extremely large, which we might well expect, since that pole had but lately been involved in a whole twelve months' darkness and absence of the Sun: but in 1783 I found it considerably smaller than before." Herschel's sagacious surmise has been abundantly confirmed by all subsequent observers. In addition, he determined the rotation period with considerable accuracy. In 1781 he announced it to be 24 hours, 39 minutes, 21.67 seconds; he also ascertained the axial inclination and equatorial diameter. The general conclusion which he reached as a result of his observations was that "the analogy between Mars and the Earth is, perhaps, by far the greatest in the whole solar system. Their diurnal motion is nearly the same; the obliquity of their respective ecliptics, on which the seasons depend, not very different." The planet, he concluded, "has a considerable but moderate atmosphere, so that its inhabitants probably enjoy a situation in many respects similar to ours".
The planet Jupiter did not claim so much of Herschel's attention as either Mars or Saturn. Nevertheless, his short study of the giant world marked an epoch. In his paper of 1781 on the "Rotation of the Planets round their Axes," he put forward as a suggested explanation of the atmospheric condition of Jupiter what has been known as the "trade-wind" theory. "As the principal belts on Jupiter are equatorial, and as we have certain constant winds upon our planet that regularly, for certain periods, blow the same way, it is easily supposed that they may form equatorial belts by gathering together the vapours which swim in our atmosphere and carrying them about in the same direction. This will by analogy account for all the irregularities of Jupiter's revolutions." Herschel devoted no other paper to Jupiter, but in 1797 he communicated to the Royal Society his "Observations of the Satellites of Jupiter, with a Determination of their Rotation". From his determinations of their variable brightness, he concluded that the rotation periods of all four satellites coincided with their periods of revolution round Jupiter—a conclusion confirmed by subsequent research. Herschel also made the first attempt to measure the diameters of the satellites. His conclusion—also confirmed by later observers—was "that the third satellite is considerably larger than any of the rest; that the first is a little larger than the second and nearly of the size of the fourth; and that the second is a little smaller than the first or fourth or the smallest of them all".
"The planet Saturn," Herschel wrote, "is perhaps one of the most engaging objects that astronomy offers to our view. As such it drew my attention as early as the year 1774." And it received more of Herschel's attention than any of the other planets. Six of his papers to the Royal Society dealt with Saturn, the first in 1789 and the last in 1805. He concluded in 1789 that the planet had an atmosphere of considerable density; and from the appearance of the belts he inferred that it "turns upon an axis which is perpendicular to the ring," and this view was confirmed by his detection of a considerable polar flattening. In December, 1793, he stated that the period of rotation "is probably not of a long duration"; and in the following year he confirmed his suspicion, and announced the period of rotation as 10 hours 16 minutes.
Five satellites of Saturn were known when Herschel commenced his study of the planet. His discovery of two exceedingly faint inner satellites, rendered fainter still by their proximity to the bright disc and ring of Saturn, was the result of long and continued investigation. He had entertained "strong suspicions" of the existence of a sixth satellite for a considerable time; and on 19th August, 1787, using the 20-foot reflector as a "front view," he noticed an object which he marked down as possibly a sixth moon. The possibility was rendered a certainty when on 28th August, 1789, he turned the new 40-foot on Saturn. Six satellites were manifestly visible, sharing in the planet's motion. On 17th September, he detected another satellite, still fainter, and closer to the ring, revolving in less than one terrestrial day. These moons were named Enceladus and Mimas. Two years later he announced that the fifth satellite Japetus performs its rotation, like the moons of Jupiter and our moon, in a period coincident with its revolution, These conclusions led him to an important generalisation, that "a certain uniform plan is carried on among the secondaries of our solar system; and we may conjecture that probably most of the moons of all the planets are governed by the same law".
The ring-system fascinated Herschel. In 1789 he suspected its division into two, and this suspicion was confirmed in 1791. "Its division into two very unequal parts can admit of no doubt." He appears to have seen the inner dusky or "crape" ring, but he did not recognise it as a portion of the system. In common with Laplace and contemporary astronomers, he believed the rings to be solid structures. In August, 1815, Herschel made his last observations of the planet, with both the 40-foot and the 20-foot telescopes.
As was to be expected, the planet Uranus was an object of special interest to its discoverer. In 1783 he communicated the results of his preliminary study of the new planet to the Royal Society. He then estimated that "the real diameter of the planet must be between four and five times that of the Earth". In a later paper, in 1788, he announced the diameter as 34,217 miles—a remarkably accurate measure. He commenced soon after the discovery to search for Uranian satellites. At first he was continually disappointed and gave over the attempt. Resuming his observations, however, with the front-view method, he detected on 11th January, 1787, two very faint stars. After careful search of these objects in motion he tells us that he deferred a final judgment as to their nature till 10th February. "And in order to put my theory of these two satellites to a trial, I made a sketch on paper to point out beforehand their situation with respect to the planet and its parallel of declination. The long-expected evening came on, and notwithstanding the most unfavourable appearance of dark weather, it cleared up at last, and the heavens now displayed the original of my drawing by showing in the situation I had delineated them, the Georgian planet attended by two satellites."
He determined the times of revolution of these moons; estimated their size as probably not less than that of the satellites of Jupiter; and noted the great inclination of their orbits. In 1797 Herschel announced the discovery of four additional satellites, but this was never confirmed, and there can be little doubt that what he actually observed were small faint stars; although it is just possible that he may have glimpsed the very faint inner pair detected in 1847. A suspicion that Uranus was encircled by a ring similar to Saturn was finally negatived by his investigations in 1792, which he described as "very decisive against the existence of a ring".
Herschel never discovered a comet. This branch of astronomy he left to his sister, who detected no fewer than eight during the course of her observing career. The first of these, discovered 1st August, 1786, was the subject of a short communication to the Royal Society. Other comets, too, seem to have been closely studied by him. The great comet of 1811 called forth a long communication to the Royal Society. In this paper, he emphasised the transient nature of comets.
Herschel missed discovering the first four asteroids, which were detected by Piazzi, Olbers and Harding, when the great astronomer of Slough was at the height of his powers as an observer. He closely studied them after their discovery and endeavoured to measure their discs, but his measures were not very exact. In a paper read before the Royal Society, on "Observations on the two lately discovered celestial bodies," he suggested the name "asteroids" for the new worlds. Brougham in the "Edinburgh Review" took exception to the name, and insinuated that Herschel had deliberately coined it for the purpose of keeping the discoveries of Piazzi and Olbers on a lower level than his own discovery of Uranus, Herschel made no direct reply to the attack. He merely referred to the fact that he had "incurred the illiberal criticism of the 'Edinburgh Review,'" and in 1804, in a subsequent paper on the asteroids, after the discovery of Juno, he said that "the specific difference between planets and asteroids appears 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".