William Herschel and his work/Chapter 10
PLANETS AND COMETS
The first of the heavenly bodies to which Herschel really turned his eyes with the longing of a traveller in an untrodden land of romance, appears to have been the planet Saturn. He was then forging the instruments which were destined to disclose the hidden things of creation, and to give an impulse to the study of them, that has gone on from wonder to wonder till the present day. He was keeping a journal, making entries of what he saw, and laying a foundation for future progress. But his method of writing was somewhat peculiar. His papers were to a large extent copies of entries made in his journal, or the impressions he received at the moment while sun or star or planet was under his eye. There was thus room for mistakes, which it is not surprising that he fell into; the wonder is that he fell into so few. Of mistakes resulting from this hasty method of working he was himself conscious; but it led to another inconvenience. He did not delay publishing his views till he was perfectly sure of their accuracy. The result was diffuseness of statement and unnecessary returning to the same subject. To give his views in the order of time would thus be wearisome and useless. We shall keep to the order of subjects, bringing them, as far as possible, to a focus.
The planet Mercury did not receive much attention from Herschel; but, slight though his interest in it seems to have been, he could not make it a field of observation without shedding light on things then unknown, and afterwards forgotten. As a transit or passage of the planet over the sun's face was due at Windsor in the early morning of November 9, 1802, and Herschel's "apparatus for viewing the sun was then in the highest perfection," he was on the watch for what might happen. The weather proved as favourable as he could wish, and more than forty dark spots were counted on the sun's disc. A little black pea traversing the disc among dark spots of vastly greater size, it might have been feared, would be lost to view or only seen now and again. On the contrary, the black dot was easily seen during the four hours that remained of its passage across. As the sun rose higher, "the corrugations of the luminous solar surface up to the very edge of the planet" were visible with a 10-feet reflector. "When the planet was sufficiently advanced towards the largest opening," or spot, "of the northern zone, he compared the intensity of the blackness of the two objects; and found the disk of Mercury considerably darker, and of a more uniform black tint, than the area of the large opening." As it approached the edge of the sun, the whole of its disc was "as sharply defined as possible; there was not the least appearance of any atmospheric ring, or different tinge of light, visible about the planet." As the black dot vanished on leaving the bright body of the sun, there was not the slightest distortion of the sun's limb or in its own figure. The planet was snuffed out at once on leaving the sun's body. Things were somewhat different with the planet Venus.
Venus had for many years been the object of close research by Schroeter, a most painstaking observer of Lilienthal, then a well-known observatory in the duchy of Bremen. Her appearance had also been carefully studied by Herschel for nearly twenty years. The former made out that he had measured on her surface lofty mountains six times higher than Chimborazo, or about twenty-three miles in height. The latter could see nothing of the kind, and poked some grave scientific fun at his friend, who complained, in a learned paper, that he could not "reconcile it to the friendly sentiments which the author has always hitherto expressed towards me, and which I hold extremely precious; though perhaps to others it may not have the same appearance." Boscovich's epigram on the planets had come true in the case of these astronomers—
" 'Twixt Mars and Venus as this globe was hurled,
'Tis plain that love and war must rule the world."
Schroeter attacks Herschel for misrepresenting, or, on insufficient grounds, rejecting his views. Herschel appears not to have retorted any more than he did when attacked elsewhere by others. It was wise; but he found that the Lady Venus may be as much a source of quarrel, when she walks in unsurpassed brightness among the stars, as when she awakens the feelings of mortal hearts on earth.
As this was the only scientific quarrel in Herschel's life, it is worth while to show how small it was. Far different were the quarrels which caused annoyance and grief to the friends of Newton, Hooke, Flamsteed, Leibnitz, Bernoulli, Laplace, and which render their lives sometimes most unpleasant reading. A quarrel for the maintenance of truth and right is a necessity of life in a world, where falsehood and wrong seem often to have the best of it; but the meannesses and selfishness of scientific quarrels have little or nothing of this nobility about them. "The result of my observations would have been communicated long ago," Herschel wrote for the Royal Society, "if I had not still flattered myself with the hopes of some better success, concerning the diurnal motion of Venus; which, on acccount of the density of the atmosphere of this planet, has still eluded my constant attention as far as concerns its period and direction. Even at the present time I should hesitate to give the following extract from my journals, if it did not seem incumbent on me to examine by what accident I came to overlook mountains in this planet, which are said to be of such enormous height, as to exceed four, five, and even six times the perpendicular elevation of Cimboraça, the highest of our mountains. The same paper which contains the lines I have quoted, gives us likewise many extraordinary accounts, equally wonderful: such as hints of the various and singular properties of the atmosphere of Saturn." Then he proceeds to speak of Schroeter's measures as "defective"; the mirror of the 7-feet reflector used as "considerably tarnished"; and the "calculations (as) so full of inaccuracies, that it would be necessary to go over them again." The Lilienthal observer did not like this plain speaking.
To these somewhat sharp, but perhaps deserved criticisms, Schroeter replied in 1796. "Though it is a satisfaction to me that Dr. Herschel last year found my discovery of the morning and evening twilight of Venus's atmosphere to be confirmed, as I could not hope to have obtained such an important confirmation so early, considering the excellent telescopes required, and that a favourable opportunity for such observations occurs but seldom: yet the paper on the planet Venus, which this great observer has inserted in the Phil. Trans. for 1793, contains unreserved assertions, which may be easily injurious to the truth, for the very reason that they have truth for their object, and yet rest on no sufficient foundation." And Schroeter then endeavours to show that Herschel's paper contains misrepresentations or unsatisfactory proof of mistakes committed by him.
It was a small quarrel at the worst, in which these two friends engaged, a very different quarrel from the disputes and angry encounters that disgraced Leibnitz, and Bernoulli, and Flamsteed, and did not leave Newton altogether unscathed. Schroeter had perhaps the best of it. His mountains, twenty or twenty-three miles high on the surface of Venus, may be a myth, but there is no doubt that his measure of the length of her day, 23h 21m, is somewhat grudgingly accepted by Herschel, while his estimate of the size of Venus, as rather less than the earth, is preferred to Herschel's, who believed he had proved Venus to be a little larger than the earth. At the same time it must be admitted that Herschel had sometimes cause to complain. Writing of one astronomer in 1799, he says, "the same author's account of my double stars is extremely erroneous."
As early as 1777, while toiling at the daily work of a musician in Bath, Herschel "found that the poles of Mars were distinguished with remarkable luminous spots." He believed that, by observing them carefully, he might secure a key to a knowledge of the planet, and its history, the length of its day, its atmosphere, its seasons. These observations were continued during six or seven years. Sometimes he saw a well-marked lucid spot on Mars: "it is its south pole, for it remains in the same place, while the dark equatorial spots perform their constant gyrations: it is nearly circular." It was not only circular; "it was very brilliant and white." At other times he saw also another "lucid spot" at the planet's north pole. Occasionally both spots were seen, but the one was "thicker," or "much thicker," than the other, while the thinner was, or seemed to be, longer. After six years of watching he writes, "The white polar spot increases in size; it is very luminous." The conclusions he drew from these notes in his journal, and from his calculations to ascertain the seasons on Mars, must have been listened to by those who first heard them read as if they were a page or two from a romance by Fielding or Smollett. We give them in Herschel's own words.
"The analogy between Mars and the earth is, perhaps, by far the greatest in the whole solar system.... If then we find that the globe we inhabit has its polar regions frozen and covered with mountains of ice and snow, that only partly melt when alternately exposed to the sun, I may well be permitted to surmise that the same causes may probably have the same effect on the globe of Mars; that the bright polar spots are owing to the vivid reflection of light from frozen regions; and that the reduction of those 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 twelvemonth's darkness and absence of the sun; but in 1783 I found it considerably smaller than before, and it decreased continually from the 20th of May till about the middle of September, when it seemed to be at a stand. During this last period the south pole had already been above eight months enjoying the benefit of summer, and still continued to receive the sunbeams; though, towards the latter end, in such an oblique direction as to be but little benefited by them. On the other hand, in the year 1781, the north polar spot, which had then been its twelvemonth in the sunshine, and was but lately returning to darkness, appeared small, though undoubtedly increasing in size." The length of the year in Mars is nearly two of our years, and the distance from us varies from about 230 to 50 millions of miles.
Astronomers, previous to Herschel's time, had found that Mars was surrounded by an atmosphere like the earth. One of them, Cassini, seems to have suspected the existence of an atmosphere of great density, and rising to a height of about 70,000 miles above the planet's surface. Herschel used the same means as Cassini to determine the height of the atmosphere of Mars by watching the fading or going out of starlight, when a star came up to its limb. At a distance of 30,000 miles there was no indication of an atmosphere. "It appears, however, that the planet is not without a considerable atmosphere. For besides the permanent spots on its surface, I have often noticed," he says, "occasional changes of partial bright belts and also once a darkish one in a pretty high latitude. And these alterations we can hardly ascribe to any other cause than the variable disposition of clouds and vapours floating in the atmosphere of that planet." From the fact that the dark belts or spots and the red colour of Mars manifestly belong to the surface of the planet, we may accept Herschel's idea "that its inhabitants probably enjoy a situation in many respects similar to ours." It has been shown in our own day that the vapour of water, and with that we may associate clouds, is present in the atmosphere of Mars. But there is reason to believe that the atmosphere of Mars is comparatively rare.
Jupiter was not one of the planets from which Herschel reaped an ungathered harvest. The field had been so thoroughly worked by others in searching for a method of easily discovering the longitude at sea, that it does not seem to have presented the same attractions to him as other planets did. A paper which he wrote on Jupiter in 1797—and he wrote no other—gives many curious quotations from his journal regarding the planet and its satellites. So minute are the discoveries made of change of colour and apparent size of the satellites that if the Red spot, detected on the planet in 1878, had been visible in his day, he could scarcely have failed to see it. The bands or belts on the body of the planet, the white and dark spots they showed, the length of day they indicated, and the rotation of the four satellites round their primary were the principal points attended to by him. The results he arrived at were very near the reality.
Time of rotation of Jupiter on his axis—
h. m. s.
9 55 49
Time of revolution in its orbit of —
If the white spots on the belts were connected with drifting masses in Jupiter's atmosphere, they would drift as well as rotate. Herschel was aware of this, and, since his day, the amount of drift has been estimated at 270 miles an hour in the same direction as the rotation. In other words, they would take 42 days to go round the planet from this cause alone. Herschel was also persuaded that the four satellites revolve on their axes in the same period as they revolve round Jupiter, resembling in this respect our moon. Laplace was disposed to accept this conclusion.
For more than a century and a half the planet Saturn had been the object and, it may be said, the despair of every astronomer's curiosity, mainly in consequence of the ring which the telescope had shown it to possess, and the singular shapes the ring was found to assume. Five moons were also discovered to be circling round the planet, and Messier, viewing the planet in 1766 with what he calls "an achromatic reflector of 10 feet 7 inches focus," "perceived on his globe two darkish belts, extremely faint and difficult to be discerned, directed, however, in a right line parallel to the longest diameter of the ring." However, till Herschel applied his 40-feet reflector to its system, discovery may be said to have reached its limits. To "the liberal support, whereby our most benevolent King has enabled his humble astronomer to complete the arduous undertaking of constructing this instrument," Herschel writes, was due the discovery of other two moons or satellites, a fuller knowledge of the nature of the ring, and, in short, a new era in our knowledge of that wonderful system. An object so engaging drew Herschel's attention as early as the spring of 1774, long before he was known to fame. On the 17th of March that year, with a 51-feet reflector, he saw the ring "reduced to a very minute line," and the planet looking like a ball with a knitting-needle projecting through it on both sides. About a fortnight after, the ends of this axis had vanished, and a dark band or shadow crossed the planet's equator from side to side. In the following year he saw the ring gradually open out, with a "dark zone contained between two concentric circles," as if there were two rings with an open space between them. For ten years he continued watching the planet with telescopes of various powers, suspicious that it had not told astronomers all the story of its ring and satellitea The ten years' watch lengthened out to twenty, and the twenty to thirty or more, but this eager watcher still kept guard, ready to take advantage of the slightest lifting of the curtain which concealed a world of wonders from view.
As soon as his great mirror was finished, he turned it on Saturn, and "the very first moment he saw the planet, on August 28, 1789, "he was presented with a view of six of its satellites," in such a situation and so bright as rendered it impossible to mistake them or not to see them." Five of these satellites had been known for more than a century: a sixth was thus added. Constantly continuing his watch on the planet, he was rewarded, three weeks after, with discovering a seventh so close to the planet that the telescopes, previously in use, had failed to find it. Even in his great mirror "it appeared no bigger than a very small lucid point," and it lies so near the planet and its ring that "except in very fine weather, it cannot easily be seen well enough to take its place with accuracy." But he learned from experience, and taught others the lesson, that it is easier to find a small body which has been once seen, and whose place has been marked, than to detect it for the first time amid a crowd of other heavenly bodies. The heavens teach wisdom even in the littlest things, but the lessons they teach are sometimes forgotten as soon as learned. He found also that the time of a sidereal revolution round the planet is 22 hours, 37 minutes, 22 seconds. Both it and the other moon he discovered revolve so near and so parallel to the ring, that he had "repeatedly seen them run along its very minute arms "at the rate of 9 or 10 miles a second! He was looking from Windsor across a gulf in space about nine hundred millions of miles in width. It was a romance of the heavens—one of many.
On ascertaining that his great telescope was not required for these observations on the ring and moons of Saturn, he "made ten new object specula and fourteen small plain ones for his 7-feet reflector, having already found that the maximum of distinctness might be much easier obtained than where large apertures are concerned." During his long-continued watch of Saturn he saw sometimes a northern belt on the body of the planet, sometimes two belts at the equator. In a couple of days the entry in his journal became "a bright belt over a dark one"; and, nine days later, "one dark and one very faint white belt." The last entry he quotes in 1790 is, "The bright belt close to the ring and two dark equatorial belts." These belts would be about one hundred thousand miles in length: what were they? Similar belts or bands had long been seen and studied on the planet Jupiter. It was agreed among observers that they were probably due to cloudy masses floating in Jupiter's atmosphere. If the same explanation hold for the belts of Saturn, the changes, seen on them by Herschel, would be explained by "a very considerable atmosphere," in which they take place. He not only adopted this conclusion, but confirmed it by another observation. When the two nearest of the moons—the two he discovered in 1789—came, in their progress round the planet, to the edge of the disc, they did not disappear at once, but continued "to hang to the disk a long while before they would vanish" The seventh or innermost (Mimas) thus hung on the disc for twenty minutes, and the sixth for fourteen or fifteen. Had there been no atmosphere, both of the moons would have been at once hid behind the planet. This takes place when a star comes up to our moon, and vanishes behind it. The star is seen to go out at once; and the conclusion drawn is that this could not happen unless there were no atmosphere or very little of it in the moon to keep the star in sight for us after it had really vanished. Our atmosphere gives us twilight, morning and evening, and enables us to see the sun some minutes before he rises, and for as long after he has set. Ultimately Herschel perceived a quintuple belt, two dark and three bright, on Saturn. Sometimes also he noticed a whitish light at the poles similar to the polar spots on Mars, and due, he believed, to the same cause. But what these belts really are is a problem still unsolved. The vast body of Saturn is lighter than the same volume of water, and would float in it like cork. Our earth is about five times heavier than a globe of water of the same size, and would sink in water like lead. Whether Saturn is still a heated mass, slowly cooling down, and these clouds arise from streams of gas given off, remain problems for the future to solve.
With improved mirrors and a less powerful telescope, he watched the movements and changes of the ring. Between 1790 and 1806 he wrote seven papers for the Royal Society on Saturn and his system. Slowly he came to the conclusion, which he dismissed at first as improbable, that the ring was not single, but double, with a gulf twenty-five hundred miles  in width between the two parts. The black disc or belt was not in the middle of the ring's breadth. "It is a zone of considerable breadth," which was always seen permanently in the same place. As it was not, what some seem to have supposed, the shadow of a vast range of mountains on the ring's surface, he resolved to wait till the planet came into a position which would enable him to see the stars through the black belt, if it really were a division in the ring, a window, as it were, through which he could look out into space beyond. He does not appear to have been successful in this quest, and it has not been done by others. That there were two unequal rings, separated by this black line, he was satisfied. They were bright rings, but the inner was the brighter of the two. Near the outer edge of the outer ring, he observed and figured "a black list," fainter than the dividing gulf. He did not consider it a division in the outer ring, but it is now a recognised feature, traceable all round. Herschel also saw the edge of the ring as a thin rim of light, and, from some spots seen on it, inferred that it rotated round the planet in 10 hours, 32 minutes, 15 seconds. The planet itself revolves in 10 hours, 14 minutes, 23 seconds.
Highly interesting was the story thus told by the planet; but Herschel wrung from it other details. He suspected that an eighth satellite existed, but it was reserved for others to discover an eighth, and, it is now said, a ninth, at great distances from the planet. But the rings continued to be a puzzle, which baffled solution. He observed lucid points, different from the satellites, coming between the ring and his eye, and moving along it in their orbits. If they were not satellites, what were they? He was not mistaken in "the frequent appearance of protuberant and lucid points on the arms of the ring of Saturn." They were realities, not illusions, not an enchantment lent by the vast distance at which he saw them. "Many of these bright points," he writes, "were completely accounted for by the calculated places of the satellites"; but there were many more which remained inexplicable. He could not entertain the idea that these points "would denote immense mountains of elevated surface." He rather inclined to the belief that the ring was in a state of rotation round the planet, and that one at least of the shining spots might be a moon bedded in or somehow connected with the ring, floating, it might be, in a fluid like water, or running in "a notch, groove or division of the ring to suffer the satellite to pass along." He was perhaps not far from the truth in these romantic imaginings. But the light of the ring is generally brighter than that of the planet, and he even imagined that the shining spots may owe "their existence to inherent fires acting with great violence." "Nay, we have pretty good reason to believe," he said, "that probably all the planets emit light in some degree; for the illumination which remains on the moon in a total eclipse cannot be entirely ascribed to the light which may reach it by the refraction of the earth's atmosphere." This idea is not borne out by recent observations.
The first two papers Herschel wrote on Saturn, containing the record of more than fourteen years' work, cover nearly ninety pages quarto. Fifty of these pages are merely extracts from his journal, showing the nightly work in which he was engaged, jottings, it may be, all of which required from him time and care, before they could be put down on paper. Here is a specimen of two nights' work, done shortly before midnight:—
"Nov. 7: 22, 9. At the end of the p. arm is a place that is brighter than nearer to the body.
"23, 12. The preceding arm has still the appearance of a small protuberant point towards the south, near the end of the arm.
"Nov. 8: 23, 40. There is a protuberant point on the preceding arm besides the 7th sat.; so that at present I cannot tell whether the satellite be the nearest or farthest of them."
By patient, long-continued labour, carried on at all hours of the day and night, is a way prepared for advancing the boundaries of human knowledge, though few are capable of estimating, far less of bearing, the cost in time and comfort, by the sacrifice of which it is purchased for mankind.
That Herschel was surprised by the brightness of the rings, the greater brightness of the shining points he saw on them, and the yellowish light of the planet, is quite clear. Whether he ever suspected a light or phosphorescence of its own in the system of Saturn, as some observers have now come to think exists, is another matter. But he was on the threshold of that discovery, if discovery it be. He entertained no such idea in 1789 when he classed all the planets "under one general definition, of bodies not luminous in themselves," though two years of farther reflection and observation may have wrought a change in a man of his clear perception and quickness. On another view developed since his day he almost anticipated recent research. He denied that the ring was subdivided by many dark lines into a series of concentric rings, "as has been represented in divers treatises of astronomy." He firmly held to only one division; but he was not far from the modern view, which represents the ring as a mighty mass of revolving satellites, kept in position by the gravity of the planet and the velocity of their rotation round him.
Herschel's memoirs on Saturn cover about one hundred and seventy pages quarto, and the plates that accompany them give a distinct idea of what he saw. By comparing letterpress and plate we may better understand the relation in which he stood to his followers in this field of research and discovery. With one of the new specula, which he ground apparently for the purpose of observing the ring of Saturn more carefully, he got views that he speaks of as "uncommonly distinct." Of these views he writes: "The outer ring is less bright than the inner ring. The inner ring is very bright close to the dividing space, and at about half its breadth it begins to change colour, gradually growing fainter, and just upon the inner edge it is almost of the colour of the dark part of the quintuple belt." A little after he adds: "The shadow of the ring upon Saturn, on each side, is bent a little southwards, so that the apparent curve it makes departs a little from the ring." Looking at these singular companions of the planet across a gulf eight or nine hundred millions of miles wide, it is not surprising that an astronomer prays for "light, more light," to resolve this puzzle of the bright and the dark. It is only an outline of the ring, at the best, that we can expect to obtain from the most careful drawings. But what Herschel did not suspect or imagine about the ring, it would be natural for him to confound with, other features that took a greater hold of his fancy. Of the inner ring he says: "At about half its breadth it begins to change colour," that is, it passes from "very bright" to the darkness of the quintuple belt. Now this was said of the ring as seen and figured in 1794. Compare it with the three rings in the three figures shown in 1792. They are unlike that of 1794. Either the ring had changed, or Herschel was in 1794 looking on two inner rings, a bright or very bright ring, and a dark. This was Professor Bond's discovery in 1850, "a crape ring" half the breadth of the very bright inner ring, between it and the body of the planet. There are thus three well-marked rings in the system of Saturn, a somewhat dark outer, a very bright inner, and a "crape" or slate-coloured ring nearer still to the planet. Did Herschel not see and figure all three, only failing to observe the interval between the very bright and the "crape" ring? We can only express our surprise if one so quick of eye, and so careful to observe, ascribed to the bright ring in 1794, what he did not see or delineate on it in 1792, if the "crape" existed then as it exists now.
Fifty years after, Sir John Herschel, when at the Cape of Good Hope, made a careful search for the two moons discovered by his illustrious father. He had all but given it up in despair when, looking for the other five "with the 20-feet reflector," which he took with him to South Africa, "and a polished new mirror, there stood Mr. Sixth! . . . Next night it was kept in view long enough for Saturn to have left it behind by its own motion, had it been a star. . . . So this is at last a thing made out," he writes. "As for No. Seven, I have no hope of ever seeing it."
Since Herschel's time the minds of men have become familiar with strings of meteorites, millions of miles in length, through which our earth plunges in its yearly journey round the sun. If they form, or come in time to form, a continuous ring about the sun, one hundred thousand miles in breadth, we may have on a vastly larger scale a parallel to the rings of Saturn. The breadth of the latter is only about one-third of the breadth of one well-known stream of meteors, and their length is not a quarter of a million of miles. If then these rings of the planet are similarly composed of separate masses, great and small, and are not continuous rings, perhaps 250 miles in thickness, a satellite "floating in a fluid like water, or running in a notch, groove or division of the ring," while it ceases to be a fanciful, becomes also an unnecessary conception.
Such are the main features of the romance of Saturn since Herschel began his study of it one hundred and twenty-five years ago. In the hundred and twenty-five years that preceded, there had also been mystery and romance about the planet and his ring. All the riddles presented by this system have not been yet read, and it is likely that, when improvements in telescopes or observation enable man to read the riddles that face him to-day, they will raise new riddles and give birth to other romances for the amazement or delight of future ages. On one point science is still in doubt. Does the fifth satellite of Saturn, like our moon, always show the same face to the planet, or, in other words, turn on its axis in the same time that it takes to revolve round him? Herschel believed he had proved, or almost proved, that it "turns once on its axis, exactly in the time it performs one revolution round its primary planet."
It was only fitting that the discoverer of Uranus should pay special attention to that planet: but five or six years elapsed before his patient watchfulness was crowned with any success. Unlike Jupiter and Saturn, the light of Uranus is very faint. He does not invite pursuit; he flies from it into darkness: and the light of his moons is fainter still. Herschel suspected, perhaps hoped, that if he searched for satellites he would find them. And so he did. On January 11, 1787, he saw "some very faint stars" near the planet, "whose places he noted down with great care." Next evening two of them were missing. As the haziness, that was about, might have caused their disappearance, he noted "all the small stars near the planet the 14th, 17th, 18th, 24th of January, and the 4th and 5th of February." On the 7th of February he kept one star in view for nine hours, from six in the evening till three next morning. His journal records that he saw it "faithfully attend its primary planet." On the second night after, he was so satisfied of having caught sight of a second moon, that he delineated on paper what he expected to see the following evening. And he saw in the clear heavens what he sketched sixteen or seventeen hundred million of miles away, "The Georgian Planet, attended by two satellites." Oberon and Titania are the fairy names by which they are now known. "I confess," he adds, "that this scene appeared to me with additional beauty, as the little secondary planets seemed to give a dignity to the primary one, which raises it into a more conspicuous situation among the great bodies of our system. For upwards of five hours I saw them go on together, each pursuing its own track." It was the heroic age of astronomical research. A hero there and a hero here were wrestling with difficulties and winning triumphs in the world of stars. They were men of extraordinary skill and unwearied endurance. It was nearly fifty years after their discovery before the fairies, Oberon and Titania, again condescended to show themselves to a mortal, the son of their discoverer. And it enabled his aunt, then ninety years of age, to write: "These folks would not have called the Herschelian construction useless, if they had seen the struggle, during the years from 1781 to '86, to get a sight of the satellites of the Georgium Sidus, when, after throwing aside the speculum, they stood broad before us."
From observations continued on Uranus for fifteen years, Herschel first suspected, and then became convinced that other satellites besides the two, which he discovered in 1787, attend the planet on its journey round the sun. It was labour of love not lost, or grudgingly given, but the fruits it yielded were Dead Sea apples with a fair outside and rottenness within. He believed he saw other four moons circling round Uranus apparently in an opposite direction to other planets, that is, from east to west, not from west to east. He also suspected that it had a ring round it, or two rings; then he gave up the idea; then he entered in his journal, "When the satellites are best in focus, the suspicion of a ring is the strongest"; and nine months after he adds, "The planet is not round, and I have not much doubt but that it has a ring." He used "successively powers rising from 240 to 2400," more than two years after, "without any suspicion of a ring." A fortnight later he tried magnifying powers of 2400 and 4800. In conclusion he believed in the four new satellites, but gave the ring up. A traveller in unexplored regions of the heavens may thus be as much the victim of a mirage as a wanderer in the thirsty deserts of earth. But a singular thing was observed: these moons of Uranus became invisible when they approached the planet, which those of Jupiter and Saturn never did till the planet got between them and us. What was the reason?
The cause is in the eye of the observer himself. It requires to adapt itself to the light which falls on the retina. Now "the planet is very faint; and the influence of its feeble light cannot extend far with any degree of equality. This enables us to see the faintest objects, even when they are only a minute or two removed from it. The satellites are very nearly the dimmest objects that can be seen in the heavens; so that they cannot bear any considerable diminution of their light, by a contrast with a more luminous object, without becoming invisible. If then the sphere of illumination of our new planet be limited to 18" or 20", we may fully account for the loss of the satellites when they come within its reach; for they have very little light to lose, and lose it pretty suddenly." This view of a weak light extinguishing a weaker, though a commonplace now, received a very poetical interpretation in a paper written by Herschel three years after. "This increased sensibility," he says, "was such, that if a star of the 3rd magnitude came towards the field of view, I found it necessary to withdraw the eye before its entrance, in order not to injure the delicacy of vision acquired by long continuance in the dark. The transit of large stars, unless where none of the 6th or 7th magnitude could be had, has generally been declined in my sweeps, even with the 20-feet telescope. And I remember, that after a considerable sweep with the 40-feet instrument, the appearance of Sirius announced itself, at a great distance, like the dawn of the morning, and came on by degrees, increasing in brightness, till this brilliant star at last entered the field of view of the telescope, with all the splendour of the rising sun, and forced me to take the eye from that beautiful sight." To increase this sensibility of the eye he was on these occasions in the habit of excluding light from surrounding objects by wearing a black hood.
Herschel was not content with wresting from Uranus this novel part of his story. He continued to watch the planet. Unfortunately, the same success did not crown his efforts to read its history. A great number of observations on imaginary rings and supposed moons, that were found to be stars, or not moons but probably moving, planetary bodies of the asteroid nature, demanded his attention, and deceived his hopes. It was such a tantalising pursuit, that even "the direction of a current of air alone may affect vision." At last he came to the conclusion that no ring, similar to Saturn's, girdles Uranus; but that, certainly, four additional satellites accompany him on his long journey of eighty-four years round the sun. Astronomers who came after his time failed to find these four moons, but, later still, two satellites have been added to the original two discovered by Herschel. One of the additions is suspected to belong to the four he believed he had seen circling round the planet. Of the four recognised satellites the most distant, Oberon, performs its round in 13⋅46 days, or, as Herschel found, 13 days, 11 hours, 5 minutes, 11 seconds. Other information, which by careful watching he wrung from Uranus, has been verified or corrected by those who came after him; but to this unwearied observer belongs the credit of showing that the two satellites he discovered, unlike other members of the solar system, revolve in orbits nearly at right angles to the ecliptic, and that their course is retrograde, or from east to west, not direct, that is, from west to east. These were two singular and outstanding discoveries made by Herschel in the system of Uranus.
The two small planets, Ceres and Pallas, discovered in 1801 and 1807, have strangely given the tooth of envy an opportunity of wounding the good name of Herschel. As he found their discs like those of fixed stars, spurious and not measurable; as they "resembled small stars so much as hardly to be distinguished from them even by very good telescopes," as he imagined them from the haziness he saw around them to be "comets in disguise," he considered planet a misnomer as applied to them, and proposed to call them asteroids. Strange to say, the friend of Piazzi and Olbers, who discovered these small bodies, was charged with intending, by the suggestion of this diminutive, to cast a slight on the achievement of his friends, in comparison with his own glory as the discoverer of the great planet, Uranus. A more stupid slander of a most generous heart could scarcely be imagined. He predicted that the association of astronomers which had been formed on the Continent to hunt for more of them would be successful: "Many may soon be discovered," he informed the Royal Society. Two were caught within the next five years, Juno and Vesta, but the "many" foretold by Herschel in 1802 remained an unfulfilled prediction for more than forty years. He himself joined in the hunt, and failed: "I have already made five reviews of the Zodiac without detecting any of these concealed objects." Yet he was slandered as envious of the fame of others who had done what he confessed he had failed in doing, although in 1813 he told Thomas Campbell, the poet, that " there will be thousands—perhaps thirty thousand more—yet discovered." The discovery of the fourth, called Vesta, he pronounced "an event of such consequence" as to "engage his immediate attention." He called it "a valuable addition to our increasing catalogue of asteroids"; and he spoke of the "celebrated discoverers" as inducing "us to hope that some farther light may soon be thrown upon this new and most interesting branch of astronomy." Dr. Olbers himself wrote to Herschel that Vesta "was not to be distinguished from a fixed star"; while Schroeter, the countryman and neighbour of Olbers, had already communicated a paper to the Royal Society in which he said: "Its image was, without the least difference, that of a fixed star of the 6th magnitude with an intense radiating light; so that this new planet may with the greatest propriety be called an asteroid" That one scientific man should attack, or rather slander, another for giving to these small bodies a scientifically appropriate name, on the ground that he thereby intended to derogate from the credit of his own friends, whom he publicly extolled as "celebrated discoverers," seems incredible. Yet it was done.
By a most ingenious contrivance he managed to obtain approximate values for the diameters of Ceres and Pallas. The former he found to be 161⋅6 miles; the latter smaller, 147 or 1101 miles. So small is Pallas that it would require many thousands equally small to make up a planet no larger than Mercury. The colour of Ceres he found to be "ruddy, but not very deep"; that of Pallas, "milky whitish."
In 1807 Herschel concluded one of his papers in these words: "I find that out of the sixteen comets which I have examined, fourteen have been without any visible solid body in their centre, and that the other two had a very ill-defined small central light, which might perhaps be called a nucleus, but did not deserve the name of a disk." In the end of September that year a comet was discovered by Mr. Pigott, to which Herschel at once turned his attention in the hope of wresting from it information regarding its nature. By careful observations, continued over five months, he felt himself warranted in claiming for it "a visible, round and well-defined disk," 538 miles in diameter, and "shining in every part of it with equal brightness." He came also to the conclusion "that the body of the comet on its surface is self-luminous, from whatever cause this quality may be derived." He inferred besides that "the changes in the brightness of the small stars, when they are successively immerged in the tail or coma of the comet, or cleared from them, prove evidently, that they are sufficiently dense to obstruct the free passage of star-light." The tail of this comet, three weeks after its discovery, was more than nine millions of miles in length, and Herschel was inclined to think that it "consisted of radiant matter, such as, for instance, the aurora borealis." It was not bifid or split in two, as that of the comet of 1769 had been, but it presented a peculiarity seen also in others of these bodies: "The south-preceding side, in all its length, except towards the end, is very well defined: but the north-following side is everywhere hazy and irregular, especially towards the end; it is also shorter than the south-preceding one, . . . even to the naked eye,"
If Herschel had not known this body to be a comet, he would have described its head, as "a very large, brilliant, round nebula, suddenly much brighter in the middle." He says that he would have added, "The centre of it might consist of very small stars." So struck was he with this singular idea that he directed a telescope "with a high power to the comet." He then saw "several small stars shining through the nebulosity of the coma." The terror which once surrounded the appearance of these bodies in the heavens is gone; the awe remains, and, as knowledge increases, the mysteries that attend their birth, their growth, their flight through space, have become greater and more wonderful problems awaiting solution.
- The mirror of the reflector used on this occasion was made of glass.
- Thirty-six semi-diameters of the planet. The atmosphere of the earth is now supposed to be about 500 miles in height.
- The great red spot gives 9 h. 65 m. 34 s.
- System of the Worlds, Bk. I. chs. viii. vii.
- Phil. Trans., 1769., vol. lix. p. 459.
- One discovered by Huyghens in 1655, and four by Cassini in 1671 and onwards.
- Compare the ease with which observers detected the small companion of Sirius, and the "crape" ring of Saturn after they were once detected (Ball, Story of the Heavens, p. 387).
- The dimensions of Saturn and his rings are, according to Proctor (Encyc. Brit., "Astronomy," p. 783)—
Diameter of the planet
milesBetween planet and "crape" ring "Breadth of "crape" ring ""of inner bright ring ""of division between bright rings ""of outer bright ring "
The diameter of the ring system is thus about 165,000 miles. Herschel made it about (204,883) 205,000 miles in diameter. He believed that the breadth of the ring is to the space between the ring and the planet as 5 to 4 (Phil. Trans., 1806, p. 463). If the "crape" be left out of account in measuring the ring, the proportion is about 5 to 3⋅2 (Phil. Trans. for 1792). He estimates the vacant space between the outer and inner rings at nearly 2513 miles.
- In the proportion of 805 to 280, while the space between was reckoned 115.
- Phil Trans., 1790, p. 485 (vol. lxxx.). The seventh and sixth, though last discovered, are nearest to the planet. The longer-known five used to be named in the order of their distance from it.
- Phil Trans., 1794, pp. 54, 57.
- Holden, Life wnd Works of W, H., p. 143. But see Caroline's Memoirs, pp. 261, 305.
- Phil. Trans. for 1802, pp. 228-30.
- Letter from Dr. Olbers, April 20, 1807.
- Phil. Trans., 1807, p. 260.
- Phil. Trans., May 28, 1807, p. 246.