William Herschel and his work/Chapter 5

From Wikisource
Jump to navigation Jump to search

CHAPTER V

THE DISCOVERY OF URANUS

The third paper sent by Herschel to the Royal Society was in the form of a letter to Dr. Watson from Mr. William Herschel of Bath, dated October 18, 1780. It was a record of observations made in the three years from 1777 to 1779, with the view of determining whether our day is of the same length year after year. A point so difficult could be settled, he thought, only by observing the length of the day in other planets. This had been done, or attempted, for Venus and Jupiter, by watching the time it took for a spot on the face of the planet to return to the same position. But in Venus, on account of her exceeding brilliance, it had been done so imperfectly that her day was put down roughly as of 23 hours' length. For Jupiter the time of rotation on his axis was set down more precisely at 9 hours 56 minutes, a result arrived at by keeping careful watch on spots that may not be fixed points on his disc, but movable on what we may call trade-wind belts of clouds in his equator. These spots "change so often that it is not easy, if at all possible, to ascertain the identity of the same appearance for any considerable length of time." Sometimes a bright, at other times a dark spot, or belt, was observed, but the time of its revolution round the planet varied so much that no reliance could be placed on the result as a means of ascertaining whether our day remains the same from age to age.

Herschel considered the planet Mars a more favourable field for experiment than Jupiter. On Mars he saw spots of a different nature: "Their constant and determined shape, as well as remarkable colour, show them to be permanent and fastened to the body of the planet. These will give the revolution of his equator to a great certainty, and by a great number of revolutions, to a very great exactness also." A circumstance, with which Herschel was not acquainted, materially helped him in his observations on Mars. The atmosphere on that planet is not nearly so dense as the earth's, and similar trade-wind belts to those on Jupiter do not seem to exist. By these means he concluded that the length of a day on Mars is a little longer than our day, or 24 hrs. 39 min. 5 sec.[1] The value of an accurate measure of the length of day in other planets he conceived to be this: "Future astronomers may be enabled to make some estimate of the general equability of the rotatory motions of the planets. For if in length of time they should perceive some small retardation in the diurnal motion of a planet, occasioned by some resistance of a very subtle medium in which the heavenly bodies perhaps move, or, on the other hand, if there should be found an acceleration from some cause or other, they might then ascribe the alteration either to the diurnal motion of the earth, or to the gyration of the other planet, according as circumstances, or observed phenomena, should make one or the other of these opinions most probable." This man could think, could reason and observe: he had also unusual powers of imagination: but he was only beginning his travels through the infinitudes of space and time.

Three papers for the Royal Society in the course of ten months! The musician of Bath puts himself at once on a level with the first men of science in the kingdom. He is modest, but he has in him the confidence of true genius. In his retirement he had been collecting facts from the heavens for six or seven years. A chance of speaking out what he saw and had gathered together was presented to him. He seized it with all eagerness, and was making his voice heard. In these papers he has been speaking to the Royal Society, of which he was not even a member. When he speaks next, about three months after, it is not as the musician of Bath, but as a member of the Royal Society; and he speaks to the whole world and to all time. This paper, which was read on April 26, 1781, and,is headed "Account of a Comet," was really the beginning of modern astronomy. It fills only ten pages of the Transactions.

He had been engaged for some time in an attempt, not altogether novel, but certainly demanding great labour, to find out the distance of the fixed stars. His thoughts and plans were high, for though more than a century has passed since then, the distances of not more than twenty or thirty out of many millions can be said to be known, or perhaps safely guessed. While thus engaged, rummaging among the stars, "on Tuesday, 13th March, between ten and eleven in the evening, he perceived a star, in the neighbourhood of H Geminorum, that appeared visibly larger than the rest. Being struck with its uncommon magnitude, he compared it to H Geminorum and the small star in the quartile between Auriga and Gemini, and finding it so much larger than either of them, suspected it to be a comet. . . . The sequel has shown that my surmises were well founded, this proving to be the comet we have lately observed." By the method he followed he was "enabled to distinguish the quantity and direction of the motion of this comet in a single day, to a much greater degree of exactness than could have been done in so short a time by a sector or transit instrument; nay, even an hour or two were intervals long enough to show that it was a moving body, and, consequently, had its size not pointed it out as a comet, the change of place, though so trifling as 21/4 seconds per hour, would have been sufficient to occasion the discovery." Satisfied that he had done all he could do, Herschel concluded his paper by saying, "I failed not to give immediate notice of this moving star, and was happy to surrender it to the care of the Astronomer-Royal and others, as soon as I found they had begun their observations upon it" The moving star was not a comet. It was a wanderer, who had been seen before and classified as a fixed star. The planet was what is now called Uranus.

The announcement of the discovery sent a flutter of excitement through all the observatories of Europe, which went on increasing when it was found that they could not agree on what or who the stranger was. Almost from its first appearance English astronomers believed it to be a planet that had long been wanted to account for difficulties in their art. The French astronomers held to their faith in a comet moving round the sun in an orbit nearly circular. Herschel, praised everywhere as an observer "of great ardour and ingenuity," stood aside from the friendly strife. All observers were in debt to Bode, who found that a star, marked No. 964 in Mayer's catalogue, had been observed by him in 1756, had then been lost sight of, and was probably the stranger. Abbé Boscovich is said to have been the first to prove that the orbit was an ellipse; but to Lexell, Professor of Astronomy at St. Petersburg, is assigned the honour of showing that the newly found body was not a comet, but a planet, distant from the sun about nineteen times as far as the earth.[2] All with a name for science, from Laplace downward, took part in the friendly strife.

It has been said that this discovery was an accident; it has been also said that, if Herschel had not made it at the time he did, some other observer would before long have had the luck to fall in with the stranger. These criticisms are not creditable to those by whom they were made. Call it accident or chance, the fact remains that this novice, looking out for what he could find in the heavens, and with instruments improved by himself, discovered an unknown planet, and extended the boundaries of the solar system to twice the distance that had been received for thousands of years. Such accidents bring fame, and are only called luck by the envious.

One of the last-found planets of our solar system was discovered about a year ago, also by accident, but to the great honour of the discoverer. He was looking for something else; he found what he was looking for, and a new planet besides. What he was looking for was one of the so-called nuisances of the heavens, an asteroid, one of about 450, named 433d. To search for it as Herschel had to do, even though its whereabouts was known, called for labour and time. The astronomer, who was on the lookout for it, lessened both by exposing a photographic plate to the starry sky. He was spreading a net to catch planets and comets. A fixed star does not change its place during the exposure of the plate, or, rather, the plate moves as the star moves: a moving body, be it planet or comet, does change its place. A point will thus representa fixed star; a line, however short, and however faint the trace, represents a moving body. When Herr Witt examined the exposed plate, he saw at once the trace left by the asteroid he was in search of; but another, a fainter and a longer trace of a moving body, was also seen on the plate. It was the trace of a planet hitherto unknown. An examination of the stranger resulted in the discovery that he was a ball twenty miles in diameter, and, excepting our moon, the nearest of the planets to us, so near that he may be made to tell us the exact distance we are from the sun. His discoverer called him Eros, Love or Cupid, evidently from his childish size.[3] Herschel had no such short-cuts to discovery in his day.

An immense impulse was given to the study of the stars by Herschel's discovery. It was not merely what he achieved by being on the spot and on the lookout. It was also by the lesson he taught astronomers to do as he did. A band of twenty-four observers, suspecting, and with good reason, that a well-kept watch would reveal unknown wonders in the depths of space, undertook to search for other planets. Had photographic plates or charts then been part of the equipment of an observatory, the work would have been easy, and the reward certain. But plates and star-charts were not known; and the twenty-four workers laboured and toiled in vain. An outsider carried off first honours on the first day of the century—Piazzi of Palermo, who had visited Slough, had talked with Herschel and his sister, and perhaps drawn a breath of inspiration from them and their surroundings. The beaten twenty-four astronomers did not retire from the field. Two years later, Dr. Olbers, of Bremen, discovered another asteroid, Pallas; and two years later still, Harding, in the same neighbourhood, discovered a third, Juno. Olbers, wisely using imagination in the pursuit of science, came to the conclusion that these small bodies were pieces of a planet which had burst or exploded, and that other pieces would be found floating about in space. He acted on the idea, and rediscovered Piazzi's Ceres, which had been lost again, as well as a fourth asteroid, Vesta. Then the hunt for more pieces of the disrupted planet ceased, till, about forty years later, it again received a fresh impetus from Hencke's discovery of Astræa, and was continued by Mr. Hind at the Regent Park Observatory in London, and others, with such success that floating pieces have been netted by hundreds, grumbled at as nuisances, and assigned the honour of having been thrown off direct by the sun himself, not blown into space by a disrupted planet. One of these pigmy planets was named Lucretia, after Herschel's sister. Such were some of the fruits of William Herschel's earliest studies among the stars.

The nature of the wandering stranger discovered on March 13, 1781, was not fully known for some months. Herschel had surrendered the care of his new world to the astronomers of Europe, and they could not make up their minds about it, till Lexell of St. Petersburg led the way by showing that it was an outlying primary planet. A whole year elapsed, and Herschel had resumed his observations on this "singular star" before he thought of giving it a name. Events had happened during the interval which affected his view of the name it should bear: he had become Royal Astronomer to George iii., had received from him a yearly pension, was pursuing a profitable trade as a maker of telescopes under the King's patronage, and was housed under the shelter of Windsor Castle. It should cause no surprise, therefore, that, evidently after long consideration, he addressed the following letter to Sir Joseph Banks, President of the Royal Society:—

"To Sir Joseph Banks, Bart., P.R.S.

"Sir,—By the observations of the most eminent astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a primary planet of our solar system. A body so nearly related to us by its similar condition and situation, in the unbounded expanse of the starry heavens, must often be the subject of the conversation, not only of astronomers, but of every lover of science in general. This consideration then makes it necessary to give it a name, whereby it may be distinguished from the rest of the planets and fixed stars.

"In the fabulous ages of ancient times the appellations of Mercury, Venus, Mars, Jupiter, and Saturn were given to the planets as being the names of their principal heroes and divinities.[4] In the present more philosophical æra, it would hardly be allowable to have recourse to the same method, and call on Juno, Pallas, Apollo, or Minerva for a name to our new heavenly body. The first consideration in any particular event, or remarkable incident, seems to be its chronology: if in any future age it should be asked, when this last-found planet was discovered? it would be a very satisfactory answer to say, 'In the Reign of King George the Third.' As a philosopher then, the name of Georgium Sidus presents itself to me, as an appellation which will conveniently convey the information of the time and country where and when it was brought to view. But as a subject of the best of Kings, who is the liberal protector of every art and science;—as a native of the country from whence this Illustrious Family was called to the British throne;—as a member of that Society, which flourishes by the distinguished liberality of its Royal Patron;—and, last of all, as a person now more immediately under the protection of this excellent Monarch, and owing everything to His unlimited bounty;—I cannot but wish to take this opportunity of expressing my sense of gratitude, by giving the name Georgium Sidus,

'Georgium Sidus
—jam nunc assuesce vocari' (Virg. Georg.),

to a star, which (with respect to us) first began to shine under His auspicious reign.

"By addressing this letter to you, Sir, as President of the Royal Society, I take the most effectual method of communicating that name to the Literati of Europe which I hope they will receive with pleasure. I have the honour to be, with the greatest respect, etc.,

"W. Herschel."

When Herschel discovered the planet Uranus he had received no favour and no bounty from King or people. Nor did the King extend his patronage to him till fifteen months had elapsed. Galileo was in receipt of a handsome allowance from the Grand Duke of Tuscany, when he discovered the satellites of Jupiter, and called them the Medicean Stars. It was not only pardonable to do this; it was most natural. But science refused to endorse the flattery: and scientific men, especially on the Continent, were equally unwilling to accept the name proposed by Herschel for the newly discovered planet. For many years it continued to be called the Georgian Star, or the Georgium Sidus, in this country, though not without strong protests. While scientific men in Britain allowed that "George the Third has many titles to be remembered by the friends of science, to which few of his contemporaries have any pretensions," they maintained, "We shall therefore do well to anticipate the decision of posterity, by at once adopting a term that must ultimately prevail." No one thinks of perpetuating the name Georgian now. Uranus has displaced it, and justly. The judgment of posterity has gone against the name proposed by the discoverer and that of Herschel, generously proposed by Lalande. Heathenism and antiquity have carried the day. Everyone must decide for himself whether this was right, or whether the same rule should hold among the stars as has been allowed to hold on earth, where an adventurer gives his name to a New World, and the real discoverer has to rest content with naming a province of it, perhaps a province of little worth.

In writing this letter to the President of the Royal Society, William Herschel could plead more grounds for justification than we might be disposed, at first sight, to allow. That he was recognised by the King as a discoverer and a leader of thought was a great honour, recommending him at once to the nation and to the whole world. That he was paid a salary out of the King's or the nation's purse, and was placed by the King near the palace and brought into close relations with the Royal Family is also manifest. We are bound to give due weight to these considerations in the mind of an upright and honourable man, who deeply respected his sovereign, and knew best the amount of his own indebtedness. But history tells more than one story, that goes far to justify Herschel's name for the newly discovered star. It was not an uncommon thing to exalt an earthly prince to a throne in earthly skies. Probably we shall all admit that this was a mistake, perhaps a degradation of true science, which knows no distinction between king and beggar, and whose boundaries have been extended, to quote the words of Galileo, a hundred thousand fold by those whom popes and princes despised. But the fact is beyond dispute. The hair of Berenice, the Queen of Egypt and the murderess of the lover by whom she was slighted, was carried off from the temple of Venus, to whom it was vowed, and placed by Conon as a constellation among the stars. Sobieski, the valiant deliverer of Eastern Europe from the Turkish power, got a similar honour done him by Hevelius in the then invented constellation called Sobieski's Shield. Galileo felt himself under such obligations to the ducal house of Tuscany that he named the four moons of Jupiter, which he discovered, the Medicean Stars, a name they long continued to bear. The honour of a place in the heavens was great. It was also much sought after, so much so that Galileo was told "he would do a thing just and proper in itself, and at the same time render himself rich and powerful for ever," if he "named the next star which he should discover after the name of the great star of France, as well as the most brilliant of all the world," Henry of Navarre. Fortunately, in this respect at least, he had not the chance, otherwise we might have had the starry heavens peopled with the princely nonentities of earth. Royer, in 1679, did a similar honour to Louis XIV., by forming a constellation,called The Sceptre, for that monarch's glory; Messier, after the astronomer of that name, was another recently invented constellation on which Boscovich made the lines—

"Sidera, non Messes, Messerius iste tuetur;
Certe erat ille suo dignus inesse polo."

But no one would have expected a man of science so famous as Edmund Halley, to invent a constellation in honour of Charles II., The Oak, in memory of his escape after Worcester, or that Flamsteed would have placed so rotten a thing as the "Heart of Charles II." among the stars.[5]

While we are satisfied that there is no ground for finding fault with Herschel's name for the new planet he discovered, we are more satisfied that, by the mouth of Bode, the jury, to whom he required to appeal, disallowed the flattery, and called the planet Uranus, not even Herschel, as Lalande proposed. The next planet that was discovered, the first of the asteroids, was named by its discoverer Ceres Ferdinandea after a contemptible King of Naples, but Ceres has long since swallowed Ferdinandea up. Even at the time an amused cynicism, speaking in the Letters of Horace Walpole, was saying, "Must not that host of worlds be christened? Mr. Herschel himself has stood godfather for His Majesty to the new Sidus. His Majesty has a numerous issue; but they and all the princes and princesses in Europe cannot supply appellations enough for twenty millions of new-born stars."[6]

In the year 1782 Herschel not only continued to prosecute the studies he had begun, but ventured into new and almost untrodden fields of research. Two or three months were cut out of the working time of that year by a summons to Windsor to see the King and hear what he might do for him. But his activity and enjoyment in work made up for lost time. In 1780 he contributed two papers, or twenty-five large pages, to the Philosophical Transactions of the Royal Society; in 1781 he contributed two papers, or thirty-five pages; and in 1782, notwithstanding the loss of two months, four papers, or nearly one hundred pages—a good year's scientific work for any man, more especially for one who was giving thirty or even thirty-eight music lessons to his pupils per week; groaning and fretting under the incapacity of not a few of them—a man who had to be in his place conducting a band or a concert, and supervising a church's music, or who, instead of seeking rest in sleep, when the day's weary work was done, would often spend the night in observing the stars. His sister, who was his invariable companion in these night watches, had ample reason to say of him, "He did in one season more than anyone else could have done, and would have resumed the hunt [for Saturn's satellites] the next fifteen years, if nothing had interfered."

The new path on which he entered, and which led him into other and most attractive fields of inquiry, was the distance of what are called the fixed stars from the solar system. He knew that at the distance of the nearest of them, twice the sun's distance from the earth, immense though it seems, appears no bigger than a needle point, and cannot be used as a base line for measurement, or, indeed, as a line at all. He gave up the thought of attempting to solve the problem from that, the most natural and the easiest side. It was good for neighbours so near us as Mars and Venus. It was useless for Sirius or Arcturus. Following, perhaps, the example of Galileo, he believed that observations on stars so close together that neither the naked eye nor ordinary telescopes could separate them, and make two out of one, would lead to a discovery of their distance. He did not succeed in his purpose, but he was "introduced to a new series of observations and discoveries." He resolved to examine every star in the heavens with the utmost attention and a very high power, that he might collect such materials for this research as would enable him to fix his observations upon those that would best answer his end. The subject promises so rich a harvest that he cannot help inviting every lover of astronomy to join him in observations that must inevitably lead to new discoveries. He took some pains to find out what double stars had been recorded by astronomers; but "Nature, that great volume, appeared to him to contain the best catalogue upon this occasion."

The results of this search of the heavens appeared a month later in a Catalogue of Double Stars. They were "not only double stars, but also treble, double-double, quadruple, double-treble, and multiple." And he noticed, in a strangely prophetic vein of inspired imagination, not shrined in the temple of fact for more than twenty years after, "It is much too soon to form any theories of small stars revolving round large ones." Of 269 of the suns contained in this catalogue only 42 had been previously observed. While pursuing researches so laborious and so delightful, he was driven to devise ingenious improvements on the micrometer, as the contrivance was called that is used for measuring small spaces. But Herschel's thoughts were turned into other channels in the summer of 1782. He was raising questions we are only getting answers to now.

While Herschel was thus rapidly rising into fame, he was not forgetful of the sister who generously sacrificed her own wishes and prospects as a singer to advance his as an astronomer. During the time she was free from her numerous engagements as the thrifty housekeeper, the careful secretary and time-keeper, the reviser and reducer of observations, she amused herself by sweeping the heavens for comets with a five-feet reflector, of which her brother had made her a present. She was so successful that her fame soon sounded over Europe. "Miss Herschel," one writer reports, "sister of the celebrated astronomer, has observed a comet, and its orbit has been calculated. This is the seventy-third comet of which we know the period." This celestial visitor was talked of in Windsor Castle as the Lady's Comet. Unfortunately, the name was not retained, as it ought to have been, or at least given to a later discovery by Miss Herschel. Between 1786 and 1797 she discovered eight comets altogether, but of only five was she the first discoverer. The seventh, seen by her on November 7, 1795, was specially worthy of this name, but is now known as Encke's Comet. Her value as an assistant to her brother, besides her own personal merit as a woman of science, got for her a pension of £50 from the Civil List, granted to the King by Parliament. It was sufficient for the modest wants of a woman who not only handled a telescope with the dexterity of a practised observer, but, when sixty years of age, spent some of the last days of her stay at Slough" in painting and papering the rooms she was to occupy in a small house of her brother's, attached to the Crown Inn, to which she removed."

Year after year, from 1780 to 1812, the active mind and the prolific pen of William Herschel enriched the Proceedings of the Royal Society with one or more papers, which astonished the world of science and attracted the attention of mankind. The years 1813, 1816 were blanks, but 1814, 1815, 1817, and 1818 showed no feebling of hand or eye, although for years his strength had been failing: under the pressure of burdens laid on him as King's Astronomer—unnecessary burdens. Without including the diagrams, often in themselves a heavy labour, these papers are spread over two thousand quarto pages, an extraordinary record of hard, honest, earnest work. His first two papers were said to be "communicated by Dr. Watson, Jr., of Bath, F.R.S., and written by Mr. William Herschel of Bath." The same designation of the astronomer appears again in the Proceedings for 1781; but in the end of the year it is replaced by Mr. Herschel, F.R.S. In 1783-84-85 we find, William Herschel, Esq., F.R.S. But from 1786, the year in which he received the degree of LL.D. from the University of Edinburgh,[7] to 1815, the style is, William Herschel, LL.D., F.R.S. In 1817, 1818, it becomes Sir William Herschel, Knt Guelp., LL.D., F.R.S. The musician of Bath had made good his right to rank with the noblest and the most learned of men.

  1. Time of rotation determined since Herschel's days, 24 hrs. 37 min. 22·7 secs.
  2. Robison, Edin., Phil. Trans. i. 305.
  3. Nineteenth Century, April 1899, p. 612.
  4. Herschel might have known better than write this: see M. de Lalande's Astronomy, secs. 639, 640.
  5. Lalande, i. 283, 284.
  6. Letters, vi 259.
  7. Professor Holden, in his Life, writes (p. 47): "It was only in 1786 that he became 'Dr. Herschel,' through the Oxford degree of LL.D." This Oxford degree of LL.D. has of late been changed in his case into D.C.L. The Oxford "Catalogue of all graduates . . . between Oct. 10, 1659, and Dec. 31, 1850," does not contain his name, except as the father of Sir John Herschel, on whom the degree of D.C.L. was conferred. The date of the Edinburgh degree is April 10, 1786, and is the only ground I can discover for the title LL.D., that he takes in all his papers from 1786 to 1818. The honour of LL.D. from Oxford was first claimed for Herschel in 1798-9. See Public Characters, i. 396.