1911 Encyclopædia Britannica/Herschel, Sir Frederick William
HERSCHEL, SIR FREDERICK WILLIAM (1738-1822), generally known as Sir William Herschel, English astronomer, was born at Hanover on the 15th of November 1738. His father was a musician employed as hautboy player in the Hanoverian guard. The family had quitted Moravia for Saxony in the early part of the 17th century on account of religious troubles, they themselves being Protestants. Herschel's earlier education was necessarily of a very limited character, chiefly owing to the warlike commotions of his country; but being at all times an indomitable student, he, by his own exertions, more than repaired this deficiency. He became a very skilful musician, both theoretical and practical; while his attainments as a self-taught mathematician were fully adequate to the prosecution of those branches of astronomy which he so eminently advanced and adorned. Whatever he did he did methodically and thoroughly; and in this methodical thoroughness lay the secret of what Arago very properly termed his astonishing scientific success.
In 1752, at the age of fourteen, he joined the band of the Hanoverian guard, and with his detachment visited England in 1755, accompanied by his father and eldest brother; in the following year he returned to his native country; but the hardships of campaigning during the Seven Years' War imperiling his health, his parents privately removed him from the regiment, and on the 26th of July 1757 dispatched him to England. There, as might have been expected, the earlier part of his career was attended with formidable difficulties and much privation. We find him engaged in several towns in the north of England as organist and teacher of music, which were not lucrative occupations. But the tide of his fortunes began to flow when he obtained in 1766 the appointment of organist to the Octagon chapel in Bath, at that time the resort of the wealth and fashion of the city.
During the next five or six years he became the leading musical authority, and the director of all the chicf public musical entertainments at Bath. His circumstances having thus become easier, he revisited Hanover for the purpose of bringing back with him his sister Caroline, whose services he much needed in his multifarious undertakings. She arrived in Bath in August 1772, being at that time in her twenty-third year. She thus describes her brother's life soon after her arrival: "He used to retire to bed with a bason of milk or a glass of water, with Smith's Harmonics and Ferguson's Astronomy, &c., and so went to sleep buried under his favourite authors; and his first thoughts on waking were how to obtain instruments for viewing those objects himself of which he had been reading." It is not without significance that we find him thus reading Smith's Harmonics; to that study loyalty to his profession would impel him; as a reward for his thoroughness this led him to Smith's Optics; and this, by a natural sequence, again led him to astronomy, for the purposes of which the chief optical instruments were devised. It was in this way that he was introduced to the writings of Ferguson and Keill, and subsequently to those of Lalande, whereby he educated himself to become an astronomer of undying fame. In those days telescopes were very rare, very expensive and not very efficient, for the Dollonds had not as yet perfected even their beautiful little achromatic of 23 in. aperture. So Herschel was obliged to content himself with hiring a small Gregorian reflector of about 2 in. aperture, which he had seen exposed for loan in a tradesman's shop. Not satisfied with this implement, he procured a small lens of about 18 ft. focal length, and set his sister to work on a pasteboard tube to match it, so as to make him a telescope. This unsatisfactory material was soon replaced by tin, and thus a sorry sort of vision was obtained of Jupiter, Saturn and the moon. He then sought in London for a reflector of much larger dimensions; but no such instrument was on sale; and the terms demanded for the construction of a reflecting telescope of 5 or 6 ft. focal length he regarded as too exorbitant even for the gratification of such desires as his own. So he was driven to the only alternative that remained; he must himself build a large telescope. His first step in this direction was to purchase the débris of an amateur's implements for grinding and polishing small mirrors; and thus, by slow degrees, and by indomitable perseverance, he in 1774 had, as he says, the satisfaction of viewing the heavens with a Newtonian telescope of 6 ft. focal length made by his own hands. But he was not contented to be a mere star-gazer; on the contrary, he had from the very first conceived the gigantic project of surveying the entire heavens, and, if possible, of ascertaining the plan of their general structure by a settled mode of procedure, if only he could provide himself with adequate instrumental means. For this purpose he, his brother and his sister toiled for many years at the grinding and polishing of hundreds of specula, always retaining the best and recasting the others, until the most perfect of the earlier products had been surpassed. This was the work of the daylight in those seasons of the year when the fashionable visitors of Bath had quitted the place, and had thus freed the family from professional duties. After 1774 every available hour of the night was devoted to the long-hoped-for scrutiny of the skies. In those days no machinery had been invented for the construction of telescopic mirrors; the man who had the hardihood to undertake polishing them doomed himself to walk leisurely and uniformly round an upright post for many hours, without removing his hands from the mirror, until his work was done. On these occasions Herschel received his food from the hands of his faithful sister. But his reward was nigh.
In May 1780 his first two papers containing some results of his observations on the variable star "Mira" and the mountains of the moon were communicated to the Royal Society through the influential introduction of Dr William Watson. Herschel had made his acquaintance in a characteristic manner. In order to obtain a sight of the moon the astronomer had taken his telescope into the street opposite his house; the celebrated physician happening to pass at the time, and seeing his eye removed for a moment from the instrument, requested permission to take his place. The mutual courtesies and intelligent conversation which ensued soon ripened this casual acquaintance into a solid and enduring regard.
The phenomena of variable stars were examined by Herschel as a guide to what might be occurring in our own sun. The sun, he knew, rotated on its axis, and he knew that dark spots often exist on its photosphere; the questions that he put to himself were—Are there dark spots also on variable stars? Do the stars also rotate on their axes? or are they sometimes partially eclipsed by the intervention of opaque bodies? And he went on to enquire, What are these singular spots upon the sun? and have they any practical relation to the inhabitants of this planet? To these questions he applied his telescopes and his thoughts; and he communicated the results to the Royal Society in no less than six memoirs, occupying very many pages in the Philosophical Transactions, and extending in date from 1780 to 1801. It was in the latter year that these remarkable papers culminated in the inquiry whether any relation could be traced in the recurrence of sun-spots, regarded as evidences of solar activity, and the varying seasons of our planet, as exhibited by the varying price of corn. Herschel's reply was inconclusive; nor has a final solution of the related problems yet been obtained.
In 1781 he communicated to the Royal Society the first of a series of papers on the rotation of the planets and of their several satellites. The object which he had in view was not so much to ascertain the times of their rotation as to discover whether those rotations are strictly uniform. From the result he expected to gather, by analogy, the probability of an alteration in the length of our own day. These inquiries occupy the greater part of seven memoirs extending from 1781 to 1797. While engaged on them he noticed the curious appearance of a white spot near to each of the poles of the planet Mars. On investigating the inclination of its axis to the plane of its orbit, and finding that it differed little from that of the earth, he concluded that its changes of climate also would resemble our own, and that these white patches were probably polar snow. Modern researches have confirmed his conclusion. He also discovered that, as far as his observations extended, the times of the rotations of the various satellites round their axes conform to the analogy of our moon by equalling the times of their revolution round their primaries. Here again we perceive that his discoveries arose out of the systematic and comprehensive nature of his investigation. Nothing with such a man is accidental.
In the same year (1781) Herschel made a discovery which completely altered the character of his professional life. In the course of a methodical review of the heavens he lighted on an object which at first he supposed to be a comet, but which, by its subsequent motions and appearance, averred itself to be a new planet, moving outside the orbit of Saturn. The name of Georgium Sidus was by him assigned to it, but has by general consent been laid aside in favour of Uranus. The object was detected with a 7-ft. reflector having an aperture of 61 in.; subsequently, when he had provided himself with a much more powerful telescope, of 20 ft. focal length, he discovered, as he believed, no less than six Uranian satellites. Modern observations while abolishing four of these supposed attendants, have added two others apparently not observed by Herschel. Seven memoirs on the subject were communicated by him to the Royal Society, extending from the date of the discovery in 1781 to 1815. A noteworthy peculiarity in Herschel's mode of observation led to the discovery of this planet. He had observed that the spurious diameters of stars are not much afIected by increasing the magnifying powers, but that the case is different with other celestial objects, hence if anything in his telescopic field struck him as unusual in aspect, he immediately varied the magnifying power in order to decide its nature. Thus Uranus was discovered; and had a similar method been applied to Neptune, that planet would have been foundfat Cambridge some months before it was recognized at Berlin.
We now come to the beginning of Herschel's most important series of observations, culminating in what ought probably to be regarded as his capital discovery. A material part of the task which he had set himself embraced the determination of the relative distances of the stars from our sun and from each other. Now, in the course of his scrutiny of the heavens, he had observed many stars in apparently very close contiguity, but often differing greatly in relative brigntness. He concluded that, on the average, the brighter star would be the nearer to us, the smaller enormously more distant; and considering that an astronomer on the earth, in consequence of its immense orbital displacement of some 180 millions of miles every six months, would see such apair of stars under different perspective aspects, he perceived that the measurement of these changes should lead to an approximate determination of the stars' relative distances. He therefore mapped down the places and aspects of all the double stars that he met with, and communicated in 1782 and 1785 very extensive catalogues of the results. Indeed, his very last scientific memoir, sent to the Royal Astronomical Society in the year 1822, when he was its first president and already in the eighty-fourth year of his age, related to these investigations. In the memoir of 1782 he threw out the hint that these apparently contiguous stars might be genuine pairs in mutual revolution; but he significantly added that the time had not yet arrived for settling the question. Eleven years afterwards (1793), he remeasured the relative positions of many such couples, and we may conceive what his feelings must have been at finding his prediction verified. For he ascertained that some of these stars circulated round each other, after the manner required by the laws of gravitation, and thus demonstrated the action among the distant members of the starry firmament of the same mechanical laws which bind together the harmonious motions of our solar system. This sublime discovery, announced in 1802, would of itself suffice to immortalize his memory. If only he had lived long enough to learn the approximate distances of some of these binary combinations, he would at once have been able to calculate their masses relative to that of our own sun; and the quantities being, as we now know, strictly comparable, he would have found another of his analogical conjectures realized.
In the year 1782 Herschel was invited to Windsor by George III., and accepted the king's offer to become his private astronomer, and henceforth devote himself wholly to a scientific career. His salary was fixed at £200 per annum, to which an addition of £50 per annum was subsequently made for the astronomical assistance of his sister. Dr Watson, to whom alone the amount was mentioned, made the natural remark, "Never before was honour purchased by a monarch at so cheap a rate." In this way the great astronomer removed from Bath, first to Datchet and soon afterwards permanently to Slough, within easy access of his royal patron at Windsor.
The old pursuits at Bath were soon resumed at Slough, but with renewed vigour and without the former professional interruptions. The greater part, in fact, of the papers already referred to are dated from Datchet and Slough; for the magnificent astronomical speculations in which he was engaged, though for the most part conceived in the earlier portion of his philosophical career, required years of patient observation before they could be fully examined and realized.
It was at Slough in 1783 that he wrote his first memorable paper on the "Motion of the Solar System in Space,"—a sublime speculation, yet through his genius realized by considerations of the utmost simplicity. He returned to the same subject with fuller details in 1805. It was also after his removal to Slough that he published his first memoir on the construction of the heavens, which from the first had been the inspiring idea of his varied toils. In a long series of remarkable papers, addressed as usual to the Royal Society, and extending from the year 1784 to 1818, when he was eighty years of age, he demonstrated the fact that our sun is a star situated not far from the bifurcation of the Milky Way, and that all the stars visible to us lie more or less in clusters scattered throughout a comparatively thin, but immensely extended stratum. At one time he imagined that his powerful instruments had pierced through this stellar stratum, and that he had approximately determined the form of some of its boundaries. In the last of his memoirs, having convinced himself of his error, he admitted that to his telescopes the Milky Way was "fathomless." On either side of this assemblage of stars, presumably in ceaseless motion round their common centre of gravity, Herschel discovered a canopy of discrete nebulous masses, such as those from the condensation of which he supposed the whole stellar universe to have been formed,—a magnificent conception, pursued with a force of genius and put to the practical test of observation with an industry almost incredible.
Hitherto we have said nothing about the great reflecting telescope, of 40 ft. focal length and 4ft. aperture, the construction of which is often, though mistakenly, regarded as his chief performance. The full description of this celebrated instrument will be found in the 85th volume of the Transactions of the Royal Society. On the day that it was finished (August 28, 1789) Herschel saw at the first view, in a grandeur not witnessed before, the Saturnian system with six satellites, five of which had been discovered long before by C. Huygens and G. D. Cassini, while the sixth, subsequently named Enceladus, he had, two years before, sighted by glimpses in his exquisite little telescope of 61 in. aperture, but now saw in unmistakable brightness with the towering giant he had just completed. On the 17th of September he discovered a seventh, which proved to be the nearest to the globe of Saturn. It has since received the name of Mimas. It is somewhat remarkable that, notwithstanding his long and repeated scrutinies of this planet, the eighth satellite, Hyperion, and the crape ring should have escaped him.
Herschel married, on the 8th of May 1788, the widow of Mr John Pitt, a wealthy London merchant, by whom he had an only son, John Frederick William. The prince regent conferred a Hanoverian knighthood upon him in 1816. But a far more valued and less tardy distinction was the Copley medal assigned to him by his associates in the Royal Society in 1781.
He died at Slough on the 25th of August 1822, in the eighty-fourth year of his age, and was buried under the tower of St Laurence's Church, Upton, within a few hundred yards of the old site of the 40-ft. telescope. A mural tablet on the wall of the church bears a Latin inscription from the pen of the late Dr Goodall, provost of Eton College.
See Mrs John Herschel, Memoir of Caroline Herschel (1876); E. S. Holden, Herschel, his Life and Works (1881); A. M. Clerke, The Herschels and Modern Astronomy (1895); E. S. Holden and C. S. Hastings, Synopsis of the Scientific Writings of Sir William Herschel (Washington, 1881); Baron Laurier, Éloge historique, Paris Memoirs (1823), p. lxi.; F. Arago, Analyse historique, Annuaire du Bureau des Longitudes (1842), p. 249; Arago, Biographies of Scientific Men, p 167; Madame d'Arblay's Diary. passim; Public Characters (1798-1799), p. 384 (with portrait); J. Sime, William Herschel and his Work (1900). Herschel's photometric Star Catalogues were discussed and reduced by E. C. Pickering in Harvard Annals, vols. xiv. p. 345, xxiii. p. 185, and xxiv.