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In his diary, under the date 1766, Herschel has the following entries: "February 19th. Wheatley. Observation of Venus." "February 24th. Eclipse of the Moon at 7 o'clock a.m. Kirby." These are the first indications of his interest in astronomy. He had from early years inherited from his father a taste for what might be called star-gazing, and along with this he had had from youth a bent towards mathematical as well as philosophical research. Even in his busiest years, he never ceased to read and study in his leisure hours. During the first years of his stay in England, his foremost care was to master the English language. Next he acquired Italian, which he believed to be essential to his profession. From this he passed on to the study of Latin and Greek. The latter language, however, he dropped, "as leading me too far from my other favourite studies by taking up too much of my leisure. The theory of music being connected with mathematics induced me very early to read in Germany all that had been written upon the subject of harmony; and when, not long after my arrival in England, the valuable book of Dr. Smith's 'Harmonics' came into my hands, I perceived my ignorance, and had recourse to other authors for information, by which I was drawn on from one branch of mathematics to another." After perusing Smith's "Harmonics," he became possessed of a copy of the same author's "System of Optics". The study of optics passed into that of astronomy, which was stimulated by "Astronomy Explained upon Sir Isaac Newton's Principles," by James Ferguson, the Scottish astronomer. Herschel's early interest in the latter science seems to have been revived. So enthusiastic did he become that, in his own words, "I resolved to take nothing upon trust, but to see with my own eyes all that other men had seen before". At the time of his sister's arrival at Bath, Herschel had plunged in earnest into the study of astronomy. "He went to sleep," she tells us, "buried under his favourite authors; and his first thoughts on rising were how to obtain instruments for viewing these objects himself of which he had been reading." In May, 1773, he procured some object-glasses which he fitted into pasteboard tables. Caroline Herschel, who at that time had no interest whatever in telescopic astronomy, tells us: "I was much hindered in my musical practice by my help being continually wanted in the execution of the various contrivances, and I had to amuse myself with making the tube of pasteboard for the glasses which were to arrive from London". At length, Herschel completed this instrument, 4 feet long, which magnified forty times. With this, he records, he observed Jupiter and its satellites. Afterwards he made other two refractors—15 and 30 feet long respectively. Herschel soon discovered for himself the great weakness of the refracting telescope—the long tubes which were then necessary in order to counteract the effect of chromatic aberration. Finding the long tubes almost "impossible to manage," he turned his attention to the reflecting telescope, and in September hired a two-foot Gregorian reflector, which he found much more convenient. He decided to acquire a mirror of his own, for a tube 5 or 6 feet long. On enquiry he found there were none in the market of so large a size. "A person," his sister tells us, "offered to make one at a price much above what my brother thought proper to give." Herschel, however, was not discouraged. He became acquainted with a Quaker resident in Bath, who had in his leisure hours amused himself with efforts at the construction of mirrors. This individual had apparently failed in his endeavours—"his knowledge," Herschel noted, in that indifferent English which characterised him throughout his lifetime, "being very confined"—and had decided to dispose of his tools and half-finished mirrors. Herschel accordingly purchased his stock, and plunged at once into the work of telescope-making.

All through the winter 1773-4 and the following spring and summer, Herschel laboured at his new line of work, in the midst of his busy professional life. By 21st October, he had succeeded in casting mirrors for a two-foot reflector, and by the middle of December, "it became necessary to think of mounting these mirrors". Early in the new year, he placed a 5½-foot mirror into a square wooden tube, but finding the adjustment of the Gregorian telescope very troublesome, he decided to have recourse to the Newtonian form. On 1st March, 1774, he made his first entry in his astronomical journal, stating that he had viewed "the lucid spot in Orion's sword belt" and the ring of Saturn, which appeared "like two slender arms". The success of this instrument encouraged Herschel to construct other telescopes of the Newtonian form, and at length he succeeded in making a 7-foot telescope with "many different object mirrors". On 1st May, 1776, he observed Saturn's ring and two belts "with great perfection".

The memoirs of his sister give us some idea of his unflagging energy. With sorrow, natural from the house-keeper's point of view, Caroline saw almost every room in the house transformed into a workshop. A tube and stand were set up in a handsomely-furnished drawing room; while Alexander Herschel, the younger brother, who had now come to reside in Bath, erected a huge turning-machine in a bedroom, for turning eye-pieces and grinding glasses. "Every leisure moment was eagerly snatched at for resuming some work which was in progress, without taking time for changing dress, and many a lace ruffle was torn or bespattered by molten pitch." From now onwards, Caroline proved herself the devoted assistant and helper of her brother, keeping house for him, assisting him in music and in his work of telescope-making. "By way of keeping him alive," she says, "I was constantly obliged to feed him by putting the victuals by bits into his mouth". This was once the case when, in order to finish a seven-foot mirror, he had not taken his hands from it for sixteen hours together. "In general he was never unemployed at meals, but was always at those times contriving or making drawings of whatever came in his mind. Generally I was obliged to read to him whilst he was at the turning lathe or polishing mirrors, 'Don Quixote,' 'Arabian Nights' Entertainments,' the novels of Sterne, Fielding, etc.; serving tea and supper without interrupting the work with which he was engaged. . . . and sometimes lending a hand. . . . My brother Alex, was absent from Bath for some months every summer, but when at home he took much pleasure to execute some turning or clockmaker's work for his brother."

Herschel was not content with the construction of one or two telescopes. The work went on during the remainder of his stay in Bath. How he contrived to continue the construction of telescopes while making long-continued surveys of the heavens—all in the spare time which he was able to snatch from his busy career as a professional musician—must always remain more or less of a mystery.

His earliest observations were on the Moon and planets. In 1776 he made a number of examinations of the lunar surface, and three years later was engaged in measuring the heights of the lunar mountains. From 1774 onwards, he carefully observed Saturn, and in April, 1777, he commenced to make sketches in pen and ink of the markings on the surface of Mars, and in the following year of those on Jupiter. His attention was not confined to the members of the Solar System, and in 1777 he commenced a series of observations on the variable star, Omicron—or Mira—Ceti. As early as 1778 he had conceived the idea of attempting to determine by measures of close doubles the annual parallax or apparent displacement of a star in the sky due to the earth's change of position as it moves in its orbit. From this parallax, the distance of a star is determined. Before Herschel's time, astronomers had made many attempts to measure stellar parallax, but owing to the great distance of even the nearest stars and the consequent smallness of the displacement, and also because of the comparative imperfection of astronomical instruments, all these efforts had failed. Herschel was particularly interested in the problem, and made several attempts to solve it, but was unsuccessful. It was not until 1838 that the first determinations of stellar parallax were made. Herschel's attempts were not altogether in vain, however, for while tackling the problem, he was led to the study of double stars, which resulted, after many years, in one of his greatest discoveries. At the same time he executed his first review of the heavens with his 7-foot Newtonian reflector. The review only extended to the first four magnitudes: it was in itself a mere beginning, but it was the starting-point of his work on stellar distribution.

Herschel soon became known to the cultured public of Bath, not only as a prominent musician, but also as an amateur astronomer of considerable eminence. His introduction to literary and scientific circles in the town was a somewhat unconventional one. In his diary, under the date December, 1779, there occurs the following entry: "About the latter end of this month, I happened to be engaged on a series of observations on the lunar mountains, and the Moon being in front of my house late in the evening, I brought my 7-foot reflector into the street, and directed it to the object of my observations. While I was looking into the telescope, a gentleman coming by the place where I was stationed stopped to look at the instrument. When I took my eye off the telescope, he very politely asked if he might be permitted to look in, and expressed great satisfaction at the view. Next morning, the gentleman, who proved to be Dr. Watson, jun. (now Sir William), called at my house to thank me for my civility in showing him the Moon, and told me that there was a Literary Society then forming at Bath and invited me to become a member, to which I readily consented."

About the middle of January, 1780, this society—the Philosophical Society of Bath—began its meetings. Herschel not only attended these, but contributed a considerable number of papers—thirty-one in all—in the course of the next two years. One or two of these dealt with astronomical subjects, such as the height of the lunar mountains, and the variable star Mira Ceti; but most of them were on metaphysical and physical subjects, such as "The Utility of Speculative Enquiries," "On the Existence of Space," and "Experiments in Light". These were nearly all published in 1912 in the "Collected Scientific Papers of Sir William Herschel". Through Dr. Watson's influence, Herschel was introduced to scientific circles in London, and he forwarded four communications to the Royal Society before he was elected a Fellow. These were, a paper on "Mira Ceti," read 11th May, 1780; on the "Mountains of the Moon," read the same day; on the "Rotation of the planets on their Axes, with a view to determine whether the Earth's diurnal motion is perfectly equable,"—a remarkable and ingenious paper bearing the stamp of his own peculiar methods of investigation—read 11th January, 1781; and "Account of a Comet," read 26th April, 1781.

The last-named paper gave an account of the discovery which marked the turning-point in Herschel's career. On 17th August, 1779, he commenced his second review of the heavens. This review was made with his 7-foot telescope of 6·2 inches aperture, and included stars down to the eighth magnitude; its main purpose was to register double stars. On Tuesday, 13th March, 1781, in the course of this review, he jotted down in his journal the following note, in somewhat doubtful English: "In the quartile near Zeta Tauri, the lowest of two is a curious either nebulous star or perhaps a comet. A small star follows the comet at two-thirds of the field's distance." In the paper afterwards communicated to the Royal Society, he explained that he perceived a star which appeared "visibly larger than the rest: being struck with its uncommon magnitude, I 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". On Saturday, 17th March, he wrote, "I looked for the comet or nebulous star, and found that it is a comet, for it has changed its place". By Monday, the 19th, he found that the supposed comet "moves according to the order of the signs, and its orbit declines but little from the ecliptic".

The discovery was soon communicated to the Observatories of Greenwich and Oxford. Maskelyne, the Astronomer-Royal, wrote to Dr. Watson on the 4th April that he had observed the strange object, "very different from any comet I ever read any description of or saw". On 23rd April he wrote to Herschel: "It is as likely to be a regular planet moving in an orbit nearly circular round the Sun as a comet moving in a very eccentric ellipsis"; and he immediately notified the French astronomers of the discovery. Messier, the most famous observer of comets, commenced observations on 16th April, and his example was followed by Lalande, Lemonnier, and other astronomers in France and by Bode in Germany. Efforts were made to calculate its orbit, on the assumption that it actually was a cometary body. Orbits were calculated by Méchain, De Saron, Laplace, and others. These efforts were fruitless. On 8th May De Saron announced that the "comet" was much more distant from the Sun than had been supposed. Laplace independently reached a similar conclusion. Meanwhile, Lexell, the St. Petersburg mathematician, who happened to be in England when the discovery was made, informed the St. Petersburg Academy that the object discovered by the Bath musician was probably not a comet at all, but an exterior planet, revolving at twice the distance of Saturn, thus confirming Maskelyne's sagacious surmise. Later it transpired that the planet had been observed no fewer than seventeen times between 1690 and 1781 by able observers such as Flamsteed, Bradley, Lemonnier, and Meyer, all of whom failed to differentiate it from an ordinary star, either in regard to its appearance or its motion. Lemonnier, indeed, had the discovery almost within his grasp, for he observed the planet on four consecutive days in January, 1769, but his carelessness robbed him of the distinction of detecting a new celestial body.

No little excitement was aroused by Herschel's achievement.It was the first planetary discovery within the memory of man—Mercury, Venus, Mars, Jupiter, and Saturn having been known from prehistoric times. More wonderful still, the discovery had been made, not by the leading astronomers of the day, but by an unknown amateur. At one bound Herschel leaped from obscurity to fame. The Royal Society of London awarded him the Copley Medal in November, 1781, and elected him a Fellow in December, exempting him from payment of subscriptions, as a mark of esteem. The discovery had caused a stir in still higher circles, and on 10th May, 1782, Herschel was informed that the King— George III—expected to make his acquaintance. On 8th May he left Bath to join his friend, Dr. Watson, at Lincoln's Inn Fields, taking with him his instruments, star-catalogues, maps, tables, etc. In a letter to his sister, dated 25th May, he stated that he had had an audience of the King, to whom he presented a drawing of the Solar System. On 2nd July Herschel noted in his diary: "I had the honour of showing the King and Queen and the Royal Family the planets Jupiter and Saturn, and other objects".

Herschel was now seriously considering the possibility of abandoning the profession of music and devoting himself to astronomy. There can be little doubt that after George III expressed interest in the discovery, Herschel indicated that he was anxious to be made "independent of music". The result of his interview with the King was his appointment as King's Astronomer. The appointment is referred to by Herschel himself in his journal in the following terms: "It was settled by His Majesty that I should give up my musical profession and, settling somewhere in the neighbourhood of Windsor, devote my time to astronomy". The salary attached to the new office was fixed at £200—certainly not a large sum. Indeed, when Herschel's intimate friend, Dr. Watson, was informed as to the exact amount, he exclaimed, "Never bought monarch honour so cheap". Writing to her nephew, Sir John Herschel, in April, 1827, Caroline attributed the "close bargains" made between George III and her brother to the "shabby, mean-spirited advisers" of the King. Undoubtedly Herschel made a pecuniary sacrifice in accepting the offer, but "the prospect of entering again on the toils of teaching, etc," his sister tells us, "which awaited my brother at home, appeared to him an intolerable waste of time". Doubtless Herschel believed it to be the best policy to close with the King's offer and thus be free to devote himself to the study which had become the master-passion of his life. The meagreness of the allowance has been often commented upon, but it must be remembered, in justice to George III, that, firstly, the purchasing power of money was considerably greater then than now, and, secondly, the regular duties attached to the office were at that time very few. There is no doubt that George III, by his creation of this new post, made possible Herschel's long career of investigation and discovery: for no man, not even Herschel, could possibly have stood the strain of a life of professional activity and scientific investigation combined. As one of his biographers has well said, "The astronomer of Slough was the gift to science of the poor mad king".