Sir William Herschel
1738 - 1822



The German-born English astronomer Sir William Herschel discovered the planet Uranus, the intrinsic motion of the sun in space, and the form of the Milky Way.

William (originally Friedrich Wilhelm) Herschel was born in Hanover on Nov. 15, 1738. His father was a musician in the Hanoverian guard, which William joined at the age of 14.

In 1757 Herschel went to England. In Yorkshire he conducted a small military band, and from 1762 to 1766 he was a concert manager in Leeds. His notebook of 1766 has these laconic entries: "Feb. 19. Wheatly. Observation of Venus" and "Feb. 24. Eclipse of the moon at 7 o'clock A.M. Kirby." These are the first signs of Herschel's future interests. By the end of the year he became organist at the fashionable spa town of Bath. In 1772 his sister, Caroline Lucretia Herschel, came to live with him at Bath. She collaborated with her brother on astronomical researches.

Not until 1773 is there another scientific entry in Herschel's notebooks: "April 19. Bought a quadrant and Emerson's Trigonometry." That this entry heralded a new phase in his life is shown by the fact that it is followed by others of a similar nature: "Bought a book of astronomy … bought an object glass … bought many eye glasses … hire of a 2 feet reflecting telescope." These entries show that he was proposing to make his first (metal) telescope mirror.


Herschel's First Telescope

Obsessed with astronomy, Herschel progressed through pasteboard and tin-tubed telescopes to a hired Gregorian reflector. When he tried to buy a much larger reflecting telescope in London, he could find nothing suitable which he could afford. For this reason he began to build his own. By September 1774 he was observing the heavens with a (Newtonian) reflecting telescope of 6-foot focal length of his own construction.

Herschel now entered into a long and tedious period of his life, when he and his brother and sister worked away at grinding and polishing telescope mirrors. He had to keep the mirror moving unceasingly on the grinding tool for long periods of time. His sister fed him as he worked. Some idea of his astonishing industry may be had from his statement, made in 1795, that he had made "not less than 200 7 feet, 150 10 feet and about 80 20 feet mirrors." Of the various mountings he devised for these, he was very pleased with a 7-foot Newtonian telescope stand, completed in 1778.


Early Observations

Herschel began to keep a record of what he saw in the heavens from March 1, 1774. He observed the rings of Saturn, the moons of Jupiter, and the markings of the moon. It is interesting to see how in his eagerness to make novel discoveries he was deluded into thinking that he had found signs of a forest on the moon, even supposing that he could make out the shadows cast by the trees at the edge of the wood. His next lunar observations were 3 years later, when he began to calculate the height of the lunar mountains.

This self-taught astronomer of Bath was by his own efforts soon to be transmuted into the world's leading observational astronomer. He possessed instruments as powerful as any to be found and all the perseverance needed to use them effectively. In 1777 he began observations of a well-known but neglected star, Mira Ceti, which varies in brightness periodically. Soon he had the idea of determining the annual parallax of stars (the shift in the apparent relative positions of the stars as the earth goes around the sun). Whether the stars were so far away as to make this apparent movement unobservable was not then known. In fact, no annual parallax was measured until 1838, when Friedrich Wilhelm Bessel measured that of star 61 Cygni. Herschel, nevertheless, observed the relative positions of pairs of stars close together (called double stars). He measured hundreds of double stars, but in March 1778 he recorded his disappointment at finding "the stars in the tail of Ursa Major just as I saw them three months ago, at least not visibly different."


Discovery of Uranus

In recording double stars systematically, on March 13, 1781, Herschel entered a pair of which "the lowest of the two is a curious either nebulous star or perhaps a comet." Four days later he looked for the object and found that it had moved. He recorded the new position of the "comet" and proceeded to follow it regularly. What he had discovered was the planet Uranus, as it is now known - the first planet to be discovered in historical times.

Herschel was given the Copley Medal of the Royal Society and elected a fellow. Col. John Walsh wrote to him that he had spoken with the king, George III, and had taken "occasion to mention that you had a twofold claim as a Native of Hanover and a Resident of Great Britain, where the Discovery was made, to be permitted to name the Planet from his Majesty." The planet was thus at first called "Georgium sidus" ("star George"), and it appears in this form on early maps and models.


King's Patronage

George III asked Herschel to move his telescope to an observatory the King had built in the Deer Park at Richmond. Herschel moved to Windsor, near the King's residence, and in due course was given the patronage for which he had long hoped - a salary for himself and his sister, upkeep for the telescope, and later a very large sum for a 40-foot telescope, the largest ever made before the mid-19th century.

Herschel eventually settled at Slough, where he wrote the paper announcing his second great discovery, "Motion of the Solar System in Space" (1783). He carefully noted the proper motions of seven bright stars and showed that the movement in the intervening time seemed to converge on a fixed point, which he interpreted correctly as the point from which the sun is receding. Other discoveries followed. He found that "Georgium sidus" had satellites. Some of those he discovered are now known to be spurious, but the difficulties of observing, especially with the crude mounting available to him, were very
great.


Structure of the Universe

Many double stars are seen as such merely because they happen to be in a straight line as seen from the earth. Herschel reasoned that if one member of a double-star system was much brighter than the other this must be the result of such a coincidence, the brighter star of the pair being much the closer of the two. He continued to record the relative positions of all such systems, and in 1782 and 1785 he presented long lists of his observations. He was, of course, assuming that the stars were all more or less uniformly bright, intrinsically speaking, and that they were uniformly distributed throughout space. This being so, he believed that by taking counts of stars over a given small area of sky the number of stars visible would give him the extent of the Milky Way in that direction. He thus formulated a picture or map of the Milky Way, which was quite remarkable in his time, and which even now is not wildly wrong.


Later Years

In 1788 Herschel married Mary Pitt, a wealthy widow, by whom he had his only son. Herschel was able to make a useful additional income by selling telescopes, and he invested money in building machines to help grind mirrors. He corresponded with the leading astronomers of England and Europe and received many distinguished visitors at Slough who were anxious to see the telescope he had completed on Aug. 28, 1789; it had a 40-foot focal length and 4-foot aperture.

Herschel was knighted in 1816 and received honours from states and academies the world over. He died at Slough on Aug. 25, 1822.

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For uncounted ages those who watched the skies had been aware of the existence of the five old planets-Jupiter, Mercury, Saturn, Venus, and Mars. It never seems to have occurred to any of the ancient philosophers that there could be other similar objects as yet undetected over and above the well-known five. Great then was the astonishment of the scientific world when the Bath organist announced his discovery that the five planets which had been known from all antiquity must now admit the company of a sixth. And this sixth planet was, indeed, worthy on every ground to be received into the ranks of the five glorious bodies of antiquity. It was, no doubt, not so large as Saturn, it was certainly very much less than Jupiter; on the other hand, the new body was very much larger than Mercury, than Venus, or than Mars, and the earth itself seemed quite an insignificant object in comparison with this newly added member of the Solar System. In one respect, too, Herschel's new planet was a much more imposing object than any one of the older bodies; it swept around the sun in a majestic orbit, far outside that of Saturn, which had previously been regarded as the boundary of the Solar System, and its stately progress required a period of not less than eighty-one years.

King George the Third, hearing of the achievements of the Hanoverian musician, felt much interest in his discovery, and accordingly Herschel was bidden to come to Windsor, and to bring with him the famous telescope, in order to exhibit the new planet to the King, and to tell his Majesty all about it. The result of the interview was to give Herschel the opportunity for which he had so long wished, of being able to devote himself exclusively to science for the rest of his life.

The King took so great a fancy to the astronomer that he first, as I have already mentioned, duly pardoned his desertion from the army, some twenty-five years previously. As a further mark of his favour the King proposed to confer on Herschel the title of his Majesty's own astronomer, to assign to him a residence near Windsor, to provide him with a salary, and to furnish such funds as might be required for the erection of great telescopes, and for the conduct of that mighty scheme of celestial observation on which Herschel was so eager to enter. Herschel's capacity for work would have been much impaired if he had been deprived of the aid of his admirable sister, and to her, therefore, the King also assigned a salary, and she was installed as Herschel's assistant in his new post.

With his usually impulsive determination, Herschel immediately cut himself free from all his musical avocations at Bath, and at once entered on the task of making and erecting the great telescopes at Windsor. There, for more than thirty years, he and his faithful sister prosecuted with unremitting ardour their nightly scrutiny of the sky. Paper after paper was sent to the Royal Society, describing the hundreds, indeed the thousands, of objects such as double stars; nebulae and clusters, which were first revealed to human gaze during those midnight vigils. To the end of his life he still continued at every possible opportunity to devote himself to that beloved pursuit in which he had such unparalleled success. No single discovery of Herschel's later years was, however, of the same momentous description as that which first brought him to fame.

Herschel married when considerably advanced in life and he lived to enjoy the indescribable pleasure of finding that his only son, afterwards Sir John Herschel, was treading worthily in his footsteps, and attaining renown as an astronomical observer, second only to that of his father. The elder Herschel died in 1822, and his illustrious sister Caroline then returned to Hanover, where she lived for many years to receive the respect and attention which were so justly hers. She died at a very advanced age in 1848.

On Sundays he played the organ, to the great delight of the congregation, & on week-days he was occupied by giving lessons to private pupils, and in preparation for public performances. He thus came to be busily employed, and seems to have been in the enjoyment of comfortable means.

From his earliest youth Herschel had been endowed with that invaluable characteristic, an eager curiosity for knowledge. He was naturally desirous of perfecting himself in the theory of music, & thus he was led to study mathematics. When he had once tasted the charms of mathematics, he saw vast regions of knowledge unfolded before him, and in this way he was induced to direct his attention to astronomy. More and more this pursuit seems to have engrossed his attention, until at last it had become an absorbing passion. Herschel was, however, still obliged, by the exigency of procuring a livelihood, to give up the best part of his time to his profession as a musician; but his heart was eagerly fixed on another science, & every spare moment was steadily devoted to astronomy. For many years, however, he continued to labour at his original calling, nor was it until he had attained middle age and become the most celebrated astronomer of the time, that he was enabled to concentrate his attention exclusively on his favourite pursuit.

It was with quite a small telescope which had been lent him by a friend that Herschel commenced his career as an observer. However, he speedily discovered that to see all he wanted to see, a telescope of far greater power would be necessary, & he determined to obtain this more powerful instrument by actually making it with his own hands. At first it may seem scarcely likely that one whose occupation had previously been the study and practice of music should meet with success in so technical an operation as the construction of a telescope. It may, however, be mentioned that the kind of instrument which Herschel designed to construct was formed on a very different principle from the refracting telescopes with which we are ordinarily familiar. His telescope was to be what is termed a reflector. In this type of instrument the optical power is obtained by the use of a mirror at the bottom of the tube, and the astronomer looks down through the tube TOWARDS HIS MIRROR and views the reflection of the stars with its aid. Its efficiency as a telescope depends entirely on the accuracy with which the requisite form has been imparted to the mirror. The surface has to be hollowed out a little, and this has to be done so truly that the slightest deviation from good workmanship in this essential particular would be fatal to efficient performance of the telescope.

The mirror that Herschel employed was composed of a mixture of two parts of copper to one of tin; the alloy thus obtained is an intensely hard material, very difficult to cast into the proper shape, and very difficult to work afterwards. It possesses, however, when polished, a lustre hardly inferior to that of silver itself. Herschel has recorded hardly any particulars as to the actual process by which he cast and figured his reflectors. We are however, told that in later years, after his telescopes had become famous, he made a considerable sum of money by the manufacture and sale of great instruments. Perhaps this may be the reason why he never found it expedient to publish any very explicit details as to the means by which his remarkable successes were obtained.

Since Herschel's time many other astronomers, notably the late Earl of Rosse, have experimented in the same direction, and succeeded in making telescopes certainly far greater, and probably more perfect, than any which Herschel appears to have constructed. The details of these later methods are now well known, and have been extensively practiced. Many amateurs have thus been able to make telescopes by following the instructions so clearly laid down by Lord Rosse and the other authorities. Indeed, it would seem that any one who has a little mechanical skill and a good deal of patience ought now to experience no great difficulty in constructing a telescope quite as powerful as that which first brought Herschel into fame. I should, however, mention that in these modern days the material generally used for the mirror is of a more tractable description than the metallic substance, which was employed by Herschel and by Lord Rosse. A reflecting telescope of the present day would not be fitted with a mirror composed of that alloy known as speculum metal, whose composition I have already mentioned. It has been found more advantageous to employ a glass mirror carefully figured and polished, just as a metallic mirror would have been, and then to impart to the polished glass surface a fine coating of silver laid down by a chemical process. The silver-on-glass mirrors are so much lighter and so much easier to construct that the more old-fashioned metallic mirrors may be said to have fallen into almost total disuse.

In one respect however, the metallic mirror may still claim the advantage that, with reasonable care, its surface will last bright and untarnished for a much longer period than can the silver film on the glass. However, the operation of re-silvering a glass has now become such a simple one that the advantage this indicates is not relatively so great as might at first be
supposed.

Some years elapsed after Herschel's attention had been first directed to astronomy, before he reaped the reward of his exertions in the possession of a telescope, which would adequately reveal some of the glories of the heavens. It was in 1774, when the astronomer was thirty-six years old, that he obtained his first glimpse of the stars with an instrument of his own construction. Night after night, as soon as his musical labours were ended, his telescopes were brought out, sometimes into the small back garden of his house at Bath, and sometimes into the street in front of his hall-door. It was characteristic of him that he was always endeavouring to improve his apparatus. He was incessantly making fresh mirrors, or trying new lenses, or combinations of lenses to act as eye-pieces, or projecting alterations in the mounting by which the telescope was supported. Such was his enthusiasm that his house, we are told, was incessantly littered with the usual indications of the workman's presence, greatly to the distress of his sister, who, at this time, had come to take up her abode with him and look after his housekeeping. Indeed, she complained that in his astronomical ardour he sometimes omitted to take off, before going into his workshop, the beautiful lace ruffles which he wore while conducting a concert, and that consequently they became soiled with the pitch employed in the polishing of his mirrors.

This sister, who occupies such a distinct place in scientific history is the same little girl to whom we have already referred. From her earliest days she seems to have cherished a passionate admiration for her brilliant brother William. It was the proudest delight of her childhood as well as of her mature years to render him whatever service she could; no man of science was ever provided with a more capable or energetic helper than William Herschel found in this remarkable woman. Whatever work had to be done she was willing to bear her share in it, or even to toil at it unassisted if she could be allowed to do so. She not only managed all his domestic affairs, but in the grinding of the lenses and in the polishing of the mirrors she rendered every assistance that was possible. At one stage of the very delicate operation of fashioning a reflector, it is necessary for the workman to remain with his hand on the mirror for many hours in succession. When such labours were in progress, Caroline used to sit by her brother, and enliven the time by reading stories aloud, sometimes pausing to feed him with a spoon while his hands were engaged on the task from which he could not desist for a moment.

When mathematical work had to be done Caroline was ready for it; she had taught herself sufficient to enable her to perform the kind of calculations, not, perhaps, very difficult ones, that Herschel's work required; indeed, it is not too much to say that the mighty life-work which this man was enabled to perform could never have been accomplished had it not been for the self-sacrifice of this ever-loving and faithful sister. When Herschel was at the telescope at night, Caroline sat by him at her desk, pen in hand, ready to write down the notes of the observations as they fell from her brother's lips. This was no insignificant toil. The telescope was, of course, in the open air, and as Herschel not infrequently continued his observations throughout the whole of a long winter's night, there were but few women who could have accomplished the task which Caroline so cheerfully executed. From dusk till dawn, when the sky was clear, were Herschel's observing hours, and what this sometimes implied we can realize from the fact that Caroline assures us she had sometimes to desist because the ink had actually frozen in her pen. The night's work over, a brief rest was taken, and while William had his labours for the day to attend to, Caroline carefully transcribed the observations made during the night before, reduced all the figures and prepared everything in readiness for the observations that were to follow on the ensuing evening.

But we have here been anticipating a little of the future which lay before the great astronomer; we must now revert to the history of his early work, at Bath, in 1774, when Herschel's scrutiny of the skies first commenced with an instrument of his own manufacture. For some few years he did not attain any result of importance; no doubt he made a few interesting observations, but the value of the work during those years is to be found, not in any actual discoveries which were accomplished, but in the practice which Herschel obtained in the use of his instruments. It was not until 1782 that the great achievement took place by which he at once sprang into fame.

It is sometimes said that discoveries are made by accident, and, no doubt, to a certain extent, but only, I fancy to a very small extent, this statement may be true. It is, at all events, certain that such lucky accidents do not often fall to the lot of people unless those people have done much to deserve them. This was certainly the case with Herschel. He appears to have formed a project for making a close examination of all the stars above a certain magnitude. Perhaps he intended to confine this research to a limited region of the sky, but, at all events, he seems to have undertaken the work energetically and systematically. Star after star was brought to the centre of the field of view of his telescope, and after being carefully examined was then displaced, while another star was brought forward to be submitted to the same process. In the great majority of cases such observations yield really nothing of importance; no doubt even the smallest star in the heavens would, if we could find out all about it, reveal far more than all the astronomers that were ever on the earth have even conjectured. What we actually learn about the great majority of stars is only information of the most meagre description. We see that the star is a little point of light, and we see nothing more.

In the great review which Herschel undertook he doubtless examined hundreds, or perhaps thousands of stars, allowing them to pass away without note or comment. But on an ever-memorable night in March, 1782, it happened that he was pursuing his task among the stars in the Constellation of Gemini. Doubtless, on that night, as on so many other nights, one star after another was looked at only to be dismissed, as not requiring further attention. On the evening in question, however, one star was noticed which, to Herschel's acute vision seemed different from the stars which in so many thousands are strewn over the sky. A star properly so called appears merely as a little point of light, which no increase of magnifying power will ever exhibit with a true disc. But there was something in the star-like object which Herschel saw that immediately arrested his attention and made him apply to it a higher magnifying power. This at once disclosed the fact that the object possessed a disc, that is, a definite, measurable size, and that it was thus totally different from any one of the hundreds and thousands of stars which exist elsewhere in space. Indeed, we may say at once that this little object was not a star at all; it was a planet. That such was its true nature was confirmed, after a little further observation, by perceiving that the body was shifting its place on the heavens relatively to the stars. The organist at the Octagon Chapel at Bath had, therefore, discovered a new planet with his home-made telescope.

I can imagine some one will say, "Oh, there was nothing so wonderful in that; are not planets always being discovered? Has not M. Palisa, for instance, discovered about eighty of such objects, and are there not hundreds of them known nowadays?" This is, to a certain extent, quite true. I have not the least desire to detract from the credit of those industrious and sharp-sighted astronomers who have in modern days brought so many of these little objects within our cognizance. I think, however, it must be admitted that such discoveries have a totally different importance in the history of science from that which belongs to the peerless achievement of Herschel. In the first place, it must be observed that the minor planets now brought to light are so minute that if a score of them were rolled to together into one lump it would not be one-thousandth part of the size of the grand planet discovered by Herschel. This is, nevertheless, not the most important point. What marks Herschel's achievement as one of the great epochs in the history of astronomy is the fact that the detection of Uranus was the very first recorded occasion of the discovery of any planet whatever.

For uncounted ages those who watched the skies had been aware of the existence of the five old planets-Jupiter, Mercury, Saturn, Venus, and Mars. It never seems to have occurred to any of the ancient philosophers that there could be other similar objects as yet undetected over and above the well-known five. Great then was the astonishment of the scientific world when the Bath organist announced his discovery that the five planets which had been known from all antiquity must now admit the company of a sixth. And this sixth planet was, indeed, worthy on every ground to be received into the ranks of the five glorious bodies of antiquity. It was, no doubt, not so large as Saturn, it was certainly very much less than Jupiter; on the other hand, the new body was very much larger than Mercury, than Venus, or than Mars, and the earth itself seemed quite an insignificant object in comparison with this newly added member of the Solar System. In one respect, too, Herschel's new planet was a much more imposing object than any one of the older bodies; it swept around the sun in a majestic orbit, far outside that of Saturn, which had previously been regarded as the boundary of the Solar System, and its stately progress required a period of not less than eighty-one years.

King George the Third, hearing of the achievements of the Hanoverian musician, felt much interest in his discovery, and accordingly Herschel was bidden to come to Windsor, and to bring with him the famous telescope, in order to exhibit the new planet to the King, and to tell his Majesty all about it. The result of the interview was to give Herschel the opportunity for which he had so long wished, of being able to devote himself exclusively to science for the rest of his life.

The King took so great a fancy to the astronomer that he first, as I have already mentioned, duly pardoned his desertion from the army, some twenty-five years previously. As a further mark of his favour the King proposed to confer on Herschel the title of his Majesty's own astronomer, to assign to him a residence near Windsor, to provide him with a salary, and to furnish such funds as might be required for the erection of great telescopes, and for the conduct of that mighty scheme of celestial observation on which Herschel was so eager to enter. Herschel's capacity for work would have been much impaired if he had been deprived of the aid of his admirable sister, and to her, therefore, the King also assigned a salary, and she was installed as Herschel's assistant in his new post.

With his usually impulsive determination, Herschel immediately cut himself free from all his musical avocations at Bath, and at once entered on the task of making and erecting the great telescopes at Windsor. There, for more than thirty years, he and his faithful sister prosecuted with unremitting ardour their nightly scrutiny of the sky. Paper after paper was sent to the Royal Society, describing the hundreds, indeed the thousands, of objects such as double stars; nebulae and clusters, which were first revealed to human gaze during those midnight vigils. To the end of his life he still continued at every possible
opportunity to devote himself to that beloved pursuit in which he had such unparalleled success. No single discovery of Herschel's later years was, however, of the same momentous description as that which first brought him to fame.

Herschel married when considerably advanced in life and he lived to enjoy the indescribable pleasure of finding that his only son, afterwards Sir John Herschel, was treading worthily in his footsteps, and attaining renown as an astronomical observer,
second only to that of his father. The elder Herschel died in 1822, and his illustrious sister Caroline then returned to Hanover, where she lived for many years to receive the respect and attention which were so justly hers. She died at a very advanced age in 1848.
 

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Friedrich Wilhelm Herschel was born in Hannover (Germany) in 1738 as son of Issak Herschel (1707-1767), a musician in the regimental band of the Foot-Guards, and Anna Ilse (b. Moritzen). F.W. Herschel himself became also musician (an oboist) and joined his father and his brother Jacob in that band. In 1759, after experiencing the 1757 battle at Hastenbeck, he and Jacob went to England. Jacob returned to Hannover after two years, but Wilhelm (called William in England) stayed. After teaching music for some time, he became organist at Halifax in 1765, and organist and conductor at Bath in 1766.

In 1772, William took home in Bath, and was joined by his sister Caroline. On May 10, 1773, at age 35, William Herschel purchased a copy of James Ferguson's book, Astronomy (Ferguson 1756), and found interest in astronomy. His early books also included Robert Smith's Opticks (Smith 1738) and Harmonics (Smith 1749). Consequently, he started to become a skilled maker of the most powerful telescopes of his time: After 1774, he had acquired skills to make specula mirrors superior to any which had been made before. Moreover, he started to observe the heavens; among his first objects, observed on the 4th of March, 1774, was the Orion Nebula, which he had found mentioned in Smith's Opticks.

On March 13, 1781 William Herschel discovered what he first thought to be a comet, but was later found to be planet Uranus. In recognition of this discovery, he was elected to the Royal Society on December 7, 1781, and awarded an annual grant by King George III of England, which enabled him to give up his career in music (on May 19, 1782) and concentrate on astronomy as the Court Astronomer of the King.

On December 7, 1781, the day of his election to the Royal Society, his friend, William Watson, presented a copy of Messier's (and Méchain's) Catalog of Nebulae and Star Clusters to William. This catalog stimulated his interest in clusters and nebulae. At that time, he had only observed four nebulae: The Orion Nebula together with its companion M43 (1774), globular cluster M13 in Hercules (1779), and the Andromeda "Nebula" M31 (1780). In August 1782, he started to investigate Messier's objects with his superior telescopes; his first observation was that of globular cluster M5 in Serpens. Soon, he "surmised" (to say it in his own words), "that several nebulae might yet remain undiscovered." After a first finding (of the Saturn Nebula, NGC 7009) following September 1782, he started his sytematical and extensive survey of the skies visible from his location in England in March, 1783, always assisted by his sister Caroline. After initial trial-and-error attempts, he started with regular, systematical sky "sweeps" on October 23, 1783, with his 18.7-inch (47.5 cm) aperture, 20-foot focal length reflector, with standard magnification 157 and a field of view of 15'4". He made his next discovery on October 28,
1783: NGC 7184, Herschel's H II.1, a little conspicuous galaxy in Aquarius of 11.2 mag. Only 1 1/2 years later, he had cataloged 1,000 new objects (W.H. 1786), completed a second 1,000 in 1789 (W.H. 1789), and a final additional 500 objects in 1802 (W.H. 1802), so he ended up in discovering about 2500 new "nebulae" and star clusters in about 20 years.

In 1783, Herschel published his observations leading to the discovery of the Solar Motion. He determined that our solar system is moving between the neighboring stars in the direction of the star Lambda Herculis; he introduced the term Solar Apex for this dierction (W.H. 1783).

In 1787, William Herschel discovered two moons of Uranus, Titania and Oberon.

On May 8, 1788, William Herschel married Mary Pitt, former Baldwin, the widow of the wealthy London merchant John Pitt, who had died in 1786. On March 7, 1792, their only son, John Herschel, was born in Slough, England.

In 1789, William Herschel completed his largest telescope, a 48-inch (1.2-meter) aperture, after efforts of about two years. On August 28, on the occasion of first light for this instrument, he discovered Saturn's sixth known moon Enceladus, and on September 17, its seventh known moon, Mimas. This telescope was world's largest telescope for over 50 years, until Lord Rosse erected his 72-inch "Leviathan" at Parsonstown in 1845. However, this large scope was difficult to handle and thus less used than his favourite 18.7-inch reflector which was used to discover most of the nebulae. Had he used it more frequently, he soon had discovered more cosmic phenomena such as the "spiral nebulae", a discovery now left to Lord Rosse.

William Herschel visited Paris in 1801 where he met Napoleon Bonaparte as well as French scientists including Laplace and the old Charles Messier.

Sir William Herschel died on August 25, 1822 in Slough, England, and was buried in the church of Upton on September 7.
Consequently, his sister Caroline left England and returned to Hannover on October 10, 1822. His wife Mary continued to live in Slough until her death in January 1832. Their son John continued the astronomical observations of his father in Slough from 1822 to 1833.

William Herschel was honored lately by the astronomical community by naming Moon crater Herschel (5.7S, 2.1W, 40 km diameter, in 1935), together with his son John by naming Mars crater Herschel (14.9S, 230.3W, 304 km diameter, in 1973), and a crater on Saturn's moon Mimas (2.9N, 109.5W, in 1982). John and Caroline Herschel are honored with separate Moon craters. Asteroid (2000) Herschel was discovered by J. Schubart at Sternwarte Sonneberg on July 29, 1960, and provisionally designated 1960 OA; a prediscovery observation had been designated 1934 NX. The William Herschel Telescope, within the Isaac Newton Group of Telescopes on the Canary Islands, was named after him, as was ESA's "Far IR and Submillimeter Space Telescope (FIRST)," an astronomical satellite to be launched in 2007.

As the most renowned astronomer of his time, William Herschel contributed significantly to most branches of astronomy: He also investigated the proper motion of stars and derived the peculiar motion of the solar system toward the direction of constellation Hercules, modelled the Milky Way galaxy from stellar statistics, and speculated about the nature of the nebulae, including a discussion of the possibility of external island universes (galaxies) which had been brought up by Kant. He also contributed to physics (especially optics) and, e.g., discovered the infrared light.

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