Herschel is revered as a pioneer of modern astronomy, and he is universally known as Sir William on the basis that he was accorded a British knighthood towards the end of his life. From then and up to the present day, publications have referred to him as Sir Herschel. These include the many accounts of his life and work, which almost invariably mention that a knighthood was bestowed on him by the Prince Regent (later George IV) in 1816. Not surprisingly, a host of bibliographical guides, library catalogues and indexes of all descriptions and in all locations have followed suit and prefixed Herschel 's name with the knightly title of 'Sir'.Intriguingly, however, there is nothing in the formal record to indicate that Herschel was ever in fact the recipient of a knighthood, and so he was never officially Sir William. This article sets out to explore the highly unusual circumstances in which Herschel acquired the title upon his appointment by the Regent as a knight of the Royal Guelphic Order, and will show that it lacked any official or formal basis: with very few exceptions it has been widely though incorrectly assumed that Herschel derived the title of 'Sir' as a matter of form from his Guelphic knighthood. We also reveal the effects of this misunderstanding within his immediate family, and the problems this created for Herschel's son John when the latter was offered the same honour in 1831.IWilliam Herschel's first association with, and eventual settlement in Britain came about as a result of his family's residence in the north German state or 'electorate' of Brunswick-Luneburg (usually known as Hanover after its capital city), which since 1714 had enjoyed a close royal relationship with Great Britain.1 Due to the failure of the Stuart line the British throne had passed in that year to the house of Hanover, thus forming a royal 'personal union' that would last until 1837.2 Throughout this time Britain's kings continued simultaneously to rule their Hanoverian domains, though in constitutional terms the two states and their governments remained entirely separate and distinct. Strong social and cultural links between the two states were nevertheless exemplified by those such as Herschel who chose to settle and find employment in Britain. Herschel's first acquaintanceship with Britain had occurred while serving as a young bandsman in one of the regiments of the Hanoverian army.3 By about 1759 he had decided to settle permanently in Britain which he esteemed as the ideal country where he could exploit his gifts as a musician, aware of the encouragement given to music in England.4As is well known, Herschel eventually moved to Bath in 1766 where he achieved fame, first as a musician and composer, and latterly and more widely as an amateur astronomer. His discovery in 178 1 of the planet we now know as Uranus brought him to the attention of the King, George IQ, who in 1782 appointed him as astronomer to the British court at Windsor with a pension of £200. Sometimes at the Castle, but more usually in his own home, Herschel would when asked demonstrate the heavens to the Royal Family and their guests.In 1793 Herschel petitioned for, and obtained, a private Act of Parliament naturalizing him as a British subject.5 We can only speculate what may have prompted this move. His autobiographical notes6 end before this time, and his sister Caroline, the chronicler of the family and her brother's assistant in astronomy, destroyed her records for this period. In 1788 Herschel had married Mary Pitt, the widow of a wealthy merchant and neighbour (and the daughter of his landlady). After the birth of their son John in 1792, it may have seemed incongruous that whereas the infant had been born on British soil and was therefore in the eyes of the law a British subject, was still technically a foreigner (albeit a subject of the British king in his capacity as elector of Hanover). …