German-born Herschel (1750-1848) moved to the UK to be with her brother William Herschel (1738-1822) and they began as musicians before later becoming famous astronomers.
They lived at 19 New King Street in Bath, which is now the home of the Herschel Museum of Astronomy.
The visitor book is a vellum-bound volume, compiled by Caroline from the mid-1780s when she and her brother were living in Datchet, near Windsor. Written in Caroline’s own hand, it lists over a hundred names of people who came to see them – often including the king and queen, attended by members of court – to look through William’s telescopes.
Scientists, writers, artists, politicians and foreign royalty all feature. Joseph Haydn, Benjamin Franklin and Fanny Burney are listed among the guests.
The book had been on long-term loan to the museum since 1996 - kindly provided by Dr and Mrs A Koester - and the museum was recently given the opportunity to buy it.
The Friends of the National Libraries supported the deal to buy the book for £6000. It donated £5000 with the remaining £1000 coming from the museum’s reserves.
The museum is dedicated to the many achievements of the Herschels. William discovered the planet Uranus in 1781 using a telescope of his own design from the home. Later the pair were joined by William’s son John (1792-1871), who also became an astronomer.
Among Caroline’s achievements was the discovery of several comets including the periodic comet 35P/Herschel–Rigollet.