Kent Scientists

Stephen Groombridge (1755-1832)

Born in Goudhurst, lived in Blackheath

An indefatigable astronomer who catalogued the stars

Dinner guests at the Blackheath home of Stephen Groombridge in the early 19th century would sometimes see their host disappear into an adjoining room, only to reappear a few minutes later. This only happened on nights when the sky was clear.

Stephen Groombridge (Wellcome collection CC BY 4.0)

Groombridge had been born in Goudhurst in 1755. At the age of 21 he took over a linen draper’s business and continued as a merchant in London for almost forty years. From an early age he was a keen astronomer, with a small observatory at Goudhurst. As happens with hobbies, this one gradually turned into a consuming passion. At the age of 51 he embarked on an ambitious project that would occupy the rest of his life, the compilation of a star catalogue more comprehensive and precise than any of its predecessors. The Groombridge catalogue would later be described as one of the most valuable contributions to practical astronomy made during the nineteenth century.

Groombridge's transit circle. The supporting pillars are 6ft high. 

By 1806 Groombridge and his wife had moved to Blackheath and he had acquired a special purpose telescope known as a transit circle, converting the stables adjoining his home to house it. Over the next ten years he recorded some 50,000 painstaking measurements with this instrument. The purpose was to determine the exact position in the sky of more than 4000 stars, many of them too faint to see with the naked eye. All were circumpolar stars, located in the region of sky centred on the north pole where stars remain above the horizon at all times. To find the position of a given star in the sky, two measurements are needed, both made with the transit circle. The first is to time the precise moment at which the star crosses the meridian – the imaginary line dividing east from west in the sky and visible as a set of vertical hairlines inside the telescope. It was the need to catch these moments that sometimes drew Groombridge briefly away from the dinner table. The second measurement was the zenith angle, the angular distance of the star from the vertical at the moment it crosses the meridian. For every star these measurements would have to be repeated on several occasions to check and improve their accuracy.

Groombridge's home

Groombridge's home in Eliot Place in Blackheath (Google streetview)

Stephen Groombridge retired from business at the age of 60 and devoted the next 12 years – before being paralysed by a stroke – to the monumental task of correcting the measurements in his precious notebooks for all the factors that might make them even slightly inaccurate. Among these factors is the fact that the stars do not actually occupy the exact positions in the sky they appear to have, because their light is bent off course slightly as it comes down through the Earth’s atmosphere.

Groombridge was elected a Fellow of the Royal Society in 1812. He died in 1832 and is buried at Goudhurst. His ‘Catalogue of Circumpolar Stars’ was published six years later, having been completed by others, and can be found on the Internet Archive website. In his preface to the book, the Astronomer Royal, George Airy, suggests that Groombridge’s work ‘is one of the greatest which the long-deserved leisure of a private individual has ever produced’. 

By pinning down so many faint stars so precisely, Groombridge provided ‘signposts’ which later astronomers could use to locate even fainter objects in the sky. And by comparing his measurements with later ones it was possible to work out the rate at which individual stars move, slowly and steadily across the sky. (This movement, known as a star’s ‘proper motion’, means that the Great Bear constellation, for example, will look a different shape in 10,000 years’ time from how it looks today.) In this way one star first observed by Groombridge and known as Groombridge 1830 was found to be the fastest mover in the sky, travelling a distance equal to the width of the full moon in 250 years, though two faster ones have since been discovered.

There is a memorial to Stephen Groombridge in St Mary's church in Goudhurst. His transit circle is now in the Science Museum, London. Asteroid 5657, discovered in 1936, is named after him.

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