how one woman discovered eight comets

The Mechanism

Caroline Lucretia Herschel was born on 16 March 1750 in Hanover, in the Electorate of Hanover, the eighth of ten children of a bandmaster of the Hanoverian Foot Guards. She contracted typhus at 10 and smallpox at 13, and stopped growing at 4 feet 3 inches (1.30 m); her mother regarded her as unmarriageable and put her to household drudgery. In 1772, at 22, her elder brother William — by then a working musician in the English spa town of Bath — travelled to Hanover and brought her out to Bath as his housekeeper and singing pupil. She trained as a concert soprano and by 1778 was refusing every engagement that did not include William as conductor. In 1781, from the garden of 19 New King Street in Bath, William discovered Uranus with a homemade seven-foot Newtonian reflector — the first new planet identified in recorded history — and in 1782 George III appointed him King's Astronomer with a yearly salary of £200 on condition that he move within reach of Windsor. In April 1786 the Herschels moved to Observatory House on Windsor Road in Slough, twenty miles west of London, where William began constructing the great 40-foot reflecting telescope (the largest in the world at completion in 1789) and Caroline was formally instated as his assistant. William gave her, in the summer of 1782, a small Newtonian reflector he had designed specifically for comet-sweeping — a slow, systematic scan of parallel strips of sky at 25× magnification — and taught her to use it. On the night of Tuesday 1 August 1786, William was in Germany on royal business and Caroline was alone at Slough. From the flat roof of the small cottage in the garden, working her sweeper across a horizontal strip of sky in the constellation Coma Berenices at about 9:45 p.m. local time, she saw a small round fuzzy object that was not on any chart. She recorded its position and drew its motion; she observed it again the following night. On the morning of 2 August she wrote a letter — the first of many — to Alexander Aubert, William's astronomical friend at Deptford: *"In consequence of the friendship which I know to exist between you and my Brother, I venture to trouble you in his absence with the following imperfect account of a comet."* Aubert forwarded the observations to Nevil Maskelyne, the Astronomer Royal at Greenwich, and to Charles Blagden, secretary of the Royal Society. Both confirmed the sighting within days. Comet C/1786 P1 (Herschel) was hers. She read her observation paper *"An account of a new comet. In a letter from Miss Caroline Herschel to Charles Blagden, M.D. Sec. R.S."* before the Royal Society on 9 November 1786 — the first paper written by a woman and published in *Philosophical Transactions*. On 5 October 1787, in a letter drafted by Sir Joseph Banks and signed by King George III, Caroline was granted an annual salary of £50 in her own name "as assistant to the King's Astronomer" — the first salary ever paid by a British sovereign to a woman for scientific work, and by any government to a woman for astronomy. Over the following eleven years — from her own cottage roof at Slough while William slept or was travelling — she discovered seven more comets: on 21 December 1788 (35P/1788 Y1, later called Herschel–Rigollet and confirmed as periodic with a 155-year orbit), 7 January 1790 (C/1790 A1), 17 April 1790 (C/1790 H1), 15 December 1791 (C/1791 X1), 7 October 1793 (independently co-discovered with Charles Messier, C/1793 S2), 7 November 1795 (later identified as the fourth-shortest-period comet known, 2P/Encke, though the periodicity was worked out by Encke in 1819), and 14 August 1797 (C/1797 P1). Eight comets in eleven years, from a garden roof at Slough, alone, at the sweeper's small eyepiece. Between comets she also catalogued the sky. Her *Catalogue of the Stars* (1798), reduced from John Flamsteed's data and expanded with 561 additional stars observed at Slough, was published by the Royal Society and immediately became the standard working reference for late-eighteenth-century observational astronomy. Her *Reduction and Arrangement in the Form of a Catalogue in Zones of all the Star-Clusters and Nebulae observed by Sir William Herschel in his sweeps* (1828), a manuscript of 2,500 objects reduced to a single-epoch coordinate system, was the direct precursor of John Herschel's *General Catalogue* (1864) and thereby of John Louis Emil Dreyer's *New General Catalogue* (1888), the NGC identifiers still used today to name every nebula and galaxy on Earth. In 1828, aged 78, she received the Gold Medal of the Royal Astronomical Society — the first ever given to a woman — for the *Reduction*. In 1835 she was elected an Honorary Member of the Royal Astronomical Society, alongside Mary Somerville, the first two women so admitted. She died in Hanover, at her nephew John's family home on Marktstrasse, on 9 January 1848, aged 97 years and 10 months, having outlived her brother William by 26 years. On her tombstone, at her request, was engraved: *"Die Augen der so hier Verklärten waren hienieden gen den gestirnten Himmel gerichtet."* — "The eyes of her now transfigured were here below turned to the starry heavens."