Born London, England, 19 March 1799
Died Haddenham, Buckinghamshire, England, 15 February 1868
William Rutter Dawes is known for the empirical formula he devised to determine the resolving power of a telescope (Dawes Limit), his extraordinarily keen vision that earned him the sobriquet “eagle-eyed,” and the care and skill with which he conducted his observations of celestial objects. These qualities distinguished him as one of the finest observational astronomers of his day.
Dawes was born at Christ’s Hospital, where his father William Dawes was mathematical master. William Dawes had been Government Astronomer on the first expedition to Botany Bay in 1787, and had married Judith Rutter in 1792 after his return to England. William Rutter lost his mother at an early age, and following his father’s third official posting to Sierra Leone as that colony’s Governor in 1801, was sent to live with his grandfather in Portsmouth. In 1807, William Rutter’s care became the responsibility of Reverend Thomas Scott of Aston-Sandford, Buckinghamshire, with whom he resided until the return of his father in 1811, at which time he was placed in Charterhouse School. Two years later, responsibility for his welfare again reverted to the Reverend Scott when William Rutter’s father and elder sister Judith left England to take up work as antislavery missionaries in Antigua. Thus began a period of study terminated only when Scott died in 1821.
The young Dawes’ doubts about certain tenets of the Anglican church, a calling to which he seemed peculiarly suited, induced him to substitute medicine for the clerical career his father desired for him. Having passed through the normal course of study at Saint Bartholomew’s Hospital, London, Dawes settled as a medical practitioner at Haddenham, Berkshire, marrying Mrs. Scott, the widow of his tutor. In spite of the great disparity in age, the union contributed greatly to Dawes’ well-being and happiness.
In 1826, Dawes precipitously abandoned his practice at Haddenham and moved to Liverpool to attend to his sister Judith, who had returned from Antigua in desperate condition as a victim of yellow fever. There, Dawes’ interest in astronomy, inherited from his father and continued during his stay with Scott, widened and deepened. Having obtained the loan of a volume of Rees’s Encyclopedia, he copied out William Herschel ’s catalogs of double stars. Armed with those lists and a copy of the French edition of John Flamsteed ’s Atlas(given by Nevil Maskelyne to his father prior to the latter’s departure for Botany Bay in 1787), Dawes observed on almost every fine night when his uncertain health would permit. With a small refracting telescope of 1.6-in aperture, mounted at an open window of his house, Dawes made accurate diagrams of binary stars. With this arrangement he was able to distinguish the companion stars of Castor, Rigel, Polaris, γ Virginis, and many others. About this time Dawes came in contact with fellow amateur William Lassell with whom he struck up a friendship that was to last for the rest of their lives.
While in Liverpool, Dawes’ interest in holy orders revived, perhaps not only as a result of his sister’s condition and likely death but also under the influence of Dr. Raffles, who was for many years the minister at the Independent Chapel, Great George Street, in Liverpool. Although his scruples once again intervened, Dawes was eventually prevailed upon to assume charge of a small congregation at Ormskirk, a modest-sized town some 15 miles north of Liverpool in Lancashire.
At Ormskirk, Dawes erected his first observatory, a modest structure housing a 5-ft. Dollond refractor of 3.75-in aperture and equipped with a filar micrometer. His first published observation was of an occultation of Aldebaran seen from Ormskirk on 9 December 1829. However, Dawes devoted himself to the observation and measurement of double stars. This was a subject to which his acute vision and attentive habits were particularly adapted. His “Micrometrical Measurements of 121 Double Stars …,” in the Monthly Notices of the Royal Astronomical Society, documented this effort well. Dawes was elected a fellow of the Royal Astronomical Society on 14 May 1830.
But while Dawes enjoyed increasing success as an amateur astronomer, his private life fell apart. His wife, who was much older than he, died, and his own health, which had always been uncertain, broke down. Accordingly, he resigned the Ormskirk ministry, and in the autumn of 1839, he accepted the charge of the private observatory erected by the wealthy wine merchant George Bishop at South Villa, Regent’s Park, London. At South Villa, Dawes continued his double-star work, detecting orbital movement in ε Hydrae and, independent of the observers at Pulkovo, in γ Andromedae.
Three years before his death in 1868, in a communication to The Astronomical Register, Dawes gave indications of a tense relationship between himself and Bishop. Dawes objected strenuously to the fact that measurements of about 250 double stars made between 1839 and 1844, which Bishop had published in 1852 as part of his Astronomical Observations Taken at the Observatory South Villa, were in fact not made by Bishop but instead were Dawes’ own observations. The souring of this relationship may explain why, in 1844, Dawes terminated his engagement at South Villa and moved to Cranbrook, in Kent, not far from Hawkhurst where his friend John Herschel lived. That he was enabled to make this change can be ascribed to his remarriage, in 1842, to Mrs. John Welsby of Ormskirk, the widow of a wealthy solicitor.
At Cranbrook, Dawes set up an observatory that included a 2-ft-diameter transit circle by Simms, and a clockwork driven Merz & Mahler equatorial of 6.5-in aperture and 8.5-ft focal length. With these he worked tirelessly until forced by headaches and asthma to retire to Torquay, where he even thought of abandoning astronomy. In 1850, following an improvement in his condition, Dawes resumed his astronomical pursuits at Wateringbury, near Maidstone, where on 25 and 29 November, of that year, independent of George Bond in America, he detected the faint, dusky crepe ring of Saturn.
Finally in 1857, Dawes removed his observatory to Hopefield, Haddenham, near Thame, where he remained for the rest of his life. Dawes was highly regarded for the medical service he dispensed freely to the impoverished residents of the town. Here, in May 1859, he reinforced his instrumentation with an equatorial refractor of 8.25-in aperture, by Alvan Clark of Cambridgeport, Massachusetts, and 6 years later an 8-in Cooke achromatic. His second wife died in 1860, but in spite of his own rapidly deteriorating health Dawes continued to observe until 1867.
Apart from his work on double stars and a number of comets, Dawes made useful observations of Mars, from which Richard Proctor constructed an albedo map of the planet (1867). Dawes verified the reality of Encke’s Division in the outer ring of Saturn (1843), affirmed the semitransparency of the inner dusky ring, and observed ring phenomena at the edge-on presentation of 1848. Using a solar eyepiece of his original design, Dawes detected fine structure and rotary movement in sunspots, and saw a facula projected above the limb of the Sun. He also refuted the “willow leaf” aspect of the solar granulation reported by James Nasmyth , and vividly described the crimson prominences at the total solar eclipse of July 1851, which he observed from Sweden with John Hind .
As Alvan Clark’s first major customer, Dawes brought the skill of the American telescope maker to wide notice in Europe. Dawes bought five Clark lenses, including two mounted in telescopes. With them, he took his lifetime total of binary-star measurements to almost 3,000. Dawes’ “Catalogue of Micrometrical Measures of Double Stars” includes the description of what is universally known as the Dawes limit.
Dawes received the Gold Medal of the Royal Astronomical Society in 1855 and was elected a fellow of the Royal Society in 1865.