Born Redpost, Somerset, England, 25 November 1848
Died Bristol, England, 9 June 1931
Although William Denning received no formal training as a scientist, he was considered to be one of the highest ranking of British Victorian astronomers in his later life. His reputation was built on a lifetime dedicated to the study of meteor showers and the distribution of meteor shower radiants, as well as cometary observations and planetary studies, especially of Jupiter.
Denning was the eldest of four children born to Issac Poyntz and Lydia (néePadfield) Denning. Little is known about his early childhood and education. Although he may have trained as an accountant in the Bristol area, there is no indication that it was a full-time vocation. He earned some income by writing popular astronomy articles, and probably received occasional monetary contributions from family and friends before the British Government awarded him a Civil List Pension in 1904 for his services to astronomy and because of his straitened circumstances.
Denning’s first significant contribution to meteor astronomy was made in 1877 when he measured the daily radiant drift rate of the Perseid meteor shower. This result had, in fact, been long anticipated, but it was Denning who first performed the required observations and analysis. His study of meteor radiants culminated in the publication of his “General Catalogue of the Radiant points of Meteoric Showers’ in 1899. The “General Catalogue” contained information on 4,367 radiants deduced by Denning from approximately 120,000 projected meteor paths. He believed that there were some 50 meteor showers active each night of the year. For the most part, he believed they were very minor showers, delivering just one or two meteors per night, and that some meteor showers had radiant points that were stationary – fixed in their position on the celestial sphere for many months on end.
Although the “General Catalogue” marked the zenith of Denning’s career, it also brought him into conflict with other researchers in the field. Many meteor astronomers, notably Charles Olivier in the United States, felt that Denning’s radiant reduction methods were not exacting enough and that the vast majority of his claimed radiants were illusory and produced by random groupings of sporadic meteors. The stationary radiants were questioned in the sense that their existence could not be explained in terms of cometary associations. Alexander Herschel , Denning’s strongest supporter on this issue, argued that stationary radiants might be associated with interstellar meteoroid streams, but the problem was never resolved during Denning’s lifetime. Denning never wavered in his belief that stationary radiants existed, but more recent astronomy has shown that stationary meteor radiants cannot exist. A more stringent definition of a shower recently adopted by astronomers has also reduced the number of regularly identified meteor showers to about 40 per year.
As an observer of the terrestrial planets, Denning focused his attention on Mercury and Venus, and summarized his work, as well as that of many previous observers, in a small monograph. However, his dominant interest among the planets was clearly Jupiter, for which his many hours of observation were devoted to mapping transient features and timing central-meridian transits [CMT] of Jupiter’s Great Red Spot [GRS]. Denning published numerous papers on Jupiter’s rotation rate. The American astronomer George Hough favored the use of a micrometer for making measurements of Jovian markings, and challenged Denning and his contemporary Arthur Williams on the accuracy of their central-meridian transit-timing method. More recent analyses have shown that Denning, Williams, and others were fully justified in using the CMT technique and that it could produce Jovian longitudes that were quite as accurate as Hough’s micrometer measurements, although the latter were clearly preferred for Jovian latitude determinations. Using not only his own observations but also those of near contemporaries like Joseph Baxendell , William Dawes , and William Huggins , Denning was able to show that the GRS has a variable rate of motion. Moreover, he found that it was likely that the white hollow recorded by Samuel Schwabe was identical to the hollow in which the GRS typically resides. Working through historical data, furthermore, Denning made a convincing case that connected the GRS to phenomena recorded by Giacomo Maraldi (1665–1729), and even as far back as Robert Hooke (1635–1702).
Denning’s seemingly boundless enthusiasm and dedication to observing the heavens is considered the impetus for his discovery of several comets: C/1890 O2; C/1891 F1; the short-period comet 72P/1881 T1 (Denning-Fujikawa), lost until its accidental rediscovery in 1978; and the lost short-period comet D/1894 F1. He is also credited with the discovery of Nova Cygni in 1920 and with the discovery of numerous nebulae.
In his later years, Denning lived a reclusive life and preferred to maintain his extensive scientific contacts by correspondence. However, in the late nineteenth century Denning belonged to numerous societies and served in several cases as officers of those organizations. He helped found and served as secretary-treasurer of the Observing Astronomical Society during its brief existence as a haven for many of the leading amateur astronomers in the late 1860s. He was elected a fellow of the Royal Meteorological Society (1872) and a fellow of the Royal Astronomical Society (1877). Denning was president of the Liverpool Astronomical Society for their 1887/1888 session. When that organization collapsed, Denning founded and served as director for the British Astronomical Association’s Comet Section (1891–1893) and directed the Meteor Section between 1899 and 1900. Denning was elected a corresponding fellow of the Astronomical and Physical Society of Toronto (later the Royal Astronomical Society of Canada) in 1891. From 1922 to his death in 1931, Denning was the first president of the International Astronomical Union’s Commission 22 on Meteors.
In addition to the recognition implied by elections noted above, Denning received many awards during his lifetime. Denning received the Valz Prize from the French Academy of Science in 1895, and the Royal Astronomical Society Gold Medal, its highest award, in 1898. The Astronomical Society of the Pacific awarded Denning their Donahue Bronze Comet Medals for his discovery of comets in 1890, 1892, and 1894. The University of Bristol bestowed an honorary master of science degree upon Denning in 1927. Craters on both Moon and Mars have also been named in Denning’s honor. Denning never married and had no children.