Dreyer, Johann Louis Emil

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Dreyer, Johann Louis Emil

(b. Copenhagen, Denmark, 13 February 1852; d. Oxford, England, 14 September 1926)


Dreyer was the son of Lt. Gen. J. C. F. Dreyer and Ida Nicoline Margarethe Rangrup. He began studying mathematics and astronomy at Copenhagen University in 1869 and won a gold medal in 1874 for an essay entitled “Personal Errors in Observation.” He received the M.A. in 1874 and the Ph.D. in 1882. All of Dreyer’s astronomical appointments were in Ireland: from 1874 to 1878, assistant at Lord Rosse’s observatory, Parsonstown [now Birr], where he observed nebulae through the seventy-two-inch-aperture telescope; from 1878 to 1882, assistant at Dunsink Observatory, Dublin; from 1882 to 1916, director of Armagh Observatory, where he collected and reduced observations made since 1859. This work led to the Second Armagh Catalogue of 3,300 Stars (1886). He settled at Oxford after retirement.

Dreyer was a fellow of the Royal Astronomical Society from 1875, gold medalist in 1916, and president from 1923–1926. He married Katherine Hannah Tuthill of Kilmore, Ireland, in 1875; they had three sons and a daughter. Gentle and amiable in disposition, Dreyer was a good astronomical observer and a most erudite scholar, gifted in languages, accurate, painstaking, and devoted to astronomy and astronomical history.

Dreyer enriched astronomy by three monumental works of research, collection, and editing. The first is “New General Catalogue of Nebulae and Clusters of Stars” (1888). The Herschels had discovered and catalogued over 5,000 nebulae and clusters, but by the 1880’s numerous discoveries by other astronomers had made Sir John Herschel’s catalog out-of-date. Dreyer had tried to remedy this by publishing a supplement listing hundreds of items (1878). Then, at the suggestion of the Royal Astronomical Society, he used the Herschel catalog as a basis for the compilation of the New General Catalogue, renumbering all the 7,840 objects discovered up to 1888 and giving their positions and descriptions. During the following twenty years he also published two index catalogs, enumerating all additional discoveries and raising the total to over 13,000. Many galaxies, nebulae, and star clusters are still known by their NGC and IC numbers.

In 1912 Dreyer edited the scientific papers of Sir William Herschel for the Royal Society of London and the Royal Astronomical Society. The two thick volumes contain seventy-one published and some thirty unpublished papers of the elder Herschel. Dreyer prefaced the work with an excellent detailed biography, based mainly on unpublished material.

Interest in a famous figure in his country’s history, Tycho Brahe, the last great astronomer of the pretelescope era, had inspired Dreyer from boyhood to become an astronomer. In 1890 his book Tycho Brahe provided a scholarly biography rich in information about sixteenth-century astronomy and astronomers. In his last years, Dreyer completed the formidable task of collecting and editing all the works and correspondence of Tycho, including all his observations and those of his assistants. This huge collection (in Latin) was published under the auspices of the Academy of Sciences of Copenhagen between 1913 and 1929.

Among Dreyer’s other works are a History of Planetary Systems and papers including “Original Form of the Alfonsine Tables” and “A New Determination of the Constant of Precession.”


I. Original Works. Dreyer’s 1874 gold medal essay was expanded and published as “On Personal Errors in Astronomical Transit Measurements,” in Proceedings of the Irish Academy, 2nd ser., 2 . (1876). 484–528. Major works edited by Dreyer are “New General Catalogue of Nebulae and Clusters of Stars,” in Memoirs of the Royal Astronomical Society, 49 (1888), 1–237; “Index Catalogue of Nebulae Found in the Years 1888–1894 (Nos. 1–1529), With Notes and Corrections to NGC,” ibid., 51 (1895), 185–228; and “Second Index Catalogue of Nebulae and Clusters of Stars, Containing Objects Found in the Years 1895–1907 (Nos. 1530–5386), With Notes and Corrections to NGC and to First IC,” ibid., 59 (1908), 105–198. These three were republished as New General Catalogue... in 1 vol. by the Royal Astronomical Society (1953; repr. 1962). Also of considerable importance are The Scientific Papers of Sir William Herschel, 2 vols. (London, 1912); and Omnia opera Tychonis Brahe Dani, 15 vols. (Copenhagen, 1913–1929). Vols. I–IV contain the observations made by Brahe and his assistants in the following years: Vol. I, 1563–1585; Vol. II, 1586–1589; Vol. III, 1590–1595; Vol. IV, 1596–1601 and observations of comets. Volumes X–XIII are also entitled Thesaurus observationum.

Additional works by Dreyer are “A Supplement to Sir John Herschel’s General Catalogue of Nebulae and Clusters of Stars,” in Transactions of the Royal Irish Academy, 26 (1878), 381–426, which was included in the NGC; “A New Determination of the Constant of Precession,” in Proceedings of the Royal Irish Academy, 3 (1883), 617–623, which includes a historical survey of the subject; Second Armagh Catalogue of 3,300 Stars for Epoch 1875, Deduced From Observations Made at the Armagh Observatory 1859–1883, and Prepared for Publication by J. L. E. Dreyer (Dublin, 1886); Tycho Brahe. A Picture of Scientific Life in the Sixteenth Century (Edinburgh, 1890); History of the Planetary Systems From Thales to Kepler (Cambridge, 1906); and “On the Original Form of the Alfonsine Tables,” in Monthly Notices of the Royal Astronomical Society, 80 , no. 3 (1920), 243–261, a brilliant research based on medieval manuscripts.

II. Secondary Literature. Two works that draw heavily on Dreyer are A. F. O’D. Alexander, The Planet Saturn (London, 1962), which draws on Dreyer’s works on Brahe and Herschel, and The Planet Uranus (London, 1965), which uses the work on Herschel. Obituaries are in Observatory, 49 (1926), 293–294; and Monthly Notices of the Royal Astronomical Society, 87 (1927), 251–257. There is a biography in Dictionary of National Biography for 1922–1930.

A. F. O’D. Alexander

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