Born Copenhagen, Denmark, 13 February 1852
John Dreyer is noted for his meticulous compilation of the New General Catalogue of Nebulae and Star Clusters[NGC] and two supplementary catalogs, his important biography of Tycho Brahe , his collection of the papers of William Herschel , and an authoritative history of astronomy.
Dreyer spent most of his working life in Ireland, although he was born in Denmark to a family with distinguished military connections. His grandfather was an officer in Napoleon’s army, and his father, Lieutenant General John Christopher Dreyer, served in the Schleswig-Holstein War in 1864, later becoming Minister of War and Marine in the Danish government.
Dreyer attended school in Copenhagen from age 5 until 17, showing great ability in history, mathematics, and physics. At the age of 14, he happened to read a book about Brahe and his observatories on the island of Hven; this inspired him to devote his life to astronomy. Dreyer paid regular visits to Copenhagen Observatory, where he conversed with the assistant astronomer Hans Schjellerup . In 1869, Dreyer entered the University of Copenhagen, where he attended the lectures of Heinrich d’Arrest , who supervised his astronomical studies. The following year, Dreyer was presented with the key of the observatory, giving him access to the instruments. His first paper, “On the Orbit of the First Comet of 1870” (C/1,870 K1), was published in Astronomische Nachrichtenin 1872. He was awarded a Gold Medal by the University of Copenhagen for an essay on the question of personal errors in observation.
In 1874, Dreyer succeeded Ralph Copeland when he was appointed assistant to William Parsons at his observatory at Birr Castle, Ireland, where Dreyer had access to the 6-ft. diameter Leviathan, the largest telescope in the world. The following year in Birr, he married Katherine Tuthill from Kilmore in County Limerick. During the 4 years in Birr, Dreyer published several papers in the journals of the Royal Irish Academy and the Royal Dublin Society. One of these, “On Personal Errors in Astronomical Transit Observations,” examined critically the sources of error in making visual observations of transits. Dreyer used both the 6- and the 3-ft. reflectors at Birr to observe star clusters and nebulae; his results were included in the General Memoir of Observations made from 1848 to 1878, presented by Lord Rosse to the Royal Dublin Society. In 1877, Dreyer presented the Royal Irish Academy with an important paper containing additions and corrections to John Herschel ’s General Catalogue of Nebulae and Clusters.
In 1878, Dreyer was appointed assistant at Dunsink Observatory in succession to Charles Burton . The director, Robert Ball , put him in charge of meridian observations with the Pistor and Martins Circle. This work culminated in the publication of the mean positions of 321 red stars as part 4 of Astronomical Observations and Researches at Dunsink, the Observatory of Trinity College, Dublin. In 1881, Dreyer and Copeland (now Astronomer Royal for Scotland) introduced an international journal of astronomy entitled Copernicus. Although only three volumes were published, several important papers appeared. In particular, Dreyer’s paper on “A New Determination of the Constant of Precession” was later acclaimed by Simon Newcomb .
In 1882, at the age of 30, Dreyer gained his Ph.D. from the University of Copenhagen and was appointed director of Armagh Observatory in succession to Thomas Robinson . Financially, Armagh Observatory was destitute, with no prospect of replacing its aging instruments. Although Dreyer obtained a new 10-in. Grubb refractor in 1884, the lack of funding for an assistant precluded him from a continuation of traditional positional astronomy. Instead, he concentrated on the compilation of observations made earlier by Robinson since 1859, together with many of his own. The Second Armagh Catalogue of 3,300 Stars for the Epoch 1875appeared in 1886. That same year, Dreyer submitted to the Council of the Royal Astronomical Society [RAS] a supplementary catalog of nebulae.
The council proposed that it would be more convenient if the three existing catalogs were combined into a New General Catalogue of Nebulae. Dreyer accomplished this laborious task speedily, and the New General Catalogue of Nebulae and Clusters, Being the Catalogue of Sir John Herschel, Revised, Corrected and Enlargedwas published in 1888 in the Royal Astronomical Society Memoirs. The catalog contained the positions and descriptions of 7,840 nebulae for the epoch 1860. It remains the standard reference used by astronomers the world over. Dreyer later published two Index Catalogs [IC] of nebulae and clusters; the first contained 1,529 new nebulae found between 1888 and 1894 and the second contained 3,857 nebulae and clusters found between 1895 and 1907. NGC and IC numbers are still used as the names of many prominent galaxies, nebulae, and star clusters. Meanwhile, Dreyer struggled with continuing financial difficulties facing Armagh Observatory. He used the 10-in. Grubb refractor for micrometric positional measurements of nebulae with respect to comparison stars, and the results were published in the Transactions of the Royal Irish Academy.
As time went on, Dreyer became increasingly interested in the history of astronomy and especially in the life and work of his boyhood hero and countryman, Tycho Brahe. In 1890, Dreyer published a fine biography of Brahe, followed in 1906 by his classic History of the Planetary Systems from Thales to Kepler. Dreyer’s greatest historical work was a complete Latin edition of the works of Brahe, which he started in 1908; the first volume of the eventual 15-volume series appeared in 1913. This project was interrupted between 1910 and 1912 for work on an edition of the scientific papers of William Herschel , sponsored jointly by the Royal Society and the Royal Astronomical Society. Dreyer edited the two large volumes and also wrote the introductory biography.
In 1916, the RAS Council awarded Dreyer its Gold Medal in recognition of his great labors in the preparation of his Catalogue of Nebulaeand of his contributions to the history of astronomy. In September of that year, Dreyer resigned the Armagh directorship and moved to Oxford, England, where he had access to the excellent facilities of the Bodleian Library for pursuing his historical researches.
Dreyer received the degree of D.Sc. from Belfast and an honorary MA from Oxford. The International Astronomical Union named the lunar crater at 10°.0 N and 97°.9 E in his honor.
Dreyer served on the council of the Royal Astronomical Society from 1917 and as president in 1923 and 1924. During his 2-year tenure he delivered two addresses-the first advocated publishing a new edition of Isaac Newton ’s collected works and the second justified the award of a Gold Medal to Arthur Eddington for his work on star streaming, stellar structure, and general relativity. Dreyer was joint editor with Herbert Turner of the History of the Royal Astronomical Society, published in 1923; he covered the periods 1830–1840 and 1880–1920.
Dreyer combined a single-minded devotion to astronomy with a gentle and amiable disposition. He was a skilled observational astronomer, an excellent mathematician, a talented linguist, and a gifted writer. The death of his wife in 1923 was a great blow from which he never recovered properly. From the end of 1925, Dreyer’s health worsened. He was survived by one daughter and three sons.