Dreyer, John Louis Emil (1852–1926), astronomer and historian, was born 13 February 1852 in Copenhagen, Denmark, into a distinguished Danish military family, third son of Lt.-gen. J. C. F. Dreyer, minister of war and the marine, and Ida Nicolene Margarethe Dreyer (née Rangrup). He was educated at a school in Copenhagen before entering (1869) Copenhagen University, where he studied mathematics, and astronomy under H. L. d'Arrest (1822–75); he was awarded a gold medal essay prize (1874), and graduated MA (1874) and Ph.D. (1882).
All his professional appointments were in Ireland. In 1874 he was appointed assistant at the earl of Rosse's (qv) (d. 1908) observatory at Birr Castle, Parsonstown (Birr), King's Co. (Offaly). He published several papers in the journals of the RDS and the RIA, giving his first notable paper to the RIA, ‘On personal errors in astronomical transit observations’ (RIA Proc., 2nd ser., ii (Science) (1875–7), 484–528), which provided an exhaustive history of the subject. In 1877 he published a valuable supplement to John Herschel's ‘General catalogue of nebulae and clusters of stars’ (RIA Trans., xxvi, Science (1879), 381–426), listing 1,136 new nebulae and corrections to Herschel's work. Dreyer was the last regular observer to use the famous 6ft (1.83 m) reflecting telescope (known as ‘the Leviathan of Parsonstown’), which at that time was the largest in the world. He moved in 1878 to Dunsink observatory, Dublin, as assistant astronomer, where he was mainly concerned with meridian observations. With Ralph Copeland (1837–1905) he founded and edited the astronomical journal Copernicus (Dublin, 1881–4); a fascinating mixture of recent observations and astronomical history, it contained several of Dreyer's papers, including his valuable ‘A new determination of the constant of precession’, which was later republished (RIA Proc., 2nd ser., iii (Science) (1883), 617–23). However, Copernicus foundered from lack of support.
Appointed astronomer of the Armagh observatory (1882–1916), he wrote an Historical account of the Armagh observatory (1883) and published the Second Armagh catalogue of 3,300 stars . . . deduced from observations made at the Armagh observatory 1859–1883 (1886). Accepting the proposal of the Royal Astronomical Society to revise Herschel's catalogue, Dreyer made a major contribution to astronomy when he published his magisterial ‘A new general catalogue of nebulae and clusters of stars, being the catalogue of Sir John Herschel’ (Memoirs of the Royal Astronomical Society, xlix (1888), 1–237), which contained references to all recorded observations of nebulae – 7,840 objects, which were given a new number and listed in order of right ascension. By 1896 it was in daily use in most observatories, and remains a standard reference system; he published supplementary index catalogues in 1895 and 1908, which contained all additional discoveries up to 1907.
In later years his research turned increasingly to historical problems. His fascination as a schoolboy for his fellow-Dane Tycho Brahe had fired his determination to become an astronomer; in 1890 he published Tycho Brahe: a picture of scientific life and work in the 16th century, and between 1913 and 1926 his greatest historical project, a complete edition in Latin of Brahe's observations and correspondence, Tychonis Brahe Dani opera omnia (15 vols). His History of the planetary systems from Thales to Kepler (1906), marked by scrupulous accuracy and profound learning, dispelled numerous myths in the history of astronomy. Approached by the Royal Astronomical Society, he wrote a biographical introduction and edited the Scientific papers of Sir William Herschel (1912). Despite his efforts to increase the income of the observatory and his own ‘utterly insufficient’ salary (Bennett, 168), his directorship coincided with a period of financial and institutional decline; his resignation from the observatory in 1916 and his move to Oxford, England, ostensibly to facilitate his studies in the history of astronomy, was probably partly due to frustrations experienced during his directorship.
Elected fellow (1875), council member (1917), and president (1923–5) of the Royal Astronomical Society, he was awarded (1916) its gold medal for his work on the history of astronomy and for his catalogues of nebulae; that the medal should be awarded for historical work was in itself exceptional. He was also a contributor to and chief editor (1923) of the History of the Royal Astronomical Society, 1820–1920. A bibliography of his writings is recorded by R. A. Sampson and F. E. Brasch in ‘John Louis Emil Dreyer’ (Isis, xxi (1934), 131–44), and a catalogue of his library was published in 1927. He became a British citizen, was awarded an hon. D.Sc. (QUB) and hon. MA (Oxon.), was elected a member of the Royal Danish Academy of Sciences, and was created knight of the Dannebrog. His portrait hangs in Armagh observatory.
Gifted in classical and modern languages, amiable, gentle and distinguished in appearance, Dreyer died 14 September 1926 at Oxford. He married (1875) Katherine Hannah Tuthill (d. 1923) from Kilmore, Co. Limerick; they had three sons and one daughter. Their eldest son was Maj.-gen. John Tuthill Dreyer (1876–1959), CB, DSO; their second son was Adm. Sir Frederick Dreyer (1878–1956), CBE, KCB; and their third son was Maj. George Dreyer.