William Lewis Elkin, American astronomer, was a son of Lewis Elkin, a teacher, and his wife Jane, born Fitch. Elkin and his mother lived in various European countries from 1867 with the result that he became fluent in German and French. In 1876 he qualified as a civil engineer at the Royal Polytechnic School in Stuttgart, Germany. However, he had by this time developed an interest in astronomy and spent the next four years at Strasbourg Observatory in France. Meanwhile he studied at the University of Strasbourg, where he obtained his doctoral degree in 1880 with a thesis on the parallax of alpha Centauri.
During the previous year he met David Gill*, who was visiting various European observatories before coming out to South Africa to take charge of the Royal Observatory, Cape of Good Hope. Gill invited him to visit the Cape after completing his studies, to collaborate on the determination of stellar parallaxes in the southern hemisphere. Elkin arrived in January 1881 and stayed with the Gill family as a guest, receiving no salary. Using Gill's 4 inch heliometer the two painstakingly measured the parallaxes of nine first magnitude stars, including a confirmation of the large parallax of Alpha Centauri, first measured by Thomas Henderson*. Their results, constituting the first significant set of parallax measurements in the southern hemisphere, were published in the Memoirs of the Royal Astronomical Society (Vol. 48, pp. 1-194) in 1884, with Elkin as co-author.
Elkin's other astronomical activities at the Cape included collaboration with Gill's assistant, W.H. Finlay*, in measuring the orbital elements of the Great Comet of 1882. An abstract of their work was published in the Transactions of the South African Philosophical Society (1881-1883, Vol. 3, p. 14). Elkin published two more notes in the same volume of the Transactions. One of them "On the telegraphic determination of the longitude of Kimberley" (p. 26), the other on "The polarizing photometer, and its application to alpha Centauri" (p.29).
Elkin left for the United States in May 1883 to become an astronomer at Yale Observatory (in New Haven, Connecticut), but Gill and he remained lifelong friends and correspondents. Three of his subsequent projects in positional astronomy were published under the general title "Researches with the heliometer" in the Transactions of the Astronomical Observatory at Yale University (1887-1904): "Determination of the relative positions of the principal stars in the group of the Pleiades" (1887, with a revision in 1904); "Triangulation of stars in the vicinity of the north pole" (1893); and "Determination of the parallax of the ten first magnitude stars in the northern hemisphere" (1902). In 1888-1889 he again collaborated with Gill, in observations of three minor planets with a view to determining a more accurate value of the solar parallax (and hence the scale of the solar system). This work was published, with Elkin as co-author, in "A determination of the solar parallax and mass of the moon, from heliometer observations of the minor planets Iris, Victoria and Sappho made in the years 1888 and 1889 at the Royal Obsevatory, Cape of Good Hope, in cooperation with the observatories of Yale College (New Haven), Leipzig, Goettingen, Bamberg and Oxford (Radcliffe) ..." (Annals of the Royal Observatory, Cape of Good Hope, Vol. 6, 1897). Continuing his work on stellar parallaxes, Elkin eventually determined the distances of over 200 stars. Many of these were published in a catalogue of Yale parallax results, with F.L. Chase and M.F. Smith, in 1912. He also developed a photographic method to observe meteor trails, and determined the directions of meteor flight and their distances from the earth. In 1896 he was appointed the observatory's director, a post he retained to his retirement in 1910. He was a member of various astronomical societies, including the Royal Astronomical Society of London.
After his retirement Elkin did no further work in astronomy, partly as a result of poor health. His interests at this time included music (he was an accomplished pianist) and colour photography. Among the honours he received were an honorary doctorate from the University of Christiania, and the Lalande Medal of the French Academy. He was married to Catherine Adams, but they had no children.