SCHLESINGER, FRANK

(b. New York, N. Y., 11 May 1871; d. Lyme, Connecticut, 10 July 1943)

astronomy.

Schlesinger was the youngest of seven children of William Joseph Schlesinger and Mary Wagner, both of whom were German immigrants. He was first educated in the public schools of New York City. He graduated in 1890 from the City College of New York and was awarded the Ph.D. from Columbia University in 1898. His dissertation, which was based on measurements of star positions on plates photographed many years before by Lewis Rutherfurd, was a forerunner in his distinguished career in astrometry.

During this era, determinations of stellar parallaxes (which are inversely proportional to the distances of the stars) were carried out largely by the visual methods of Bessel. Experimentation with photographic techniques was progressing, and Schlesinger was eager to test the performance of the Yerkes forty-inch refractor for photographic parallax work. The parallax of a star is defined as the angle at the star subtended by the radius of the earth’s orbit. It is manifested by an apparent slight change in the direction of the star observed at intervals of six months, during which the earth moves in its orbit from one side to the other of the sun. In practice these parallactic displacements are measured relative to very distant faint stars, the motions of which are negligibly small. The uncertainties of the measurements are reduced photographically when there is a minimum disparity in the sizes of the images of the parallax star and the faint comparison stars. In order to achieve that, Schlesinger designed a rotating sector to occult the image of the bright star intermittently, while the faint stars were exposed continuously; this technique is still used extensively. By devising his time-saving “method of dependences,” Schlesinger also improved and simplified the mathematical and numerical procedures for the reductions of the measurements.

After two years at Yerkes Observatory (1903–1905), Schlesinger was called to the Allegheny Observatory in Pittsburgh as its director. Here he expended considerable effort on spectroscopic studies of eclipsing and spectroscopic binary stars, and also on the improvement of instrumentation for parallex work. In addition, he began his first investigations for the preparation of “zone catalogues” to provide accurate positions and proper motions (that is, apparent changes in position) of many thousands of stars to the ninth and fainter magnitudes. These early beginnings he pursued vigorously at the Yale University Observatory, where he was director from 1920 until his retirement in 1941. With the enthusiastic collaboration of Ida Barney, Schlesinger published ten volumes of zone catalogues, including some 150,000 stars, between declinations–30° and +30° of declination.

At Yale he extended his work on parallaxes to the southern hemisphere, where the Yale-Columbia Southern Station began operation in Johannesburg, South Africa, in 1925. Before Schlesinger began his investigations of stellar parallaxes, only a few hundred were known; during his lifetime, and primarily owing to his direct influence, the number grew to four thousand.

In 1908 E. C. Pickering, director of the Harward Observatory, had published his “Revised Harvard Photometry,” giving the positions, magnitudes, and spectral classes of all stars of magnitude 6.5 and brighter. Using this as a basis, Schlesinger in 1924 published his first edition of the “Bright Star Catalogue,” vastly extending its usefulness by adding proper motions, radial velocities, and other relevant data. The second edition, published with Louise Jenkins in 1940, became probably the most widely used of all astronomical catalogues.

BIBLIOGRAPHY

A list of 262 Schlesinger’s papers is in Dirk Brouwer, “Biographical Memoir of Frank Schlesinger,” in Biographical Memoirs, National Academy of Sciences. 24 (1945), 105–144. These works by Schlesinger include “Photographic Determinations of Stellar Parallaxes,” in Problem der Astronomie: Festschrift für Hugo v. Seeliger (1924), 422–437; “Some Aspects of Astronomical Photography of Precision,” in Monthly Notices of the Royal Astronomical Society, 87 (1927), 506–523, which is the first George Darwin Lecture; and General Catalogue of Stellar Parallaxes, 2nd ed. (New Haven, 1935), complied with Louise Jenkins.

E. Dorrit Hoffleit