Obituary for Prof. Frank Schlesinger J.H. Oort Originally published in Dutch in Hemel & Dampkring, 43, 27-30 (1945). In these days of war, in which his compatriots are struggling to bring us, we hope soon, the liberation, my thoughts go at a quiet moment, however strange this may seem so near to the battle, to a deceased friend. 1 Among the names of the astronomers of fame, who passed away from us dur- ing the war years, that of Frank Schlesinger strikes us deepest. No foreign as- tronomer was as closely connected to the Netherlands as he was. In the first place through ties of close and faithful friendship, first with the elderly, Kapteyn, whom he held in high esteem, later with his peer de Sitter and with the young: Schilt, Brouwer and Oort, who came to work at the Yale Observatory as a result of this friendship with de Sitter. He used to tell me that the most valuable thing that he remembered from his trip to Europe and the congress of the International As- tronomical Union in Rome in 1922 was the friendship with de Sitter. Whoever watched him later on during his repeated visits to Leiden could easily convince himself how true this was; once after a trip to France, which by the way was a country he was very fond of, he sighed with profound satisfaction: ‘it feels like coming home’. One of the first images that come to mind when I think of Prof. Schlesinger is how, one evening just after my arrival as a 22-year-old young man at the Yale Observatory next to me, sitting on the sofa in his living room, he showed pictures of groups of astronomers at earlier international gatherings and how with warm feeling of appreciation of others he described various figures through his words. The way he talked about these astronomers could make one think that he was talking about the closely related members of a large and scattered family. And so, I believe, he also regarded a large part of the astronomers in the whole world almost as members of his family. For that part of the ‘family’ that lived within easy reach he formed a natural center, which every month brought together a number of astronomers, a.o. Shapley from Cambridge, Slocum from Middletown, Russell from Princeton, Miller from Swarthmore, Benjamin Boss from Albany, all members of the ‘Neighborhood Club’, in New Haven. Later, when he was chosen to be president of the International Astronomical Union and become for some time the center of all astronomers, to be elected to this position for which he naturally was suited, filled him with great joy. Professor Schlesinger was a human with deeply feeling, who could be a warm and faithful friend. He had a fine sense of humor, which often enlightened difficult situations. Almost inhumanly strict in the standards of his own work, he possessed the gift of fully admiring what others had accomplished, and he was particularly mild in the face of shortages in the work of others. Schlesinger’s work is for the most part focused on the precise determination of the positions the stars. Born in New York (11 May 1871), he attended the College of the City of New York and completed his studies at Columbia University in the same city, where he obtained his doctorate in 1898 on a thesis entitled ‘The Prae- 2 sepe Group: Measurement and reduction of Rutherfurd photographs’. In his early association with this pioneer of photography, L.M. Rutherfurd, the foundation was laid for his life’s work. Already then he formulated the ideal to use his acquired experience for the determination of parallaxes of fixed stars. Parallax measure- ments were at that time the great task of astronomy; without parallaxes one could not know distances of stars. But this problem was far from being solved. In 1838, after several centuries of trying in vain, Bessel, Struve and Henderson had succeeded in measuring the parallax of a fixed star almost simultaneously, inde- pendently of each other, so that astronomers had started to concentrate system- atically on these measurements. Although, mainly due to the introduction of the heliometer, the technique of parallax measurement of a fixed star had improved enormously, the actual harvest was still small, so that in an article in 1899, entitled ‘Suggestions for the determination of stellar parallax by means of photography’, Schlesinger remarked that in the 60’s, since the first successes, no more than 25 to 30 stellar parallaxes have been determined that could be trusted at the level 000:05! In this article Schlesinger outlined his own program. He did not yet have the op- portunity to start it himself. For he got a job as an astronomer at the International Latitude Service, for which he had to build up a station at Ukiah in California. There he directed the observations of latitude variation from 1899 to 1903, when he was appointed astronomer at the Yerkes Observatory, which opened the way to execute his parallax plans with the beautiful 20 meter long telescope there. The problem was to determine the position of the star whose parallax one wanted to measure relative to a few faint stars around it. The difficulty lay in the enormous accuracy required to determine angles of only a few hundredths of a second of arc. Here Schlesinger’s great technical ingenuity and his particular talent for efficiency and organization worked together to create a model of procedures and methods, which with one stroke raised the parallax determination of fixed stars to a com- pletely new level. His working method was soon adopted by all stellar observed involved in parallax measurements, and the scheme designed by Schlesinger was so remarkably well thought out during the years 1903-’05, that I do not know of the slightest detail for which anyone has managed to suggest even a single im- provement in the years since. How the productivity in quantity as well as in qual- ity has increased as a result of Schlesinger’s initiative can be appreciated from the fact that in 1942 of about 5500 stars the parallax had been determined according to Schlesinger’s method, while the mean errors on average did not exceed ±000:01; which compared to the 25 parallaxes with uncertainty of the order of ±000:05 at the beginning of the century! Schlesinger’s own contribution to that harvest was substantial, first at the Yerkes Observatory, later as director of the Allegheny Ob- 3 servatory in Pittsburgh, where he was able to have a telescope designed entirely for this purpose. By accepting an appointment as director of the Yale Observatory, he finally found the opportunity to start in earnest the parallax measurements in the southern sky as well. In the workshop of the Yale Observatory, under his direct supervision, a large telescope was built to be established in Johannesburg. 4 Figure 1: The great telescope of the Yerkes Observatory, with which Schlesinger performed his first parallax determinations. The telescope has an objective lense of 102 cm diameter and a focal distance of over 19 meters. In the meantime, he also gave his attention and manpower to another large undertaking. In the second half of the nineteenth century the collaboration of about twenty observatories (including that in Leiden) led to a catalog with pre- cise positions of about a quarter of a million of the brightest stars, the so-called Astronomische Gesellschaft Katalog. Schlesinger saw that it was possible with the help of photographic observations to be determined these same positions with much greater accuracy and much less effort, and he started around 1920 with ex- periments in that direction. These finally led to the observation of wide zones in the sky; before his retirement, new positions had already been determined for more than a quarter of the whole sky and were published in the Publications of the Yale Observatory. The importance of this enterprise, in which the Cape Ob- servatory now also participates, lies, on the one hand, in the precise determination of the positions and, on the other hand, in the determination of the motions of the stars, which can be deduced by comparison with the original Astronomische Gesellschaft Katalog. For the first time, Schlesinger used large-field telescopes for these positional measurements, with which areas of 5◦× 5◦, later even of 10◦× 11◦, could be photographed. The Astronomische Gesellschaft itself, stirred by this example, organized in the years 1928-’32 with such instruments a re-observation of the northern part of the great catalog. In order to be able to reduce the positions measured on the plates to real po- sitions in the sky, it is necessary to know these positions from a number of stars on each plate. These have to be measured with the meridian circle, the classi- cal instrument for position determination. Here was an opportunity to collaborate with the Leiden Observatory, where Dr. Hins undertook the observing the ‘stan- dard’ stars for one of Schlesinger’s celestial zones with the meridian circle. It went beyond this cooperation between the two observatories. After the founding of the great Yale Telescope in Johannesburg in 1925 the Leiden observer in Johan- nesburg repeatedly observed with this beautiful instrument for his own programs. Conversely, at the Leiden Observatory Dr. van Herk participated in the observa- tions of asteroids in a major plan set up by Dr. Brouwer at the Yale Observatory to improve the foundations of the star positions. The Yale Station in South Africa again gave its strong cooperation to a part of Kapteyn’s ‘Plan of Selected Areas’, an undertaking that now has its center in Groningen and in Prof.