Adriaan Wesselink passed away in New Haven after a long illness and a second stroke, on Thursday 12 January 1995. He was born at Hellevoetsluis, then a Dutch naval repair center, on 7 April 1909, the youngest of four children His father was Jan Hendrik Wesselink, a medical practitioner, and his mother, Adriane Marina Nicolete Stok, a surgical nurse, who, after marriage, was, for many years, president of the local welfare board. In spite of the fact that his own father had been a minister of the Dutch Reformed Church, Adriaan's father, though a fervent believer in personal ethics, had completely rejected formal religion. As his own dispensing pharmacist he had wide interests in chemistry, botany and other sciences, including some astronomy, and would take his son on Sunday botanical excursions in the dunes. He also allowed Adriaan to make some chemical experiments using his pharmaceuticals. Adriaan's first book on astronomy, one of his father's, was Flammarion's Astronomie Populaire.
During the first World War, even though Holland was neutral, Adriaan's father moved his practice and family to the Hague when the local shipyard was closed. Adriaan ultimately entered the University of Utrecht, at the age of 17, to study physics, mathematics and astronomy, the last under A. Nijland. After meeting Hertzsprung and Oort, both from Leiden, at the quarterly astronomical colloquia, Adriaan moved there in the summer of 1929, gaining an assistantship soon afterwards. At this time, Holland, and especially Leiden, was the nursery of many astronomy students and young faculty beginning, or destined for, famous careers. Earlier students had been Bart Bok, Gerard Kuiper, Peter van de Kamp, Jan Schilt and Moll. Hertzsprung was the head of the Astrophysics Department and under him Adriaan received a rigorous training. He was awarded the Doctorandus (teaching qualification) in 1935 and his Doctorate in 1938.
The long total solar eclipse of 19 June 1936 tracked across European Russia, Siberia and Japan. Wesselink proposed a scheme for observing the total brightness of the final crescent as a way of determining the diminution of brightness towards the edge of the solar disk — the so-called limb-darkening — and with two colleagues made the observations at Belorechensk, near Maikop just northeast of the Black Sea. His primary interest, however, was pulsating variable stars, and he was soon invited to pursue it when Kuiper invited him to spend eight months in 1938 at Yerkes Observatory. Hertzsprung, however, wanted Adriaan to go ultimately to Johannesburg, where, on the grounds of the South African Government Observatory, Leiden had installed twin astrographic telescopes for positional and color observations of the southern sky, and had maintained a visiting observer for several years. Adriaan declined because he wanted to translate his doctoral dissertation into English for full publication in the Leiden Annals, including all the observational material. This decision meant that when Nazis occupied Holland in May 1940 he was there for the duration. In an AIP interview he records how, especially in the later stages of the war, in a manner all too familiar to the Dutch, he had to roam the countryside on a bicycle in search of food, to be obtained by purchase or barter, and how he had sometimes to use a prepared hiding place in the Observatory between a ceiling and a floor to avoid arrest or deportation to Germany. Almost all the official faculty of the University had resigned in protest of the Nuremberg Laws, but senior students like Adriaan were encouraged to stay on and look after the place. Adriaan corresponded with Lindblad in Sweden, sending astronomical information about Holland to America in a Newsletter.
Adriaan married Jeanette van Gogh on October 11, 1943, with whom he was to have three children, Josephine Maria, Jan Hendrik and Henriette Wilhelmina Claire. Pooling of ration tickets permitted a modest wedding celebration, and, because the bride worked as a chemical assistant in a government laboratory testing dairy products, she was able to bring home left-over samples and put them to good use. After Oort came out of hiding and took over the direction of the observatory he sent Adriaan to Johannesburg as Leiden Observator in 1946, as Hertzsprung had wished.
Following traditional practice, at Johannesburg Adriaan's duties included taking plates that would be sent to Leiden for measurement and discussion. This left him with little scope for personal initiative, and so he looked elsewhere, finding an opportunity at the Radcliffe Observatory. Founded in Oxford in the 18th Century the enterprise was transferred to Pretoria in the 1930s, but the war delayed the installation of the main 74-inch mirror, then the largest in the southern hemisphere, until 1948. After A. D. Thackeray became Director, Adriaan joined as Chief Assistant and Adjunct Director in 1950 and M. W. Feast became junior assistant in 1953. A co-operative scheme gave the Royal Observatory at the Cape access to the Radcliffe telescope and young astronomers began to be seconded for short periods from Britain. This began a marvelous time, when the triumvirate of Thackeray, Wesselink and Feast made brilliant observational advances. By 1964, however, Adriaan, whose health was not entirely robust, began to find it impossible to carry his share of the heavy observing load at this undermanned telescope. He also, it must be admitted, shared a widespread feeling that while apartheid was deplorable, what might come later would be worse. For parallax and positional work, the Yale University Observatory had maintained a large refactor designed by Frank Schlesinger on the grounds of the University of the Witwatersrand in Johannesburg for decades, but with the mushroom growth of the city this site was abandoned and the telescope was moved to Australia. Meanwhile, Yale and Columbia University set up a second southern site for astrometric work in Argentina, at El Leoncito near San Juan, in the Andean foothills. This entirely new facility consisted of a double astrograph with 50-cm lenses operated in collaboration with the University of Cuyo. It was officially dedicated in 1965, and Adriaan, who had joined the Yale staff the previous year, eventually took a close interest in the survey and proper motion work being done at El Leoncito, especially after the Yale Director Dirk Brouwer died and there was less interest in developing the nearby observing station in Bethany.
Adriaan was promoted to Senior Research Associate and Lecturer in 1966. He continued in this capacity, overseeing the work at EI Leoncito and working on accumulated data from his Pretoria years, until his retirement at the age of seventy. He supervised a number of doctoral candidates and was a highly valued observational astronomer in a department that was moving steadily toward astrophysical theory. His health also deteriorated steadily in the late 1980s and he passed away at his Bethany home at the age of 85.
Adriaall Wesselink, whom I knew over the years, sometimes as colleague, critic, or rival, always as friend, was a big man with a round cheerful face, who loved a good joke and a good party. Though a marvel with a photographic plate, he was not otherwise physically deft, and one imagined that motor cars quailed at his approach. The hardships of the war-time occupation, when regular services had collapsed and the occupying authorities were downright lethal, left him with a tendency, which he never quite outgrew, to solve many problems by the nearest and most efficient method to hand. The dominating theme of his scientific work was the use of the photographic plate as a photometric instrument all the way from his first publication in 1930 to his last, fifty-nine years later. Adriaan's favored way of coping with the fact that the response of photographic emulsions is not linear was the use of finely crafted diffraction gratings of equally spaced, meticulously identical wires placed before the telescope objective. These produced from every point source a set of secondary, or even tertiary, images on either side of the main image, of intensities reduced compared with the primary by factors computed from the grating specifications and the effective wavelength of the incident light. The largest objective grating he built was for the Radcliffe 74-inch telescope, and they were familiar to generations of students at the Bethany observing station Plates thus calibrated could be analyzed in detail with microphotometers to convert densities or total obstruction by star images into measures of incident light. Adriaan used this method for his thesis topic, the investigation of the eclipsing binary SZ Camelopardalis, a most meticulous piece of work which had to take into account limb-darkening of the stellar components and ellipticity due to mutual gravitation.
Adriaan and his colleagues designed a unique photometric system to study solar limb darkening at the 1936 eclipse. They focused two movie cameras with different color responses onto an array of small convex mirrors of different known radii which produced solar images of calculable relative intensity. As the final sliver of the solar surface disappeared its intensity was estimated by picking out the best exposed image on each frame. Solar limb-darkening was calculated, but the intensity variation proved to be mainly due to area change and not to any particular adopted limb-darkening law.
Wesselink applied his photometric expertise widely. In 1948 he deduced from the very low conductivity of lunar material implied by the work of Petit and Nicholson, that the Lunar surface must be dusty, but that the dust was not terribly deep, as some later worried it might be. In 1969 he was delighted at the televised image of Neil Armstrong's first lunar foot-print in 1969 for its shape and depth supported his analysis.
In 1946 he published a photovisual light curve of Delta Cephei based mainly on pre-war observations made at Leiden with two diffraction gratings one behind the other, placed at right angles to each other. He would often claim, with justification, that this was the best light curve ever made. He assembled photometric and radial velocity data from other sources, and assumed that if two observations had the same color-index, the surface brightness of the star must be the same and that any difference in magnitude must be due to a change of surface area of the star. This could be deduced from the radial velocity data, which, though referring to the average surface, could be plausibly adjusted to give the radius change. He derived a radius for the star of 38 times that of the Sun. Repeating the observations and having available better photoelectric data, he revised this to 52 solar radii. In 1969 he turned to the relation between angular diameter, surface brightness and apparent magnitude for a star, and using the then known 18 angular diameters, deduced an empirical relation based on (B,V) photoelectric photometry. Adopting a radius 48 times that of the Sun he deduced an absolute magnitude for Delta Cephei in accordance with the new Cepheid scale. Finally in 1972, with additional angular diameters derived from the work of Hanbury Brown, he supervised the production of a catalogue of calculated angular diameters for all the stars in the Yale Bright Star Catalog.
At Pretoria, Adriaan took part in the original program for which the observatory had been founded, namely the determination of the radial velocities of the southern B-stars as markers of the pattern of galactic rotation, and the massive results are of lasting importance. The most striking result of all was the discovery in the early fifties of RR Lyrae stars in the Magellanic Clouds, marking them out as distinct from Cepheids, instantly expanding the scale of the Universe and clearing away a morass of erroneous observational and theoretical assumptions. Baade had speculated that this might be the case, but here was the definite proof, and the work must rank as among the most significant of the present century. On top of all this, the triumvirate produced several monumental papers recording explorations of the Clouds including masses of data still not completely assimilated. In particular they demonstrated that Cloud Cepheids were normal specimens of the type, and that the Clouds rotated about their centers of mass. At Pretoria Adriaan acquired a working knowledge of photoelectric photometry, and wrote several papers on the detection of RR Lyrae stars in clusters and on absorption in the Clouds themselves. He continued to publish in this area after moving to Yale. It is fitting, however, that in what seems to have been his last publication in 1989 he returned to a comparison of diffraction grating magnitudes and colors with photoelectric data.
Adriaan was a member of four commissions of the International Astronomical Union and an active participant in their work, a member of the American Astronomical Society, a Fellow of the Royal Astronomical Society, President of the Astronomical Society of Southern Africa in 1962, and an Honorary Member of Cuyo University.
For information about Adriaan's early years I am grateful for permission to quote from two taped interviews with him in 1977 and 1978, conducted by D. H. DeVorkin for the Niels Bohr Center for History of Physics of the American Institute of Physics