Gijsbert van Herk, Dutch astronomer long associated with Leiden Observatory and a dedicated astrometrist, was born in Breda, in the southern part of The Netherlands, on October 14, 1907. He studied astronomy first at the University of Amsterdam, earning the Doctoraal Diploma (Master's degree) in 1930. His Master's thesis concerned photometry of the Magellanic Clouds. At Amsterdam, van Herk was deeply influenced by A. Pannekoek, and even after he transferred to the University of Leiden in late 1929, where he remained, with two short interruptions, throughout his career, he continued to express his indebtedness to Pannekoek's example.
Van Herk was awarded the PhD degree from Leiden in 1936. His thesis described the results of observations carried out during the first Leiden expedition to Kenya, as part of an ambitious program to determine the fundamental declinations of stars. (The scientific and practical challenges of the two Kenya expeditions, and van Herk's contributions to both, have been described by J. K. Katgert-Merkelijn in Journal for the History of Astronomy, 19, 267, 1991.) The early 20th century had seen major programs directed toward improving the accuracy of stellar positions and motions. It had become clear that the work, largely done with meridian circles, had yielded quite accurate right ascensions, but that the declinations remained relatively uncertain. The uncertainties in the declinations were attributed primarily to flexure of the telescopes and to refraction. For some decades, successive directors of Leiden Observatory had considered mounting a campaign at an observing site to be located as close as possible to the equator of the earth. Exactly on the equator, the azimuth of a star measured as it either rises or sets is equal to its declination. The idea was to carry out observations at the same very low elevation near both rising and setting, thus minimizing relative differences in telescope flexure and in refraction, and so to arrive at fundamental declinations. The instrument was to be especially designed for this program.
The concrete plans for the first Leiden expedition were developed by W. de Sitter. The intention was to determine systematic corrections to the catalogs of fundamental positions then available, specifically to Boss's preliminary General Catalog. The first expedition was led by C. H. Hins, then head of the Meridian Department in Leiden, with van Herk as his assistant. The first expedition lasted from July 1931 until April 1933. The results as described in van Herk's PhD thesis confirmed that the method worked in principle but that observations over a longer duration would be required, and that the instruments could be further refined. A second Kenya expedition was planned by de Sitter, but, after de Sitter's unexpected death from pneumonia in 1934, J.H. Oort had continued the planning and had become van Herk's advisor for his PhD. Oort undertook to find funding for a second expedition, but the Second World War intervened, and the second expedition only could depart for Kenya in 1947.
Meanwhile van Herk carried out meticulous observations on the Leiden meridian circle until work on this instrument terminated in 1957. (The meridian instrument is now on display at the Dutch National Museum for the History of Science, located in Leiden.) During this time he also made major contributions to several internationally coordinated astrometric programs. His approach always stressed that the accuracy of an astrometric instrument could not be judged in isolation, for example by studying the instrument in a laboratory environment, but depended on the sometimes very trying local conditions, with the errors frequently of external origin. His 1961 paper on "Problems in Meridian Astronomy," written together with A.J.J. van Woerkom and published in the Astronomical Journal, 61, 87, stressed that modern astrometric catalogs were characterized by errors not much less than those of older investigations, because the errors were dominated by external causes. In Leiden, the story is still told of the correction which van Herk found necessary to account for the tilt in the entire Observatory building, and thus in the telescope pillar, which followed from the delivery of coal to the coal cellar early each autumn: an abrupt systematic error followed the delivery, with the correction slowly diminishing during the course of the winter as the coal supply dwindled and the Observatory rightened. Van Herk also discovered that an east-west correction was called for: the proximity of the North Sea provided an east-west thermal gradient, amounting in winter (when most of the observations from Leiden were carried out) to a few degrees over some 50 km, causing the "optical meridian plane" to deviate significantly from the gravitational one.
The second Leiden expedition to Kenya commenced in August 1947, with van Herk as head, and continued until October 1951. (Those assisting in the second expedition to Kenya included Adriaan Blaauw and Maarten Schmidt.) The results of the observations were published by van Herk in an exhaustive paper in the 1957 Annals of Leiden Observatory, xviii, 5, and indicated corrections to be applied to the declinations listed in several fundamental catalogs of stellar positions.
When it had become clear that the damp Leiden climate, and changing scientific emphasis at the Observatory, precluded continuing the meridian-circle program and some of the other observational work, van Herk accepted two appointments for short periods, first at the US Naval Observatory in Washington, DC, from February 1958 until November 1960, and later at the Copenhagen Observatory in Brorfelde, Denmark, from February 1961 until September 1962. Following each of these appointments, he returned to Leiden. His classic paper on the structure and dynamics of the globular cluster Messier 3, written together with J. H. Oort and published in 1959 in the Bulletin of the Astronomical Institutes of the Netherlands, XIV, Nr. 491, dates from this period. His important work on the motions of RR Lyrae variables appeared in the BAN in 1965: From 1973 to 1976 van Herk was president of Commission 8, Positional Astronomy, of the International Astronomical Union.
Later in his career, van Herk's instrumental skills were called upon on several occasions. He played an important role in measuring the straight (that is, not following the curvature of the earth) baseline for the deployment of the 25-meter antennae constituting the Westerbork Synthesis Radio Telescope, which commenced operations in 1970. During the 1970s, van Herk played an important role in encouraging the preparations for the Hipparcos astrometric satellite mission. Several astronomers who later became key figures in the most modern astrometric work were introduced to the rigors of this demanding field by van Herk.
In 1983, Leiden Observatory observed the 350th anniversary of its founding. A much valued contribution to this occasion was the book, describing the history and accomplishments of the Observatory, written by van Herk, Herman Kleibrink, and Willem Bijleveld. (The book, De Leidse Sterrewacht: vier eeuwen wacht bij dag en bij nacht, Waanders/de Kler, publishers, is written in Dutch but contains an extensive summary and figure captions in English.) The astronomical instruments at the Old Observatory are now carefully maintained and enthusiastically used by dedicated amateurs. Throughout his career, van Herk gave selflessly of his time and expertise to encourage and instruct the amateur community in The Netherlands. He authored three books intended for serious amateurs.
Gijsbert van Herk died on November 10, 1999. He was a man of deep personal modesty, always gracious in encouraging others; the persistently high demands which he set on observational accuracy set a standard which will remain long revered. Minor Planet 1752 is named "van Herk" in his honor. He is survived by his wife, Dr. Heleen A. van HerkKluyver, herself a Leiden astronomer.
Photo courtesy of the University of Leiden