Pieter C. van der Kruit

Cape Photographic Durchmusterung

Kapteyn’s contacts with David Gill led him to accept a carefully orchestrated suggestion by Gill to measure the plates the latter was taking for a Durchmusterung (catalogue of star positions) of the southern skies, using photographic plates for the first time. This work, involving a new method of parallactic measurement, devised by Kapteyn, took more than twelve years and the final result was the publication of 454,875 star positions and magnitudes in the Cape Photographic Durchmusterung, completed in 1900. Kapteyn became also associated with the international Carte du Ciel project to provide star catalogues and maps of the whole sky. He found himself unable to persuade the leaders of the project to accept his method of parallactic measurement. The Carte du Ciel provided serious problems with regard to the funding of Gill’s work on the CPD, owing to obstruction by Astronomer Royal William Christie.

Studies in Utrecht

Kapteyn studied mathematics and physics at the University of Utrecht, where his most important teachers were C.H.D. Buys Ballot and C.H.C. Grinwis. In 1875 he completed his PhD thesis under the supervision of the latter on the theory of vibrating thin membranes. Astronomy professor M. Hoek died during Kapteyn’s studies, and no theses on astronomical subjects were written in Utrecht at the time. Nevertheless he gained a great deal of experience in astronomy, which is reflected in the large number of propositions on astronomical issues that accompany his thesis. In his last years in Utrecht he met his future wife Catharina Elisabeth (Elise) Kalshoven.

The Milky Way and the Local Group

The beauty and the charm of the Milky May (MW) have been celebrated by countless poets and writers of many Countries along the centuries (see e.g. the beautiful anthology of Piero Boitani 2012 ). The stellar nature of the MW was firstly observed by Galileo. In 1610 in the Sidereus Nuncius ( Galilei 1993 ) Galileo wrote that the MW is “nient’altro che una congerie di innumerevoli Stelle, disseminate a mucchi; che in qualunque regione di essa si diriga il cannocchiale, subito una ingente folla di Stelle si presenta alla vista, delle quali parecchi si vedono abbastanza grandi e molto distinte; ma la moltitudine delle piccole e del tutto inesplorabile”. In the same paragraph, Galileo remarked that observations with his telescope, for the first time, wipe out centuries of philosophical discussions about the nature of the MW. Three more centuries have been necessary to complete a second radical Copernican Revolution that displaces the solar system from being roughly at the center of the MW and project this latter in the vast Universe populated by billions of similar spiral galaxies (see Chap. 1).

The Anatomy of Galaxies

Just after WWII Astronomy started to live its “Golden Age”, not differently to many other sciences and human activities, especially in the west side countries. The improved resolution of telescopes and the appearance of new efficient light detectors (e.g. CCDs in the middle eighty) greatly impacted the extragalactic researches. The first morphological analysis of galaxies were rapidly substituted by “anatomic” studies of their structural components, star and gas content, and in general by detailed investigations of their properties. As for the human anatomy, where the final goal was that of understanding the functionality of the organs that are essential for the life of the body, galaxies were dissected to discover their basic structural components and ultimately the mystery of their existence.

An Astronomical Laboratory

Kapteyn’s ultimate aim was to determine the arrangement of the stars in space, but this could not be done unless parallaxes (distances) of stars were measured on a wholesale basis. He did extensive measurements on special plates, taken according to a scheme he had suggested by Anders Donner at Helsingfors (Helsinki). In 1896 he finally obtained his own laboratory, albeit on a temporary basis, housed in the unused residence of the Queen’s Commissioner. Kapteyn’s star, which was found during the preparations for the Cape Photographic Durchmusterung, had the highest proper motion on the sky at the time. The matter of the credit for its discovery led to some heated correspondence between Kapteyn and Gill, resulting from some melodramatic reporting in the journal ‘the Observatory’. Kapteyn came to the conclusion that stars were too distant for measuring individual parallaxes and that the way forward was through proper motions and statistical methods.

Colors and Motions

Kapteyn had drawn the attention of Simon Newcomb of the US Naval Observatory, who visited him in Groningen and at his second home in Vries. Together with his first PhD student Willem de Sitter (who had worked at the Cape), he commenced the study of the distribution of the colors of stars in different regions of the sky. At that time Kapteyn started to look at patterns of stellar proper motions on the sky, which could be used to estimate distances of stars statistically. In Groningen he was very active in the Physical Sciences Society, which promoted the understanding of scientific progress among a wide audience.

Growing Up in Barneveld

Jacobus C. Kapteyn was born in Barneveld in 1851 as the tenth of fifteen children, in a strictly religious environment. His parents ran a boarding school for boys, where he himself was also educated. He later said that he had been very lonely amidst so many other boys who needed the attention of his parents, and had often felt neglected as a result. At a relatively young age he decided to turn away from the church. He was a very bright student, who developed a strong interest in astronomy, among other subjects.

Professor in Groningen

In the spring of 1878, Kapteyn took up his appointment as a professor at the University of Groningen. It turned out that the Faculty of Mathematics and Natural Sciences had considered a total of 5 candidates, including Hugo von Seeliger. The Faculty put Kapteyn at the top of the list. So did the Curators of the University of Groningen and Kapteyn was duly supported by the Minister. The Royal Decree was published in December 1877. His brother, the mathematician Willem Kapteyn, was appointed in Utrecht on the same day. Kapteyn’s first year in Groningen saw the start of a long series of unsuccessful attempts to secure funds for an observatory in Groningen. On July 16, 1879, he married Catharina Elisabeth (Elise) Kalshoven.

Tides, Statistics and the Art of Discovery

An excursion into the theory of tides resulted in the presentation of a completely incorrect explanation. To help biologists understand statistics, Kapteyn performed a detailed study of skew frequency distributions, which resulted in a bitter disagreement with Karl Pearson, an authority in these matters. Kapteyn also published an extensive study on correlation theory. He was asked to represent the US Academy of Sciences at the tercentennial of Groningen University. Hale and Kapteyn kept up a lively correspondence in 1915 on the importance of inductive vis-a-vis deductive methods of scientific research, Kapteyn promoting the inductive approach and the Hale the deductive one. In this correspondence Kapteyn described his approach to research in great detail. The letters also contained comments on World War I. Kapteyn, being prevented from visiting Mount Wilson, had to be kept from resigning his position as Research Associate in California. Hale told him that he could do the work that came with this appointment anywhere he liked.

From Kepler to Parallax

Although lacking an observatory, Kapteyn was not idle. Some of his work was non-astronomical. He carried out extensive research looking for possible periodicities in the weather on the basis of measurements of growth-rings in trees and together with his brother Willem, a professor of mathematics in Utrecht he made detailed studies of a particular set of mathematical series. But he also performed clever astronomical work. He wrote an extensive article in which he developed a new numerical method for solving Kepler’s equation and another one on the determination of the altitude of the celestial pole free from systematic errors, vindicating the method with observations in Leiden. He also used telescopes in Leiden to measure stellar distances (parallaxes) from accurate timing of meridian passage. These measurements were surprisingly accurate when compared to current determinations. His work on polar altitude resulted in his first contact with David Gill at Cape of Good Hope.