J.C. KAPTEYN HIS LIFE AND WORKS Henriette Hertzsprung–Kapteyn

Introduced and translated with some annotations by Pieter C. van der Kruit

07.11.14Updated and corrected version, 07.11.14 Cover picture: Henriette M.A.A. Hertzsprung–Kapteyn with her father Jacobus C. Kap- teyn. Part of07.11.14 an illustration from the biography (see Figure 20.1). J.C. KAPTEYN: HIS LIFE AND WORKS

07.11.14 07.11.14 J.C. KAPTEYN: HIS LIFE AND WORKS Biography by Henriette Hertzsprung-Kapteyn

INTRODUCED AND TRANSLATED WITH SOME ANNOTATIONS BY PIETER C. VAN DER KRUIT Jacobus C. Kapteyn Professor of Astronomy, Kapteyn Astronomical Institute, University of Groningen, the Netherlands

07.11.14 07.11.14 vii

The cover07.11.14 of the original book. The size is 18.5 × 22 cm. Quand on n’a pas ce qu’on aime, il faut aimer ce qu’on a.

When you don’t have what you love, you have to love what you have.

07.11.14 Preface

In 1928 H. Hertzsprung–Kapteyn, daughter of Jacobus Cornelius Kapteyn, wrote a biogra- phy of her father, entitled J.C. KAPTEYN: ZIJN LEVEN EN WERKEN, and published it with publisher P. Noordhoff in Groningen. The book is now part of the Digitale Bi- bliotheek voor de Nederlandse Letteren (Digital Library of Dutch Literature), where a complete electronic version of the book is available1. This is the only existing biography of Jacobus Kapteyn. It is known that astronomer Willem de Sitter and historian Johan Huizinga had been planning to write a biography of Kapteyn. Willem de Sitter (1872–1934) was Kapteyn’s first PhD student (1901). He became professor of astronomy in Leiden in 1908 and was director of Leiden Observatory between 1919 until his death. De Sitter has done fundamental work (starting with his thesis) on the large satellites of Jupiter, but is among a wider audience best known for his work on cosmology and promoter of Einstein’s general relativity. He and Einstein proposed in 1932 a model for an expanding cosmos based on de Sitter’s solution of Einstein’s field equation, which is now referred to as the Einstein-de Sitter universe. Johan Huizinga (1872–1945) was appointed professor of history in Groningen in 1905, but took up a similar chair in Leiden in 1919, which he held until his detention by the Nazi’s in 1942. He was the son of Dirk Huizinga, professor of physiology in Groningen and close friend of Kapteyn. In a note, dated August 24, 1925, in the astronomical periodical the Observatory, de Sitter described their intention to write a biography of Kapteyn and appealed to colleagues for any information or help.2 They seemed to have moved most of Kapteyn’s archive of correspondence to Leiden in preparation of this. However, in the end the biography was never written. It is likely that later de Sitter’s son Aernout was planning to write the biography and had been transporting Kapteyn’s correspondence –maybe together with other papers– in a big crate to Indonesia, where he was director of Lembang Observatory. This crate was apparently lost in the bombing of Rotterdam in 1940.3 Various obituaries and short biographical articles have been written about Kapteyn as an astronomer and scientist, most notably by A. Blaauw: Kapteijn, Jacobus Cornelius4. In 1999 a ‘Legacy’ symposium has been devoted to Kapteyn with the title The Legacy of J.C. Kapteyn: Kapteyn and the development of modern astronomy. The proceedings5 have been published by Piet van der Kruit and Klaas van Berkel (editors) and will be referred to below as ‘the Legacy’. More recently a collection of ‘love letters’ that Kapteyn wrote to Elise Kalshoven at the time that she was still his fianc´eeand future wife, while Kapteyn already had moved to Groningen, has been published under the title Lieve Lize: De minnebrieven van de Groningse astronoom J.C. Kapteyn aan Elise Kalshoven, 1878-

1See www.dbnl.org/tekst/hert042jcka01 01/, where versions in .pdf and .txt formats are provided. 2See articles.adsabs.harvard.edu/full/1925Obs....48..293D. 3See the article by Petra van der Heijden in ‘the Legacy’. 4Published in: Dictionary of Scientific Biography, ed. C.C. Gillispie, Charles Scribner’s Sons, New York, 1973. I come back to the07.11.14 spelling of the name ‘Kapteyn’ or ‘Kapteijn’ below. 5Kluwer, xviii + 382 pp., ISBN 0-7923-6393-0, 2000. ix x J.C. KAPTEYN: HIS LIFE AND WORKS

1879 6 by Klaas van Berkel en Annelies Noordhof-Hoorn, referred to below as ‘the Love Letters’.

Henriette Mariette Augustine Albertine Kapteyn (Groningen: November 16, 1881 – Utrecht: October 15, 1956), known to intimates as Hetty, was the second child and second daughter of Jacobus Cornelius (Ko) Kapteyn (Barneveld: January 19, 1851 – Am- sterdam: June 18, 1922) and his wife Catherina Elisabeth (Elise) Kalshoven (Barneveld: June 9, 1855 – Amsterdam: March 2, 1945). They had married on July 17, 1879 at Utrecht after Kapteyn had taken up his professorship in astronomy and theoretical mechanics at the University of Groningen, to which he was appointed in 1878. Henriette had an older sister (Jacoba Cornelia, born in 1880), a younger brother (Gerrit Jacobus, born in 1883), while there had been a stillborn brother in 1895. In historical notes about Kapteyn, when this biography is mentioned, almost invariably the name of the author is written as Henrietta Hertzsprung–Kapteyn. The cover of the book only has her first initial ‘H’. On the other hand, there are on the WWW a few genealogies of the Kapteyn and Kalshoven families, that all spell her name Henriette7. Fortunately, the City of Groningen provides electronic, public access to its archives, in- cluding the records of births for the period since 1811 (up to one hundred years prior to the queries). These archives, named AlleGroningers (www.allegroningers.nl), contain the birth certificate of Henriette Kapteyn (see Figure 0.1)8. Clearly her name is spelled with the an ‘e’ at the end. Note also that no diaeresis or umlaut is used over the second ‘e’. The same is true for the ‘e’ in Mariette. Henriette studied in Groningen and in Amsterdam and obtained the degree ‘Candi- daats’9 in law and an ‘MO’10 in English.11 Henriette Kapteyn married Danish astronomer Ejnar Hertzsprung (1873–1967) on May 16, 1913. The marriage certificate of Ejnar Hertz- sprung and Henriette Kapteyn is also available in the archives of the city of Groningen (see Figure 0.2 and 0.3)12. There her first name is also spelled Henriette. I will adopt that spelling throughout the following. Connected to this is the issue of whether the last name is spelled Kapteyn or Kapteijn. J.C. Kapteyn always used the ‘y’ and his and her name are also spelled in that manner in the biography that Henriette wrote. Sufficient reason to adopt it throughout the following. It is also spelled Kapteyn in the birth certificate of Henriette (see Figure 0.1). However, in the marriage certificate in Figure 0.2 and 0.3 the names of Henriette herself, that of her father and her uncle Frederik Willem Hendrik Kapteyn (1853–1920) (who acted as an official witness) were spelled Kapteijn. Note that all signatures do not have dots over the ‘y’ which would be required to make it an ‘ij’. The site AlleGroningers has more official

6Univ.of Groningen, 168 pp, ISBN 978-90-367-3353-3, 2008. 7See for example aswww.genealogieonline.nl/genealogie-baert-cornelis-kalshoven/I225.php, www.middel .org/canon-van-groningen/kapteyn and www.lamartin.com/genealogy/kalshoven.htm. 8The URL is www.allegroningers.nl/index2.php?task=bladeren&id=17241&gemeente key=®istertype key&jaar=&entiteit=register&aktegegevens=&option=com¯ genealogie&Itemid=59&cp=715&pp=714 9This is the title obtained more or less halfway through academic studies, roughly comparable to what nowadays would be the Bachelors degree. 10MO stands for Middelbaar Onderwijs (secondary education) and the diploma qualified for teaching at gymnasia and high-schools. 11See: Nieuwe Deelgenoten in de Wetenschap: Vrouwelijke studenten en docenten aan de Rijksuniversiteit Groningen 1871-1919 (Groninger Historische Reeks) by Inge de Wilde, 1998) 12www.allegroningers.nl/index2.php?task=bladeren&id=818392&entiteit=akte&option=com07.11.14genealogie& Itemid=54. PREFACE xi

Figure 0.1 Reproduction of Henriette Kapteyn’s birth certificate from the archives of the city of Groningen.

documents of Kapteyn and his relatives, but most can be found by searching for the name ‘Kapteijn’. Hertzsprung was trained as a chemical engineer in Copenhagen and obtained his de- gree in 1898. Subsequently he worked in Saint Petersburg, but in 1901 he went to Leipzig University to study photochemistry. He returned to Denmark in 1902 and worked at Copenhagen University Observatory and at the (private) Urania Observatory. He was ap- pointed in 1909 by Karl Schwarzschild to work at the Astrophysikalisches Observatorium in Potsdam. Karl Schwarzschild (1873–1916) was director of Sternwarte G¨ottingen and moved to Potsdam to lead the Astrophysikalisches Observatorium in Potsdam in 1909. Schwarzschild is best known for his solution of Einstein’s field equations of general rela- tivity of a spherically symmetric case, leading to the Schwarzschild radius, which is the size of the event horizon of a black hole. In 1919 Hertzsprung went to Leiden Observatory and became its director in 1937. He retired in 1946. His greatest contribution is his work on the classification system for stars according to spectral type, stage in their develop- ment and luminosity.07.11.14 This resulted in the Hertzsprung–Russell Diagram, which has been instrumental to understand stellar evolution. xii J.C. KAPTEYN: HIS LIFE AND WORKS

Figure 0.2 Certificate of the marriage between Ejnar Hertzsprung and Henriette Kapteyn on May 16, 1913. This section shows the upper part. The scribblings on the left-hand margin are a record that the marriage was dissolved by divorce in 1937.

At the time of the marriage Hertzsprung worked in Potsdam, where Schwarzschild was director. The latter had recommended Hertzsprung to Kapteyn, who intended to introduce him to Hale and bring him along on one of his visits to Pasadena. Hertzsprung then made an extensive visit to Groningen and Kapteyn in 1911. Klaas van Berkel, notes in ‘the Legacy’ (page 161): ‘In Groningen, Hertzsprung however not only got to know Kapteyn much better, but also his daughter Henrietta [should be Henriette], and when he left for the U.S. in June 1912 the two of them were engaged.’ The marriage was not a success and they divorced formally on January 19, 1937. Klaas van Berkel, in ‘the Legacy’ writes (page 161): ‘The marriage was unhappy and practically dissolved already in 1923. It is unclear however how quickly Kapteyn came to realize just how un-social Hertzsprung was. If we have to believe Luyten, he should have known this before the couple was married. ‘For three or four month [after his departure for the U.S.] she [Henriette Kapteyn] never heard a word from Hertzsprung, and finally Kapteyn wrote to the director of Mount Wilson07.11.14 Observatory about it. Eventually Hertzsprung’s fiancee received a postcard from him on which he wrote: So sorry not to have written before, but the skies here are so PREFACE xiii

Figure 0.3 Certificate of the marriage between Ejnar Hertzsprung and Henriette Kapteyn on May 16, 1913. This section shows the lower part. The signatures below those of Ejnar and Henriette and in the middle are from the witnesses Gerardus Heymans (professor of psychology and philosophy), Carl Vollgraff (professor of Greek), uncle Frederik Kapteyn and brother-in-law Willem Noordenbos.

beautiful, and, you see, where the stars are, there I’m happy. ’ (Luyten, My First 72 Years, p.15). Since Kapteyn had traveled to the U.S. for his yearly stay at Mount Wilson together with Hertzsprung and will have returned in September or early October, this story, if true (and Luyten says he had heard it from Hetty Kapteyn herself), implies that only after returning from the U.S. Kapteyn was informed about Hertzsprung’s lack of attention.’ Henriette and Ejnar had one child, a daughter Rigel Hertzsprung, named after the bright star. After their separation in 1923, Henriette and her daughter Rigel lived for at least some years in Hilversum, close to where Kapteyn’s widow and their mother and grandmother lived. It was during this period that Henriette wrote the biography. Henriette married Joost Hudig (1880–1967), on April 17, 1937 (a few months after the divorce) in London. Hudig was a chemical engineer and specialized in pedology (soil study). He had married Sophia07.11.14 Alida Hudig in 1905 in Rotterdam, where both were born (Sophia in 1880; although they shared the family name Hudig, they were not close relatives). xiv J.C. KAPTEYN: HIS LIFE AND WORKS

Figure 0.4 Tombstone on the graves of Henriette Kapteyn and her second husband Joost Hudig on the Begraafplaats ‘Leeuwerenk’, Oude Diedenweg 64, 6704 AD Wageningen. (From Online Begraafplaatsen)

Shortly after that they moved to Groningen, where two daughters were born (Johanna Clementina13 in 1907 and Johanna Elisabeth in 1909)14. On public record are also the deaths of son Ferrand Whaleij in 1911 (3 days old) and son Joost in 1920 (7 years old). Sophia Alida died in 1924 at the age of 44 years and therefore Hudig had been a widow since then. Joost Hudig worked at the ‘Rijkslandbouwproefstation’ (State Agricultural Laboratory) in Groningen, of which he also became director. In 1929 he was appointed professor of chemistry and the science of fertilization at what is now the Agricultural University of Wageningen, where he worked until his retirement in 1949. Hudig features in Henriette’s book in Chapter 15. According to her descriptions Kapteyn and he were neighbors and Hudig acted as secretary in the ‘Natuurkundig Genootschap’ (the current Royal Natural

13Johanna Clementina Hudig studied law and became the first female judge in the Netherlands. 14The site AlleGroningers07.11.14(www.allegroningers.nl) contains only births up to 100 years before the time of consultation, so there could have been more children after 1910. PREFACE xv

Sciences Society). Henriette and her second husband have been buried in Wageningen. The tombstone is shown in Figure 0.4 15.

Both Henriette’s biography and the ‘Love Letters’ are in Dutch. The biography of Kapteyn has been translated by E. Robert Paul, first as a contribution to the journal Space Science Reviews, vol. 64, pp.1-92, 1993 (with a preface and introduction on pp. x-xix), later as a book The Life and Works of J.C. Kapteyn by Henriette Hertzsprung– Kapteyn: An Annotated Translation with Preface and Introduction by E. Robert Paul 16. It also has a list of publications about Kapteyn and remarks about astronomy in his times. E.R. Paul was definitely an authority on the subject of Kapteyn and astronomy of his days and he published a few authoritative papers and a scholarly book on the subject17. The translation of Henriette Hertzsprung–Kapteyn’s biography has been very seriously criticized. I reproduce the following paragraphs from Appendix B, authored by K. van Berkel and P.C. van der Kruit in ‘the Legacy’, where I left out the examples of poor translations: ‘In this volume and in almost any other study of Kapteyn, reference is made to the biography by his daughter ‘J.C. Kapteyn, Zijn Leven en Werken’, published in 1928. This only biography of Kapteyn has been translated into English (and annotated) by E. Robert Paul. [...] Various Dutch-speaking par- ticipants, present at the Symposium, commented on the sometimes misleading translations. Prof. Adriaan Blaauw informs us as follows: ‘In August 1993 I learned by chance from one of the associates of Kluwer Academic Publishers, Dr. K. (not an astronomer), about Robert Paul’s translation. Publication was in an advanced state, the book was just about to be printed. Dr. K. agreed to slightly delay the printing in order for me to have an opportunity to acquaint myself with the contents. I was impressed by Paul’s sympathetic effort to make Mrs. Hertzsprung– Kapteyn’s work accessible to the English speaking community and his extensive annotations about the personalities – astronomers and university colleagues – oc- curring in her narratives. However, I was startled by the poor quality of the trans- lation. I learned that Robert Paul had done this himself, and it was obvious that he had grossly underestimated the pitfalls of the Dutch language. On August 26, I sent Paul by rapid mail 60 proof pages marked with my corrections. My inspection – done under heavy pressure of other work – had been far from exhaustive. In my accompanying letter to Paul I wrote: ‘[...] The Dutch language, as it is used by some authors, can be of rather complicated structure [...] and thereby sometimes may be misleading to foreigners. Mrs. Hertzsprung’s writing [...] is no exception. There are cases (as you will note from my corrections), where the imperfect translation

15www.online-begraafplaatsen.nl/zerken.asp?command=showgraf&grafid=200874. 16Published by Kluwer, 104 pp., ISBN 978-07-923-2603-8, 1993. 17The Death of a Research Programme – Kapteyn and the Dutch Astronomical Community, J.Hist.Astr. vol. 12, 77-94, 1981; Kapteyn and Statistical Astronomy, I.A.U. Symp. 106, 25-42, 1985; Kapteyn and the Early Twentieth-Century Universe, J.Hist.Astr. vol. 17, 155-182, 1986. These papers and the journal version of his translation of the biography are available free of charge through the NASA Astrophysics Data System ADS (adsabs.harvard.edu).07.11.14 He also wrote the book The Milky Way Galaxy and Statistical Cosmology, 1890-1924, pp. 278, published by Cambridge Union, Press, ISBN 0-5213-5363-7, 1993. xvi J.C. KAPTEYN: HIS LIFE AND WORKS

then leads to just the opposite from what she meant to say. There are also among my corrections some due to the fact that a Dutch word can sometimes have more than one quite different meanings, and the alternative would have been the correct one. [...]’ My suggestions for improvements were acknowledged by Dr. K. on August 31. It was only after publication of the book that, from the complimentary copies I received, I could judge to what extent they had been followed up. It seemed to me that this was the case only for those, that did not require drastic surgery of the page-proofs. So, unfortunately, the book still contains many traces of the imperfect translation. Alas, Robert Paul died on October 12, 1994, of cancer, at the young age of 51, a little more than a year only after the publication of the book. An obituary, written by Steven J. Dick, appeared in the Bull. Am. Astron. Soc. 26, 1606 (1994). Isn’t it likely, that Paul, already suffering from his fatal disease during the last stages of the publication of the book, so shortly before he passed away, did not have the strength anymore to thoroughly revise the text? Wondering about the origin of the present, unsatisfactory situation, it has al- ways been incomprehensible for me that Kluwer accepted the text as it was. One wonders whether Kluwer’s editorial advisory committee for the series Space Sci- ence Reviews had a really serious look at the manuscript. I took the matter up again in February 1996 with Dr. K.’s successor at Kluwer, Dr. de G., and sent him two sets of copies of the pages of the manuscript – corrected and not corrected – in the hope that Kluwer would make an effort to remedy the situation. This has not been successful.’ [...] For this Appendix a new and detailed comparison was made to see in what sense Paul’s translation has misinterpreted what Henriette (or Hetty) Hertzsprung–Kapteyn wrote about her father. Does this comparison confirm Blaauw’s critical opinion concerning the quality of the translation?18 In his Preface Paul presents his work as an ‘annotated translation’. He only had grouped the original chapters under new headings, the original chapter titles now appearing as subheadings. Furthermore, he changed the way in which Hertzsprung–Kapteyn referred to her father. Whereas she speaks of her father in the third person (‘he’ or ‘Kapteyn’), for reasons of readability Paul has her refer to ‘father’ (and to ‘mother’ in the case of Kapteyn’s wife). The style of the book, however, was not altered. The original biography, Paul says, is written in a cultured and colloquial style, as it was intended for a non-scholarly audience. It also suffers from – and was enriched by, Paul admits – ‘the emotional excesses of a loving daughter writing of her very famous father’. But instead of altering the style of the book, Paul claims to have attempted to convey the original spirit and intent of the author and to remain technically close to the scientific and historical material. The reader therefore is under the impression that Paul’s translation in both style and contents remains close to the original text. This is however certainly not the case. First of all, Paul did not just translate the text and add a couple of footnotes. He in fact edited the text, splitting large

18We kindly thank07.11.14 Annemarie Houkes (Department of History), who carefully compared Paul’s translation with the original text. PREFACE xvii

sentences and grouping together others. In most cases, no harm was done to the text, but sometimes serious misunderstanding is the result. [...] While editing the text, Paul also took liberty to delete or insert sentences and information where he deemed this to be necessary. Deletions however sometimes take away the very reason for telling the story. [...] More often, however, Paul inserts additional sentences without specifying that he did so. Sometimes his insertions are completely fabricated. [...] Paul also inserts additional information, not in footnotes as he should have done, but in the text itself. [...] The situation is still worse in cases where Paul, without saying so, corrects factual information given by Hertzsprung–Kapteyn. Of course, she now and then made mistakes. [...] Our first conclusion therefore can be that Paul did not just translate Henriette Kapteyn’s biography of her father. He also edited it at almost every page. Instead of an ‘annotated translation’ what we have here is a translation and a revision of the original. We have seen that the revision was not always done very accurately, but what about the translation? Is it as bad as Adriaan Blaauw suggested? Un- fortunately we have to say: yes. But whereas Prof. Blaauw was so polite to suggest that Paul had merely been a victim of the intricacies of the Dutch language, we are forced to conclude that even his knowledge of elementary aspects of the Dutch language was insufficient. Sometimes he confuses words that are clearly listed as different words in any Dutch dictionary. [...] In most of these cases, a good Dutch–English dictionary would have been sufficient to prevent small, but embarrassing mistakes. On the other hand, it looks as if in other cases Paul relied too much on the dictionary. [...] Finally there is a long list of sometimes misleading, but sometimes just incorrect translations where it is hard to decide what the reason for misrepresenting the original text might have been. [...] There is no question that E. Robert Paul has done scientific inquiry a great service with his perceptive studies of the history of (statistical) astronomy in Kapteyn’s day. However, it is clear from these examples that his translation of Henriette Hertzsprung–Kapteyn’s biography is untrustworthy and useless for scholarly purposes. Those who are able to read Dutch do not need his translation, and those who for one reason or another would like to use it after all, will still have to consult the original to check whether Paul gave the right translation. It would be useless to try and improve this translation, since the work would have to be redone from beginning to end. Perhaps the publication of the translation made by Peter van de Kamp, which as a result of the imminent appearance of Paul’s translation never got to be published, offers a way out. But the best solution would be to have someone write a new and critical biography of Kapteyn, based not only on Henriette Hertzsprung– Kapteyn’s book, but also on the more recent literature concerning the life and works of Kapteyn, including the historical contributions to this volume.”

It seemed thus not worthwhile to attempt a new translation, certainly not in the form of yet another book,07.11.14 so I decided to edit it into an English text and make that available only as a electronic Web version. I do this also in preparation of a possible biography of xviii J.C. KAPTEYN: HIS LIFE AND WORKS

Kapteyn as a scientist, that I am contemplating to write.19 The present text has been produced in spare time between various other duties and obligations. In a book review of Paul’s translation, Derek Jones 20 writes: ‘I am not a scholar of Dutch and cannot comment on the accuracy of the translation, but the resulting English is difficult to read because of the strange usage and vocabulary.’. Paul’s translation ‘attempts to convey both the spirit and intent of the original biography and attempts to preserve the many Dutch idiomatic expressions’. Maybe the latter was not a good choice. In any case, I did try to make the text as easy to read as possible, taking only as hard boundary condition that the text should reflect what Henriette in my opinion was trying to say. This is one reason why I consider myself editor of this volume rather than translator. I did not, like Paul did, rewrite the sentences but tried to stay as close as possible to the text as it was. I regularly checked Paul’s translation and sometimes used it when I felt that it better conveyed the meaning of the author than my command of the English language would have produced. I differ also from Paul in that I do not regroup the chapters and have kept the biography in the third person. And I have included only the original illustrations. I also make anno- tations in footnotes where explanation for the reader not familiar with the Netherlands and/or astronomy is useful or wherever it seems appropriate or helpful to understand the text or the context. However, I have kept some of Paul’s footnotes where appropriate and I have indicated that as such. I do in principle document my sources. I do in any case identify each person that appears in the text with a footnote, but when appropriate I reproduce a footnote by E.R. Paul. A good further source for biographical information in case of the many astronomers that appear in the text, is the Biographical Encyclopedia of Astronomers, edited by Thomas Hockney, Virginia Trimble and Thomas R. Williams21. In the index I list every person that appears in the text; I identify them with their first given names but then further their initials, except for relatives of Kapteyn, where I list the full set of first names. Finally, I reproduce all quotes from written material in a sans serif font. When HHK quotes in Latin, German or French, I provide the English translation in the text and present the original text in a footnote.

Canberra/Groningen/Baltimore, Pieter C. van der Kruit November 2010 – June, 2011 Jacobus C. Kapteyn Professor of Astronomy

Note added November 2014: I have updated and corrected the footnotes and corrected some typographical and other small errors after completion of the Kapteyn biography. This biography is accompanied by a special Website that I am maintaining at www.astro.rug.nl/ JCKapteyn.

19This has been completed in the mean time. [Note added late 2014]] 20Observatory, vol.07.11.14 115, p. 284/5, 1995. [www.adsabs.harvard.edu/1995Obs...115..283H] 21See link.springer.com/referencework/10.1007%2F978-0-387-30400-7#section=336003&page=1 Contents

Preface ix 1 Parents 1 2 Brothers and sisters 7 3 Childhood 9 4 University 14 5 The Kalshoven family 17 6 Leiden 19 7 Groningen 24 8 The Cape Photographic Durchmusterung 28 9 Gill 37 10 The Laboratory 40 11 Newcomb 45 12 Two perfect gentlemen 49 13 Vries 51 14 The professor 55 15 A versatile life 57 16 Star Streams and Selected Areas 65 17 At home 73 18 America 07.11.1475

xix xx J.C. KAPTEYN: HIS LIFE AND WORKS 19 The World War 84 20 Leiden again 90 21 The last years in Groningen 94

Index 103

07.11.14 07.11.14 TO MY MOTHER, 07.11.14WITHOUT WHOM THIS LIFE COULD NEVER HAVE BEEN SO COMPLETE. Parents 1

Chapter 1

PARENTS

The family Kapteyn had always been educators. Throughout the centuries and generation upon generation they acted as teachers – education was in their blood and if they did not teach at a school or some other learning institute, they taught at home. The earliest known teacher in the family was Paulus Captijn, who was born in 1712 in the village of Berkenwoude, which lies in the Krimperner Waard1 in the province of South- Holland. The archives of the ‘Schepenen’2 of the village of Heukelom3 for 1756 mention him as Paulus Captijn, ‘schoolmaster here in this town’. But before him many generations probably were educators. Archives do mention names, but seldom their occupation. It was custom in those days that professions, be it teacher or anything else, was passed on from father to son and elsewhere in the family, and this happened also with the Kapteyns. They became quite well-known as educators; in the Amsterdam newspaper of July 12, 1868 for example, a Kapteyn was put forward in response to a request for suggestions for a teacher in English, with the recommendation ‘one who carries the name of a family, in which pedagogical excellence is a tradition’. Such a large reputation as an educator was obtained by G.J. Kapteyn, who owned a boarding school in Barneveld4. Gerrit Jacobus Kapteyn was born in 1812 in Bodegraven5, where his father was head of the municipal elementary school. As a child he suffered from the ‘English disease’6, which resulted in an unusually large skull. Later he used to tell the story that a peddler that saw

1The Krimpenerwaard is the rural area just south of Gouda and east of Rotterdam. It is bordered on the north by the river Hollandse IJssel, on the south by the river Lek and on the east by the stream Vlist. Krimpenerwaard obtains its name from the town located in its western corner, Krimpen aan den IJssel. ‘Krimp’ used to mean a bend in a river. ‘Waard’ is an interfluvial area between the winter dikes of different river channels, in this case the Lek and and the Hollandse IJssel. 2A ‘schepen’ is a municipal senior magistrate and ruler. 3A small municipality in the province of North-Brabant near Oisterwijk and Berkel-Enschot. 4Barneveld is a town in the province of Gelderland in the center of the Netherlands. It is known for its poultry industry and in particular for its large protestant community. It is in current times part of the so- called Bible Belt, which stretches from the province of Zeeland in the southwest, through the West-Betuwe and Veluwe in the center of the Netherlands, to the northern parts of the province Overijssel, just beyond Zwolle. 5Bodegraven is a municipality in the province of South-Holland, not far form Gouda. Wikipedia notes that it lies roughly equally distant (about 30 km) from the four major cities in the Netherlands: Amsterdam, Rotterdam, Den Haag, and Utrecht. 6Rachitis or rickets. It results07.11.14 form a deficiency of vitamin D or calcium and occurs among young children. It affects the the bones, often by softening them, leading to fractures and deformations. 2 J.C. KAPTEYN: HIS LIFE AND WORKS him as he happened to visit Bodegraven to sell his healing products, offered his mother a ‘medicine’ to help the boy get rid of the disease. This, he tells, worked miraculously, especially given the fact the the disease usually disappears spontaneously. He completely recovered, as is clear from his further life which was an example of physical and mental strength. At a young age he became an assistant in his father’s school and passed for an unusual number of exams to become a fully-qualified teacher. In those days this meant having to obtain diplomas for various levels of expertise and qualification, from fourth level up to first level. Since highschool and secondary education was not organized according to current7 structure of the educational system this was the highest level possible. This first level was awarded only in exceptional cases; only two persons passed8 and Kapteyn was one of them. In the same period he traveled regularly to Leiden to attend courses in the classical languages, Greek and Latin. He had to travel by night using a horse-drawn boat in order to arrive in time, since the lectures started at 9 o’clock in the morning. This boat transported farmers and others from the surrounding villages, who went to Leiden’s market place, where they had to arrive very early in order to build up their stalls and arrange their products and be ready in time for the market to open. As a result, Kapteyn arrived much too early for the lectures and in order to make good use of his valuable time he took organ lessons. He had a good talent for music so that he already played the organ in his local church in Bodegraven, with which he helped the widow of the former organist, as he donated his earnings to her. His organ teacher in Leiden, who felt this was much too early, refused to come out of bed for the lessons, and shouted his instructions and comments from his bed. After having attended the lectures he traveled back to Bodegraven, taught his classes and studied in the evenings. When he was appointed as assistant teacher in the boarding school of Mr. van Wijk at Kampen9, he had to give up attending the lectures and had to study the subject on his own. His daily work was intense, but still he worked every evening for his exam. Sometimes, when he was very tired and had difficulty not falling asleep, he sat with his feet in cold water to stay awake. Although his life was busy and demanding, he still found time to read the complete works of Shakespeare, especially during the hours that he was watching over pupils doing their written examinations, which shows his ability to study efficiently and his discipline to organize his activities. He had indomitable energy and perseverance, and he passed his Candidaats10 exam in the humanities in Leiden with excellent marks. When he was 25 years of age he married Elisabeth Cornelia Koomans, a 23 year old daughter of a farmer from Bodegraven. They settled in Voorschoten11, where he opened a boarding school for boys. This turned out to be too heavy a task for the young couple. The boys, that attended the school were from Den Haag, spoiled, used to luxury and difficult to master. After a few difficult years they gave up. Kapteyn applied for a position at a school in Barneveld, where he would receive a salary of f 600 and the use of a house from

7HHK’s day. 8It is not clear what was meant here, presumably it concerned two from his class or some otherwise defined cohort. 9Kampen is an old town in the province of Overijssel, at the northern end of the Bible Belt. It is situated at the end of the river IJssel, where it flows into the Zuiderzee; Kampen is an old Hanseatic town. 10Comparable to07.11.14 the current Bachelors degree. 11Voorschoten is a village between Den Haag and Leiden. Parents 3

Figure 1.1 ‘Benno’, the boarding school of G.J. Kapteyn, father of J.C. Kapteyn, in Barn- eveld.

the municipality free of charge.12 He was selected for the position and appointed, and they moved to Barneveld with their two little sons and never regretted this change. The school was founded in order to offer boys from Barneveld and surroundings the opportunity to obtain more formal education than was possible at the elementary school. The school did make quite a name for itself in subsequent years and the number of ap- plications to attend became so large that boys had to be lodged elsewhere in the village. After some time Kapteyn became unsatisfied with this situation and he decided to resign from his appointment and build his own boarding school intended for a limited number of students. He bought a large piece of land and had a stately, large house erected on it, that he christened ‘Benno’, which means ‘strong child’ (see Figure 1.1). Students came from all over the country and that was no wonder. After all, he was a natural teacher and educator (a heritage from generations of educators), a man of justice and love for his work and with an energy such that he was always fully committed.

12According to the site of the International Institute for Social History –see www.iisg.nl/hpw/calculate- nl.php– of the Royal Netherlands Academy of Arts and Sciences f 600 (Dutch guilders or florins) in 1840 amounts to about e5700 in terms of purchasing power in 2013. According to the site How much did you say? Converting Dutch Historic07.11.14 Currencies (magazine.dutchancestrycoach.com/converting-dutch-historic- currencies) it corresponds to two annual salaries of an unskilled worker. 4 J.C. KAPTEYN: HIS LIFE AND WORKS

His versatility was incredible. It was not superficial, but of a quality of broad and deep knowledge, that rarely goes together. He spoke and had a thorough knowledge of the three modern languages13 and for students that were preparing for the exam to be admitted to the Gymnasium, he also taught Latin and Greek. Furthermore he lectured in geography and history, economics and accounting. He was known widely as a mathematician. His handwriting was printed work. He never used drafts and made no mistakes. He had taught this to himself by never presenting to others anything that had corrections in it. Whenever he had made a mistake he tore up the piece of paper and had started anew. At times when Barneveld happened to be without a minister, he taught the catechism to his students and his children and did that in a exemplary way. On Sunday evenings he also gave a sort of catechism, that he called ‘lectuur’14. And after that, if it were not too late, he told stories. He was very gifted in that. His stories were so gripping and perceptible that every listener experienced the story and was held spellbound. He told mostly mythological stories, ones involving robbers and ghosts full of the most thrilling adventures. Whoever heard him tell the story of Cartouche15, the ‘eight-sided peasant’16 or the ghost that wanted to be shaved in order to be freed of a curse, never forgot these. The library on the top floor of the house was exceptionally extensive and rich. Every- thing that was fashionable in literature in those days was present: Rousseau, Corneille, Racine, Moli`ere,Goethe, Schiller, Lessing, etc. Conversations took plane in French, which were very valuable for the boys, who learned in this way to express themselves in the foreign language that was used most often. Mr. Kapteyn himself knew the French language very well, but it happened at one occasion that he was asked a particular word which he did not know, so he subsequently memorized the complete French dictionary. Those boys who knew this tried to trap him, but always failed. He knew all words. His aim was to shape his students ‘with the blessing of the Lord into virtuous and able men’. He was aware constantly that he personally had to set an example, always behaved correctly and gave much attention to good manners. He was a stately figure with a stern expression, a straight posture with a skullcap over his gray hair. The typical schoolmaster, in the best sense of the word. His students as well as his own children had great respect for him. He was one of those that possessed a natural authority based on his influence and example. He did not punish; his guidance was followed without questioning by all and that was taken for granted. If something in the boys met his disapproval he made that known by a certain coolness in his morning handshake. He had commissioned the construction of a large garden, but he still found that it was not sufficiently large for the boys, so he bought a piece of forest just outside Barneveld, where he let his workers erect a tent with benches and construct a bowling alley. On

13English, French and German. 14Literally reading material. 15Louis Dominique Cartouche (real name Bourguignon) was a well known French bandit around 1720, who robbed transports on various roads, in particular the one between Versailles and Paris. 16This was Cornelisjansz van Swieten, well-known thief in Amsterdam around 1680, who stole many precious articles07.11.14 from the homes of the rich. He was called ‘achtkanten boer’, as a superlative of square because of his tough and rude manners. Parents 5

Sunday and free half-days17 they had their breakfasts early and went there. The girls came with bread to eat and a woman from the area took care of tea and milk. The boys constructed huts, climbed trees with rope-ladders, played the most enjoyable games and in general delighted in the free atmosphere. His students were well aware that his life was dedicated to the well-being of the school for which he provided all efforts and funds necessary. For the rest of their lives they remembered fondly and with gratitude this man that had given them so much. But Mrs. Kapteyn had no lesser part in the success of the school. She had as a child learned on the farm of her parents what keeping a large household entailed. That was very useful as it was a heavy task that was demanded of her. Eventually she gave birth to 15 children, but she directed the household, that at times contained up to 70 members, with determined and able hand. She was not particularly beautiful and had those long eyelids that my father inherited from her and that gave her complexion a strangely absentminded touch. She possessed a natural calm dignity and had a strong sense of humor. In the house as well as in the school there always was a high level of order and punc- tuality. That was difficult to accomplish by herself and she was helped enormously by two of her daughters, four maids that worked with her for many years, a nanny for the small children and two servants, who took care of the garden and stocked the kitchen in abundance with vegetables and fruits. Every day 13 large loafs of bread were baked. A baker from the village came every evening to prepare them. The house possessed a real baker’s oven. The bread tasted so well that the boys were of the opinion that there was no place where it tasted better and they later wrote about it and sometimes even had loafs sent to them. The bakers themselves liked it as well and used to say ‘It seems so heavy, but it is so light’. Mrs. Kapteyn was able to realize a unbelievable variety in the meals that were provided, and understandably was famous for this. The mothers of the boys, who mentioned the variety and extraordinary taste of the meals, asked her regularly how she did that. Food never tasted to the boys at home as well as it did in school, they complained, and they had recipes sent, but in spite of that it never turned out the same. There were also two seamstresses that were deaf-mute, who had a full job of mending the clothes. The children thought that all seamstresses were deaf-mute. At a time one of them came home and was full of amazement, saying: ‘Mother, they had a seamstress that can talk!’. For mother and daughters there was more than enough needlework to be done for this large family and in the evening they mended clothing while Mr. Kapteyn was reading out stories. The boys18 were in the evening in the classroom with the assistants preparing for the lessons of the next day. This way the sons saw very little of their parents and the family life. There was no distinction between sons and students, as part of some misguided sense of justice. The more sensitive ones, such as my father, suffered from this and from the lack of warmth and togetherness, for which there was no time in this busy family. They worked hard and the days and years passed. In addition to their father and his five assistants, the older sons also started teaching at the school, while they studied for the exams at the university.

17Public holidays. 07.11.14 18The Kapteyn sons are meant here. 6 J.C. KAPTEYN: HIS LIFE AND WORKS

All year round breakfast was at 7 o’clock, during which there was a reading from the Bible, as the parents were very religious. Drawing lessons started at half past 7 and the rest of the lessons at 8 o’clock. The assistants were required to be well prepared, following the example of Mr. Kapteyn himself. The sons, who taught Latin, Greek and mathematics, also had to prepare well, since their father checked up on them regularly and often came listening to the lessons. Part of the boys were prepared for the Gymnasium, and the other part for a more practical life. They emerged well equipped with knowledge, with a sense of responsibility and punctuality. They knew what hard work was and did learn to work hard themselves, which was an invaluable lesson for life In the meantime Mrs. Kapteyn went on continuously in her heavy tasks with constant calm and order. For her a days work was never finished. ‘Strength and honour are her clothing; and she shall rejoice in time to come. She openeth her mouth with wisdom; and in her tongue is the law of kindness. She looketh well to the ways of her household, and eateth not the bread of idleness. Her children arise up, and call her blessed; her husband also, and he praiseth her. Many daughters have done virtuously, but thou excellest them all. Favour is deceitful, and beauty is vain: but a woman that feareth the LORD, she shall be praised.’19 Indeed, are not these old lines, composed four centuries ago by Fray Louis de Granada20, a Spanish mystic, as if for a woman like her? Every day, husband and wife walked in the garden, arm in arm. Many questions and issues concerning the school and the household were discussed. The cherished this hour in which they could exchange thoughts and enjoy the peace and the open air. It was for these two people a beautiful way of working together and sharing their lives, because when they grew older, a young and strong generation was well prepared to take over.

19Proverds, 31, 25-30. Translation taken from the King James Bible online (www.kingjamesbibleonline.org). 20Fray Louis de Granada, a Spanish Dominican monk from the sixteenth century. However, these liens were taken straight07.11.14 form the Old Testament! Maybe this monk was involved in a translation of the Bible from Latin into Spanish or some other modern language. Brothers and sisters 7

Chapter 2

BROTHERS AND SISTERS

Gerrit J. and Elisabeth C. Kapteyn had 15 children; the youngest was born after their silver wedding anniversary. One son died at a very young age as a result of hypothermia, but the others all grew up without problems. They all were exceptionally healthy of body and mind, which was rare in large families. Maybe it was to be expected, as a result of the excellent health of the parents. Both were strong and had a iron will and mentality. They were preparing 40 boys for life and for that one needs a strong hand and a clear head. Both were blessed with these qualities. Their children were blessed with the same qualities. They were honest and just, full of energy and intelligent. Their lives would testify to this. While they grew up, there was no strong band between the brothers and sisters. They regarded each other as students in the same school, following the example of their parents. It is characteristic for this attitude that later they remembered rather little of each other as children. They lived their own lives, and since they all were more rational than emotional, they did not suffer from this. Those that would be more sensitive by nature however, did feel often lonely and misunderstood. For my father this was the case and it would be a shadow from the past that he carried with him all of his life. They were all successful in there careers. The boys were given the opportunity to study at university. The third son, Adriaan1, wanted to become a sailor, which was strongly opposed by his father. He persisted however, not being intimidated by the threat of his father that for him that would mean the end of his chances for a academic study. He came back after one year as a sailor, sobered and disillusioned by the experience, and decided he wanted to become an engineer. The only route open to him was to become a ordinary worker in a plant. He did rise through the ranks of the organization and ended up as chief engineer with the national railroad company. The fifth son, Albert,2 was judged by his father to be unsuitable for university educa- tion. So he became an apprentice with a local blacksmith in Barneveld, and then worked at the Rhijn railway factory in Utrecht as a voluntary worker in exchange for his training. A friend, who had seen him at work, persuaded his father to give him a chance to study in Li`ege(Belgium), which he provided under the condition that he would pass his entrance exam within three months. With all his strength and energy Albert conducted his studies to this end, which was a tremendous effort for one who had no education beyond elemen-

1Adriaan Pieter Marinus07.11.14 Kapteyn (1841–1910). 2Albertus Phillipus Kapteyn (1848–1927). 8 J.C. KAPTEYN: HIS LIFE AND WORKS tary school. But he passed with the highest marks of his group, studied for three years at this technical university and followed this up with an extraordinary career. Five of the boys studied at the university of Utrecht, none of them ever failed to pass an exam. There were no sufficient funds for the two younger boys to study at a university and they went to work at a trade office in Amsterdam when they were 15 and 16. The sisters also passed for many exams and excelled as a result of their intelligence, perseverance and responsibility. They all excelled, whatever they undertook. The doctor, the engineer, the scientist, the teacher, the nurse, the business man, they all were among the best in their trade and were given many responsibilities; the name ‘Kapteyn’ had a very good tone and they all had a share in this. They were proud of each other, and after the death of their father, they made it a habit to have a reunion every year with their mother, putting her in the center of all attention. Even after her death they continued that tradition. So, that was the environment in which my father grew up. He was born in 1851, as the ninth child3 and was called Jacobus Cornelius, after the sister of his father4. That Aunt ‘Ko’, which abbreviation (that he found awful) was also used for him, was a very gifted woman. She had an excellent brain and a big heart. She lived very economically so that she could use her savings –she ran a girls boarding school– to make it possible for her second brother to go and study to become a medical doctor. Her portrait shows an noble face with a high forehead and with an intelligent look in her eyes, a calm dignified posture and strong smooth hands. A stout and energetic woman who certainly has contributed to the well-being of her namesake and godchild.

3HHK is mistaken07.11.14 here. He actually was the tenth child. 4Jacoba Cornelia Kapteyn (1818–??). Childhood 9

Chapter 3

CHILDHOOD

Jacobus Cornelius was a delicate child and the only one that gave his parents reasons for concern for his health, although he never was seriously ill. He was slender built and pale and had a high voice. In bearing and complexion he made a absent-minded and introverted impression. His long upper eyelids reinforced that impression. His fellow students do not remember much of him, except that he was very smart and went through elementary school in an incredibly short period, so that not before long there was little left for him to learn. And they remember that he was a slender, pale young man with a thin and long neck, absent-minded look and poorly kept appearance. He would not think of minding much about his clothes, there were other things that interested him much more. He would lie awake for hours making some calculations or considering problems. He was always taken up by profound thoughts, and could be so absorbed that he forgot everything around him and saw nothing except whatever was occupying his mind. One day, as he was on a walking trip with his schoolmates, he walked completely absorbed by his thoughts into the canal that ran around ‘de Schaffelaer’1, only coming to his senses when he lost solid ground below his feet, and was ridiculed without restraint by the other boys. Some other time, when he was on an excursion with his school to the monument Nellenstein2 not too far from Doorn, he was so concentrated on the details of the monument that he did a few steps backward to have a better view of it, but had forgotten that he was on top of a small hill, and he fell backwards, fortunately without serious consequences. He was a animal lover, recognized birds by their song, and kept birds and rabbits. Obtaining sufficient amounts of food for the animals was a real problem, since the many children could not be provided with large amount of pocket money. He then got the idea to sell ‘krakelingen’3, which gave him only a very small profit but nevertheless that did not deter him. For two cents a week4 he daily picked up the newspaper for the local

1This as a small castle just outside Barneveld. It is named after Jan van Schaffelaer, the cavalry officer who, in 1482, when besieged with his men in the tower of the church of Barneveld, threw himself from the tower to give his men free conduit. The castle dates from the 16th century, so later than van Schaffelaer. 2In the village of Leersum (about 25 km south-east of Utrecht) there is a tower, built in 1818 on a small hill (the ‘Donderberg’), that provides a look-out over the land of Broekhuizen. Underneath this tower the family Nellestein has a family grave. 3Krakelingen are a kind of Dutch pastry cookie, coated with sugar, rolled out into a rope and twisted into the form of a figure eight.07.11.14 4About e1.5 in current currency. 10 J.C. KAPTEYN: HIS LIFE AND WORKS minister, and he did receive small amounts of money from his mother for picking up the eggs in their hen-house. In this way he managed to get the necessary funds. Whatever he undertook, he was always successful, since he never gave up, was not discouraged by disappointments or failures and overcame all difficulties. His breeding of canaries always was a success and everyone trying to grow such birds knows that this takes a lot in terms of care and dedication. Once he had captured a squirrel, that he kept locked in a small room in the attic, where he lovingly cared for it. He brought the animal the best and most desirable food a squirrel may wish for, but then he saw the animal was in obvious stress and proceeded to sit quietly and sadly in a corner. He became very sad himself, but then suddenly he realized that the animal needed water to drink. Others had told him that squirrels, just like rabbits, do not need to drink water. He quickly brought a small bowl of water, which the squirrel drank eagerly. As soon as it finished the water it stretched itself in satisfaction and moved in no time to the top of the curtains. That happy outcome he would always remember. Some other time he had caught an owl. He had heard that these animals are blind during the day, and he wanted to confirm that. So he stretched wires criss cross through the attic room and released the owl. But it evaded all the strings, flew under or over it without touching a single one. Blindness was a just a fable, he concluded. In this manner he learned invaluable lessons, the importance of experimenting for yourself instead of unquestioningly believing in the authority of others. When he was 14 years of age, his sister Bertamie5, four years his senior, returned from England and brought a star map6 that they studied together. The celestial globe was put in the garden and stars were localized. With enormous care and great accuracy he produced a star map, onto which he indicated stars of the first magnitude with star figures f cut from gold colored paper, second magnitude stars from silver colored paper and on which he painted the positions of third magnitude star with white paint.7 It was a beautiful piece of work, that fortunately was preserved8 and it shows us, how at the age of 14 he already possessed the same love and accuracy of work as later in his astronomical research. When his father noted how seriously he studied the stars, he bought a large telescope for him, that was erected in the attic room and with which he very diligently observed the stars. It could already then be seen what this boy with his keen mind would eventually become. It was difficult to argue with him. There was a little shop in Barneveld –everyone in those days knew the shop of Jan and Maatje den Oudsten– where people came together to discuss politics and local affairs. He was able to win arguments effortlessly from older people, that certainly knew what they were talking about. This superiority was noted by others, although he was hardly aware of it himself. It remained like that the rest of his life. His older brother Hubert, who was a university student and only came home during holidays, used to play chess with his father in the evening. Ko, who was about 10 years

5Albertine Maria Kapteyn (1847–1927). 6HHK’s text mentions a map, but the next sentence seems to suggest it actually was a globe. 7Magnitudes are an astronomical way of assigning a measure to stars of their brightness. First magnitude stars are the brightest, second magnitude stars a factor of just over 2.5 fainter, etc. The faintest stars that the naked eye can see on a dark night are of magnitude 6, a factor 100 fainter than stars of magnitude 1. 8These days it resides07.11.14 in the Kapteyn Room in the Kapteyn Astronomical Institute at the University of Groningen. Childhood 11 old at the time, watched this and Hubert said: ‘Do you also want to play a game with me, little boy?’. The father Kapteyn smiled knowingly, as he knew what this little boy’s capabilities were, and the game started. In no time the student lost. That irritated him and he proposed they play a rematch. Hubert lost again and he became angry and he said: ‘If I lose another time, I will never play you again.’ He lost a third time, and he refused to ever play again against this younger, talented brother. Some other time the boy went to his father with a calculation that he was unable to perform. Since his father was busy, a friend of his oldest brother said: ‘Come here and I will help you.’ But when he saw the problem he realized it was well beyond his abilities to solve it. The brothers found it difficult to accept that they could not stand up to their younger brother, both in discussions as well as in games, and they started to ignore him. Also in school he had no real friends. Nobody really cared about him and he became quiet and shy, remained on his own most of the time and became more and more introverted and withdrawn. Still he did not enjoy being by himself, he was in reality a sociable person, not just rational and longing for warmth and love in the first place. His parents too had little time for him. Only when he was ill would his mother be loving and caring. She would sit at his bed and call him ‘my little servant’ and he would bask in this beneficence. But as soon as he got better that was over and the motherly love disappeared again behind the daily demanding tasks. This feeling of loneliness in a large family was a deep sorrow that forever had left its scars. His family probably never noticed this, since they were apparently different in this. But his life would bring him compensation, richer and fuller than in many other lives. This lack of love and the unfulfilled longing for it during his youth, is the key to understanding many aspects of his later life. It explains his joy at any display of love towards him, and his quick response to anyone who approached him with warmth. He was not very critical and analytical in his dealings with other people, he accepted and responded and gave much from the warmth of his heart. In America he later found more people much like him. He found them less critical and complicated, less prone to taking on a reserved attitude unlike the Dutch manner of first wait and see whether others can be trusted, but they gave friendship and cordiality much like children that readily put an arm around each other. It is wonderful to trust and admire others like children do. And would one be deceived that often? He did not experience too much of that. As a young man he preferred female company, probably since male contemporaries could not always stand up to him and left him alone. He was painfully aware of his shyness and his slovenly appearance. He really never cared about his clothing and appearance, but in company of young females this bothered him and he saw with silent admiration how easily his brothers and the boys from the school behaved in those circumstances. One day there was a gymnastics event in the main hall of the school, where many boys and girls had been invited. The boys performed all kinds of courageous stunts and all girls cheered; the boys talked easily and gracefully to the young beauties and made an impression of enviable grace and ease on the timid boy. He was standing in a corner and felt ignored and left out. Then he got an idea; he wanted to show he was a man as well and took a heavy pole upon his shoulders that really was much too heavy for him. With incredible effort he carried it around the hall. Only later did he understand why this act of bravery did not bring him the success he had anticipated, but at the time it was a disappointment that put him back in07.11.14 his lonely state of feeling unappreciated. 12 J.C. KAPTEYN: HIS LIFE AND WORKS

He was a brave and resolute child. Once upon a time when he had hurt himself in the leg with a small ax, he did not tell his mother, but went to the pharmacy to obtain an adhesive plaster. Of course she found the hole in his pants and the blood stains on his clothes and confronted him with that. This attitude of not wanting to complain at times of physical suffering remained typical for him throughout his life. When he was 13 years of age he had been playing in the church tower and had tied the rope of the bell down. He woke up in the middle of the night by the sound of rain beating on his window, and he remembered with fear that he had tied the rope, which he imagined might now be shrinking. He had all sorts of fearsome visions, such as that the bell might be jerked loose and would suffer enormous damage through his fault. He got up in the middle of the night, went through the sleeping village to untie the rope. He never had the courage to tell his parents about this. He also thought very seriously in those days about what reforms would be required to make society a better place. He did not know exactly how, but in his childish inexperience he decided to campaign against top hats, which he found to be silly and unworthily. When at some moment he told his very practical mother about his world-reforming plans, she said: ‘And you think you can reform the world?’. He replied in all earnest: ‘Yes mother, if only with my limited possibilities I would be able to move one straw out of the way, I would be satisfied.’ His mother looked at him surprised and questioningly and maybe for a fleeting moment was impressed by his determination and the emerging abilities of her son, who she realized she really hardly knew. He actually wore a top hat later himself and he did not reform society, but he did give his will-power to those he knew in order to help bringing about his conciliatory and idealistic ideas. He moved his straw out of the way and the world became a better place through his life and works. This quiet, but pious boy had a childish belief in the presence of God as a savior in times of trouble in life, but when he grew older he lost this belief, as he saw that it would make him rigid as it had made his parents dogmatic. Religious words remained empty words, that provided no warmth and inspiration. He became unsatisfied with the religion in which he was brought up. Every young generation battles the older ones. In those days it was particularly the battle against orthodox articles of faith like compulsory attendance of church services, reading the Bible, praying, this traditional expressions of a belief that has no real value for a young searching soul. The phrases held in high esteem throughout the centuries only because of their untouchable respectable tradition, had to loose their glamor. A strong and deep spirit is not in need of dogma, no prescription for the soul; it creates its own religion, its own ideals according to which it wants to live and this uniqueness is the divinity of man. And as he thought and did his battle, to find the only way to reach the most important thing: the supreme freedom of the soul to define its own rules and laws and to choose the manner in which to fulfill these. And with his own freedom he developed a respect for the freedom of others, a right that had to be unassailable and unquestioned. In that way all prejudices according to rank or class had to be abolished. This was not a conviction that had come forth out of progressing realization; it was entirely natural and instinctive, since his spirit went beyond all pettiness. Once, when he could not be found in the house and had been looked for all over the village, he was found in the tabernacle of the poor Jewish boy07.11.14 Sammie, sincerely taking part in the ceremonies of the Sukkot, the Jewish Childhood 13

Feast of Tabernacles. For him man was sovereign and his worth exceeded qualifications as rank, class or religion. Already at a young age these thoughts rooted themselves in his thinking. He considered everything to be natural; the strange distortions introduced by civilized society did not affect him. He remained himself throughout his life. His success and the important positions he was to hold later, invoked no change in him. He greeted his old maid as politely as he greeted the highest dignitaries, his clerks remembered with adoration, how he often raised his hat for them. The true democratic spirit lived in his soul, and this humble attitude stayed with him all his life. When he was 16 years of age, he passed the entrance exam of the University of Utrecht, but his father judged him too young to become a university student. So, he stayed in Barneveld for another year. Since he had finished school he now had much spare time, so he often just attended lessons as he felt like, went on walks with other schoolboys, worked a little for fun, but so well and focused that by the time he did start at the university he had already mastered much of the material for the first year. He was very talented in all subjects, especially mathematics, for which he developed his own methods. There was a very difficult, mathematical book by Sturm: Cours d’Analyse de l’Ecole´ Polytechnique9, that all his brothers had studied. He only examined the synopsis and not the book itself, finding the solutions to the problems in his own manner. When he had to do his examination Prof. Grinwis, professor of mathematics, disapproved of his solutions, since the methods deviated completely from the unusual ones, but in the end he had to admit that the results were correct. In the year he spent at home he read extensively in his father’s comprehensive library. If no-one knew where Ko was, one could be sure that he was there, absorbed by some book. He cited the rest of his life from the works he had read on his sixteenth, parts he would never forget, such as from Goethe, Rousseau, Lessing, etc. He had a special love for Lessing’s Nathan der Weise10, which still brought tears to his eyes on his deathbed, Der K¨onig von Sion by Hamerling11, whose stately hexameters made it so easy to recite, how familiar to us was ‘der m¨achtige Knipperdolling!’12, and somewhat later Tartarin de Tarascon13, the book with the most delicious boasting ever produced. We re-read this shortly before his death and he laughed about it as he had when he was young. Up to the end of his life he remained young of mind, with the joy and resilience of a boy.

9This book on analysis was written by Charles-Fran¸coisSturm (1803 – 1959), who was professor of analysis and mechanics at the Ecole´ Polytechnique and later at the Facult´edes Sciences, both in Paris. 10Nathan the Wise is a play by German writer Gotthold Ephraim Lessing (1729–1781), published in 1779. It advocates religious tolerance and its performance has for some time been forbidden by the church. 11Robert Hamerling (1830–1889) was an Austrian poet. The set of poems referred to is a narrative of the Anabaptist movement of 1534. 12Bernhard Knipperdolling (1495–1536) was the German leader of the Munster¨ Anabaptists. These reject- ed Christian practices; for example they refused have their children baptized. Anabaptists were heavily persecuted during the 16th07.11.14 and 17th century by Roman Catholics, but also by Protestants. 13Tartarin of Tarascon is an 1872 novel written by the French author Alphonse Daudet (1840–1897). 14 J.C. KAPTEYN: HIS LIFE AND WORKS

Chapter 4

UNIVERSITY

His parents wanted him to study theology. None of the other sons had wanted to do that and they hoped that this quiet, pious son would fulfill their deepest wish and become a preacher. They had no idea of the boy’s inner thoughts, nothing of the changes that had taken place in him. They had no idea that he had already long ago chosen to follow his own beliefs and would not be able to follow the path that they had wished him to choose. He knew that he, like his brothers, was more suited for a study in exact sciences. However, he did not have the courage to tell his very stern father, and together with one of the boys from the school, who had the same problem, he agreed that they would present their wishes to their respective fathers at the same moment. Taking support from the fact that someone else had the same difficult tasks, he found the courage. It turned out not difficult at all, since the old man Kapteyn was a wise man, who, although being thoroughly disappointed in his greatest wish, realized very well, that one should not force his children to do a thing they did not want to do. And he agreed, that also this son would study mathematics and physics. As a result he was enrolled at the University of Utrecht in 1868, where already two of his brothers were students. The brothers did not see too much of each other and lived their lives as independently as they were used to at home. Like all previous Kapteyns he was given the nickname ‘Dux’1, which was a name they were very much attached to. He studied with Professors Buys Ballot, the renowned meteorologist2, and the mathematician Grinwis3. He had little difficulty with his studies, since, as was already mentioned, he had done a lot of work at home in the previous year. So he took studying easy and enjoyed life. Since he was the only student enrolled in the Faculty of Philosophy4, he joined a social club (society) of students in law, where he encountered much fun and a free life, which brought him much happiness after his life of duty and obedience. This sudden, big change did little harm; it was common in those days that students did not take everything too seriously and Kapteyn went along with that. As soon as he would notice that the leisurely life was getting too much of a grip on him, he found the strength to quit. A great joy in those days was his friendship to Andree Wiltens. This young man was immediately charmed by the character and greatness of the boy Kapteyn and did show

1Latin for leader and origin of the later word Duke. 2Christophorus Henricus Diedericus Buys Ballot (1817–1890). 3Cornelis Hubertus Carolus Grinwis (1831–1899). 4The number of07.11.14 students in those years was very small, but Kapteyn was enrolled in the Faculty of Mathematics and Physics. It is unlikely that Kapteyn was the only first year student in that faculty. University 15 that warmly. The heart of the lonely boy was warmed with joy and he basked in the ensuing love and friendship, that was new to him but which he had longed for all his life. Wiltens was a son of a civil servant from the East Indies, whose three sons all studied in Utrecht and were members of prominent clubs. He was a noble and good person and he had a rare characteristic, namely that he could admire and love with all the warmth of his heart. And this he had taken up a big admiration and love for this talented friend, which for both of them remained important for the rest of their lives. Wiltens did have a very special place in Kapteyn’s life as the one who for the first time showed him what was most important in life, namely love. Kapteyn did not study too hard, as there was so much socializing and joy to have with friends, as student life in the Netherlands was offering then, his inherent joyful, social character enjoyed all this happily. His sense of humor and sociability, the happy side of his character, now started to come forward. He could be very sharp, which did hurt others where his criticisms were wrapped in strict logic. He did make enemies this way, which bothered him much, since he could not stand animosity and preferred to be on good terms with everybody. Later he lost this sharp side to his character in the harmonious environment, that he lived in, and many did not understand he had ever been so sharp. His first exam was only average5. In those days exams were public and the audience had come in large numbers, so that the room was almost full. His father had come as well and was sitting close to him. He did not preform brilliantly and people started to become restless. The examiner kept going on about the same subject, although it was clear there was not much to gain from that. Kapteyn’s brains were irritated by that lack of logic and he said loud and clear: ‘Professor, I don’t know anything about this!’. It was absolutely quiet for a short moment and he saw his father turn pale. The professor, however, changed the subject to one he knew a great deal about and in this manner he saved the day and obtained his degree. After the exam the father and his sons went to his room. The atmosphere was happy and lively and the largest surprise happened to the sons when they saw their very formal father taking an easy chair, propping his feet on the window sill and tipping the ashes from his cigar through the window. Was that really their formal, always correct father? ‘Yes, boys, the upbringing has been completed and I no longer have to set an example’, he said, laughing when he saw their surprise. In the evening the father went along with the friend to the student pub to celebrate the event. He was the center of attention of those present who said unanimously: ‘The old Dux will have to tell a story.’ Everyone knew his talent, and the old Dux started telling stories so that everyone lost all sense of time. A major difficulty for the young Kapteyn was to have to recite. He had a rather high, somewhat shrill voice, his intonation was not perfect and his reciting restless. Once he had in a student reciting society recited Der Taucher by Schiller, when someone in the audience had remarked: ‘I thought it impossible that anybody can recite so poorly.’ He did not take this strong criticism as an insult, but as a inspiration to practice. A few years later, when as young professor in Groningen, he had to deliver his inaugural lecture, he did all he could to be well prepared. He recited his lecture up to 30 times in front of two of his sisters, who patiently sat in the two outermost corners of the room as his audience and made comments. And like always with what he resolved to do, he overcame this difficulty, so that07.11.14 whenever he had to speak in public, he spoke with a calm tone and 5The exam referred to must have been the ‘Candidaats’ exam, nowadays Bachelors. 16 J.C. KAPTEYN: HIS LIFE AND WORKS in clear sentences, so that everyone could understand him and follow what he said with pleasure and without difficulty.

07.11.14 The Kalshoven family 17

Chapter 5

THE KALSHOVEN FAMILY

In his last year at Utrecht he made acquaintance with the Kalshoven family, which turned out to be a completely different family than the Kapteyns. They could be described as antipodes. No heavy responsibilities were laid on the shoulders of the two young daughters, they did not study or have a job, the atmosphere was cheerful and cozy, life was enjoyed in a simple and relaxed manner. Gaiety, music and courtship, all those enchanting things in a young person filled the atmosphere. Mrs. Kalshoven1 was a woman of fine, civilized manners and culture. As a young girl she had married the wood merchant Kalshoven2, who was a widower with five children. She gave birth to another three, two girls and one boy. It had been a heavy task for the lively young girl, who came from a quiet, untroubled life in Harderwijk3, where her father was conrector4 of the local Gymnasium, and now had moved to a large old-fashioned house on the Singel5 in Amsterdam. Business was not flourishing and the family decided to move to Abcoude6, where life was expected to be easier and less expensive. Mrs. Kalshoven was a strong woman, who accepted the circumstances and adjusted to them, and they enjoyed happy years after the difficult times in Amsterdam. Her stepchildren grew up and left home, and after the death of her husband Mrs. Kalshoven moved with her own children to Utrecht, where she had to live simple and economically. As she became older she had more and more difficulty walking and became restricted to her chair. She underwent these tribulations with resignation, but there were also days when she found it difficult to accept her handicap and hated her need for help and assistance. She would then complain and moan, but when she had visitors she became gay and happy so that the two healthy daughters would look at each other and conclude it was not so serious after all. As healthy youths they could not imagine what it meant to be an invalid. The mother was surrounded by a warm love, in particular by her oldest daughter Elise, who was the quietest and most serious of the two. She did her domestic duties without complaining, and the greatest joy was to sit behind the piano and to play whatever came

1Henri¨etteMari¨etteAugustine Albertine Kalshoven–Frieseman (1822–1895). 2Jacobus Wilhelmus Kalshoven (1813–1869). 3Harderwijk is a small city a little to the east of Barneveld, situated on the Zuiderzee. Between 1648 and 1811 it hosted a university. 4A conrector is the adjunct of the rector of head of a Gymnasium. 5The Singel in Amsterdam is a canal that originally served as a protective moat that ran around the city in the Middle Ages, situated07.11.14 inside the set of canals that were dug in later centuries as Amsterdam expanded. 6Abcoude is a small municipality to the south-east and just outside Amsterdam in the province of Utrecht. 18 J.C. KAPTEYN: HIS LIFE AND WORKS to her mind. She had a great talent for music and she developed that by herself, since there was no money for music lessons in this simple family. Marie7, the younger sister, had a beautiful singing voice and the two girls could spend hours playing and singing. Marie would think of a song and started singing, Elisa followed on the piano, which went without much effort, since she was a very gifted musician. She however failed to develop these talents by having lessons and much practicing. And maybe that was not really necessary, they were not persons of serious study and work. They rather lived as the birds in a forest, singing and cheering when they wanted to, when the sun is shining and the sky is blue; and music brought them happiness throughout their lives. The mother had an impulsive character too and lived one moment at a time. She could not resist a beautiful summer’s day and would then call out: ‘Children, the sun is shining, it so so nice outside. We will have a carriage prepared and we will go on a tour.’ The thrift and worries were forgotten for a moment and they went outside cheerfully. Kapteyn enjoyed visiting and he became a regular guest. He found there the carefree way of living that he had never seen at home. There it had only been duty and work, here they did not immediately switch on the lamps to hover over books, but they sat in the twilight as long as possible. Voices of young girls sang and made fun in front of the open window and all sorts of jokes were invented. It was a lot of merriment, but Kapteyn did not fail to notice the more serious side of the character of the older girl. The small dark girl with the thick braids and the striking brown eyes possessed a dignified confidence, a caring soul and an absolute lack of self-awareness, so that she did not realize how beautiful she was. Since the girls had never gone to school and the little they knew had been taught by a governess, they still had their originality, which gave them a certain charm that was noticed easily. One evening, when Kapteyn rang the doorbell of the Kalshoven house, the door was opened by Elise, who radiated happiness and joy. ‘What happened?’, he said, moved by her happiness. ‘Oh, I am so happy, my brother8 has returned from the Indies.’ Then he realized he wanted no other than this women as his wife, who personified happiness and did not require, like many others, to be made happy first. He had thought much about life, and made clear for himself what it is that constitutes value and what not. His unique, genuine common sense already made him at an early age differentiate and understand, where for others life still was a chaos of impulses and temptations. And he kept this picture of this young radiant girl in heart, abiding his time.

7Maria Gabrielle07.11.14 Kalshoven (1857–1940). 8Jacobus Wilhelmus Kalshoven (1859–1941). Leiden 19

Chapter 6

LEIDEN

So then his wonderful years as a university student drew to an end. He had worked hard, his doctoral exam1 was passed cum laude and on the 24th of January 18752 he defended his PhD thesis A Study of Vibrating Flat Membranes and became doctor of Mathematics and Natural Philosophy.3 Strangely enough it only now dawned upon him that he would have to decide what profession he wanted to choose. His father had been looking around for him and knew where he could get a job as a teacher. But he would have nothing of that. Teacher at a school, no; he felt this neither to be his calling nor his talent. His father was very angry about this refusal; after all the Kapteyns were all born to be teachers and the time had come for him to realize that he now had to find his way in life by himself. But the young man persisted, he wanted to work in a scientific environment and he thought he would be able to find a way to accomplish that. He heard that a new astronomical observatory was going to be founded in China, he made contact with the people in charge of this and they offered him to go to Peking as director. He accepted enthusiastically, there was correspondence and exchange of ideas, but then it all came to naught, as the observatory never was built. He then heard that a post as observer would become available at Leiden Observatory. He applied and was offered the position. In this way he returned to his passion as a youth, and full of determination he accepted. He remained at the Observatory in Leiden for two years and all this time he worked with the youthful strength, fortitude and determination that was so typical of him. ‘I never worked harder in my whole life’, he said later. He never took a rest, he observed at night whenever the weather allowed, came to work in the morning earlier than all the others and lived so intensely in this grand science that in those two years all sorts of plans grew in his head that never released their grip on him and on which he worked the rest of his life.

1This is the equivalent of the current Masters exam. 2HHK obviously made a mistake here; the defense took place on June 24, 1875. 3The Dutch title was Onderzoek der trillende platte vliezen. The PhD was awarded magna cum laude, see Album Promotorum Utrecht (dap.library.uu.nl/, also (books.google.com.au/books?id= YMoUAAAAIAAJ&dq=kapteyn+grinwis&source=gbs navlinks s), p. 130. The thesis supervisor was Prof. C.H.C. Grinwis. From the same source it can be learned that Grinwis was also the supervisor of Kapteyn’s older brothers Nicolaas Pieter (1845–1916) en Willem (1849–1927) on theses entitled respectively Over de rekening met symbolen en de toepassing daarvan op de integratie van differentiaal-vergelijkingen (On the analysis with symbols and the application thereof on the integration of differential equations) and Over de theorie der trillende platen en haar verband met experimenten (On the theory of vibrating membranes and their relation to experiments).07.11.14 They defended their theses on the same day June 14, 1872; Nicolaas Pieter was awarded his degree also magna cum laude and Willem cum laude. 20 J.C. KAPTEYN: HIS LIFE AND WORKS

Classical astronomy of those days was concerned only with the bodies in the Solar Sys- tem. The fixed stars only served as points of reference for observations of planets and as ‘dials of the great clockwork for the sailors to read the time, and of which the Moon is the hand”. Near the end of the eighteenth century, William Herschel4 had started to investi- gate the Sidereal System in its own right. He looked upon the Heavens as a whole, like an organism, invented methods to investigate it, and did long series of observations to eluci- date its structure and how it was organized. After Herschel, during the nineteenth century a large number of astronomers had been occupied by this problem, without however car- rying the subject any further. Kapteyn was much attracted to explore new avenues, enter new paths that had not been walked before. To get closer to the heart of this mystery, what an undertaking! It was not to please himself, it was to serve others; in him glowed the true love, the submission to science, the master, that asked dedication, patience and determination. He had been a patient worker, with a iron will to succeed and a radiant optimism that would overcome all difficulties. ‘The child is father of the man’; indeed the child had become a man, and the sacrifice and perseverance of his youth had prepared him with the strength that life had in store for him. In those days he learned and perfected his command of the theory and operation of as- tronomical instruments. His observations were of exceptional quality, in particular through his great care to avoid systematic errors. It appeared that he was not purely an obser- vational astronomer, but also not a pure theorist. The eminent astronomer Eddington5 would later say: ‘There is a region between purely observational and purely astronomy in which Kapteyn was unrivaled’. And that was because of his great accuracy and wonderful intuition. In Leiden he laid the foundations for his scientific career. But the social aspects were not neglected. He made some very good friends, who remained friends throughout his life. He ate at a table with other young men, that all were hard workers at the beginning of their careers, as enthusiastic as him, a true round table of courage and power. They were a.o. W. Burck6, the later well-known botanist, Hubrecht7, who would become professor of zoology in Utrecht and Hoek8, the later leader of North Sea marine research in Den Helder, all men of weight and importance. One day, in late 1877 –Kapteyn was 26 years of age then– when they were sitting at dinner again, Hubrecht jumped up and to everyone’s surprise ordered a bottle of cham- pagne. When the glasses were filled he said: ‘I propose a toast on the health of the brother

4The Englishman William Herschel (1738–1822) was a Hanoverian emigre and itinerant musician turned amateur astronomer. Discovering the planet Uranus in 1781, Herschel became the most celebrated astro- nomer of the early nineteenth century and is known today particularly for his work on stellar motions and for his project ‘Construction of the Heavens’. (Footnote by E.R. Paul) 5The English astronomer and brilliant mathematician A.S. Eddington (1882–1944), who was appointed Plumian Professor in Cambridge in 1913, made major contributions in statistical astronomy and stellar structure, and general relativity and cosmology, becoming one of the most influential theoretical astronomer of the twentieth century. (Footnote by E.R. Paul) 6William Burck (1848–1910). 7Hubrecht, Ambrosius Arnold Willem (1853–1915) was a professor of zoology and anatomy in Utrecht. He is well-known for his studies of embryology. 8Dr. Paulus Peronius Cato Hoek (1845-1914), well known as a fisheries expert, a carcinologist, and for his work as a secretary of the International Council for the Exploration of the Sea; he became director of both the Fishery Research Institute and the Zoological Station at Den Helder. Later this became the NIOZ, a national marine07.11.14 sciences research institute on the island of Texel (the southernmost of the West-Frisian islands at the north of the Netherlands), just to the north of Den Helder. Leiden 21 of one of us, who has been appointed professor of mathematics in Utrecht, Willem Kap- teyn’.9 That was a complete surprise for Ko, who had not yet heard of this. He rejoiced in this for his older brother, since a professorship was in their eyes the most desirable position that one could be appointed to. When the excitement settled down, Hubrecht once more ordered a bottle of champagne and when once more the glasses were filled, he said: ‘And I also want to propose a toast, but now for someone who is among us and who also has been appointed professor. To Ko Kapteyn, who has been appointed professor of astronomy in Groningen.’ That was an even bigger surprise, not in the least for Kapteyn. Indeed, Hubrecht, who was the son of the Secretary General of the Ministry of Internal Affairs, sometimes knew things before others did. Both brothers were appointed by two subsequent Royal Decrees, 14 December 1877, Nrs. 35 and 36 of the ‘Staatscourant’10. It turned into a double celebration. After dinner they all went through Leiden, singing and cheering. An old lady saw these very noisy young men and looked upon this with much misgivings. Hoek went up to her and said: ‘Would you believe that one of these is a professor?’. Laughing, he pulled the young Kapteyn, who certainly had not been the most quiet one, to the front. ‘That is difficult to believe!’ said the woman, clasping her hands together. Is that what a professor looks like? This did not agree with what she had imagined. She kept staring at them as they went on, while their voices died out in the dark evening. That same evening the older Kapteyn was lecturing for the Barneveld chapter of a reforming society called het Nut11. In the middle of his lecture a telegram arrived, saying that his son Willem had been appointed professor in Utrecht. He mentioned this full of joy to his audience and, after he had been congratulated, proceeded with his lecture. But not long after that a second telegram arrived, telling that him about the appointment of his son Ko. Great excitement ensued, while the father gleamed with joy and pride. The chairman took the floor and said: ‘Mr. Kapteyn, I want to propose that you end your lecture and tell us about your life.’ Mr. Kapteyn did just that and, the excellent story teller that he was, he started an improvised account of his life and that of his family. The young Kapteyn was only 26 years, an age where most others still had to complete or at best had recently completed their university studies. But his performance had created great expectations and no one was surprised by this extraordinary appointment. On the 20th of February of the year 1878 he presented his inaugural lecture at the Uni- versity of Groningen. However, things did not look promising for a young and enthusiastic professor in astronomy. As the professorship was newly created, he had no observatory waiting for him and not much prospects of getting one soon. And a professorship in as- tronomy without an observatory was unthinkable. But what was not here now, could come to be and with all the energy and unshakable optimism he started the task of obtaining an observatory. Many letters to the curators12 stem from this period, plans were presented and proposals made, in the course of time becoming less elaborate and for fewer funds. But it never went beyond these curators, since he did not receive any support for requests

9Willem Kapteyn (1849–1927). 10The Staatscourant is the official newspaper published by the government of the Netherlands, where new laws and other official announcements are being published. 11This is an abbreviation of ‘Maatschappij tot Nut van ’t Algemeen’ (Society for the Common Good). 12The highest official body07.11.14 running a university and deciding on all matters, including where to allocate the budget. 22 J.C. KAPTEYN: HIS LIFE AND WORKS

Figure 6.1 Jacobus Kapteyn and Elise Kalshoven in pictures taken before their marriage.

to the government. Both other directors of observatories, Prof. van de Sande Bakhuyzen13, director of Leiden Observatory and Prof. Oudemans14 of Utrecht gave negative advice. They did not like the idea of a third observatory in this small country. The response of the disappointed young astronomer was bitter: ‘On the basis of your advice a reasonable instrument in Groningen will be refused for quite some time. In all of the Netherlands the astronomer in Groningen be will almost the only one who will not have a decent work place for scientific research.’ Nothing helped, support and funds were not found and after his last unsuccessful attempt in 1890 to obtain with government funding a photographic refractor in a dome, he gave up; his drive and genius had found other means in the form of the Astronomical Laboratory. But many battles had been fought and much disappointment was endured before at came that far. In the summer of 1878 he proposed Elise Kalshoven to marriage and she accepted. Both knew it felt good, and with full confidence they looked upon the future. Their engagement lasted for a year, during which Kapteyn traveled regularly to Utrecht. They were also serious in spite of all the joy, they discussed the great dreams that Kapteyn had in mind and where Elise listened to with full attention. A new world of intellectual matters and

13Hendrikus Gerardus van de Sande Bakhuyzen (1838–1923) was professor of astronomy and director of the Observatory of the University of Leiden (1872–1908). (Footnote by E.R. Paul). 14Jean Abraham07.11.14 Cr´etienOudemans (1827–1906) was professor of astronomy at Utrecht (1875–1898). (Footnote after E.R. Paul). Leiden 23 science opened up for her, a world that would take her to a higher level than that which she had lived in during her youth. The enormous revolution that Darwin’s15 Origin of Species had brought about had left a strong mark on their times. It was the era of rationalism and a young generation had chosen against worn-out forms and dogmas. They felt like the gods in the kingdom of the intellect and old conventional traditions were thrown aside as dead branches on the perpetual green tree of life. In the Netherlands the voice of Multatuli16 thundered and swept along all that was young and ablaze and not yet asleep under the safe care of old familiar tradition. Kapteyn revered Multatuli and agreed enthusiastically with ‘Freedom, everybody’s greatest right!.’ He introduced his future wife to this world of new ideas. The first book he suggested her to read was Drapers history of the conflict between religion and science17. These problems were too difficult for her, but still they did have a large influence on her thinking and filled her with admiration. She felt lifted in an atmosphere, that appealed to her earnest and pure soul as the most beautiful way to live, searching for truth and freedom from narrow traditional ways that mankind had suffered for so long. Together they bought the complete Household Edition of Dickens’ work, which became a source of happiness throughout their lives18. The bust of Dickens later decorated their living room and his spirit lived in the family. Late in the summer of 1879 they married. And he had chosen well, because with this happy young girl he brought happiness into his home. And she found a happy place at the side of a great, good man.

15Charles Darwin (1809–1882), who published his On the Origin of Species by Means of Natural Selection in 1859, was naturalist who independently co-discovered along with Alfred Russell the modern theory of evolution. (Footnote by E.R. Paul) 16Eduard Douwes Dekker (1820–1887; pseud. Multatuli) was a novelist and essayist and the most brilliant Dutch writer of the nineteenth century. His masterpiece is Max Havelaar of de Koffij-veilingen der Neder- landse Handelsmaatschappij (1860). (Footnote by E.R. Paul) It is a strong protest against the poor living conditions at the coffee and tea plantations in the East Indies and against oppression and exploitation of the indigenous populations in colonies in general. 17The two most influential books in the Anglo-American world of the nineteenth century were written by John W. Draper (1811-1882), American immigrant son of an itinerant Methodist minister, who published History of the Conflict between Religion and Science (1875), and by the American Andrew D. White (1832– 1918), president and founder of Cornell University, who published History of the Warfare of Science and Theology (1896). (Footnote by E.R.Paul) 18The complete Works of07.11.14 Charles Dickens (1812–1870) is fully twenty volumes in length.(Footnote by E.R. Paul) 24 J.C. KAPTEYN: HIS LIFE AND WORKS

Chapter 7

GRONINGEN

Groningen was at that time a rather isolated city as a result of the poor transportation possibilities, and in Holland1 it was considered with some pity a provincial town, not really part of Dutch civilization. The earnest, somewhat stiff Groningers, who were always behind in fashion and strongly pronounced the ‘n’ at the end of words, were not regarded as first class citizens. The young Mrs. Kapteyn did not feel at home there at all. It seemed as if she had emigrated to a different country. The Groningers spoke and dressed differently as in Utrecht, they were stiff and rigid and put on airs. The last thing one could say about the Kapteyn’s was that they put on airs. The first thing they did was to rent against all unwritten rules a house on the Winschoterkade in a neighborhood that was occupied by sailors and common folks. It had never occurred to any professor to choose such an ordinary neighborhood. But they had little money and a lot of common sense and preferred this large house in humble quarters over a small one in a higher-class part of town, and lived very happy there. The group of professors consisted of mostly older, sedate people. They were astonishing- ly young to be admitted to such a respectable circle. Young Mrs. Kapteyn would stamp her feet from irritation and from impatience with the stiff and drab attitude of these people. Why should professors and their wives have to be so pompous and formal, she thought. And instead of being impressed they felt doubly happy in their youth and cheerfulness, glad that they were still able to be merry and live their lives following their own opinion and wishes, not bothered by what were the conceived demands of rank and class. They were young and cheerful, sometimes exuberant like children who had just become their own lord and master. It was all sunshine and joy and they made themselves in short time good friends, that were attracted by the natural, simple but still dignified young couple. Elise Kalshoven had been a quiet, calm girl, but Elise Kapteyn developed into a spon- taneous, lively woman. Once, after having stayed with them for some time, Marie, the younger sister saw her and was very surprised. When she came home she told her mother: ‘You would never recognize Lise, she is a completely different person.’ However, that was not true. Her originality and lively spirit had been over shadowed at home by the others, but in her own environment these qualities were allowed to freely come to the fore. It rep-

1‘Holland’ is the western part of the Netherlands, encompassing the provinces of North- and South-Holland (and sometimes07.11.14 Zeeland) and the major cities of Amsterdam, Rotterdam and Den Haag (The Hague). In this context HHK might have included the province of Utrecht, where Mrs. Kapteyn grew up. Groningen 25 resented a new style that the Kapteyns were displaying and many in Groningen noticed with a smile, that may have had a bit of envy in it, this young, happy couple. After a year their first child was born, a little daughter2. They had together acquainted themselves with the principles of child care. That was exceptional in those days and they were ahead of their peers with their ways of thinking, so that often they would correct a know-all midwife, who was used to be the uncontested ruler according to antique custom. This was no minor thing, since in those days midwifes ruled with iron hand and did not leave any room for other opinions. Kapteyn also took up the doctor. He would only take the advice from an established authority if his common sense agreed. When the child, only a few months old, developed a serious intestinal infection and the doctor prescribed a mix of proteins, he had the courage and the determination to disregard this prescription. His common sense told him that the weak stomach of a almost dying child would not be able to cope with proteins. He used a diet of sugar-water and that at an absolute minimum. The father and mother watched with fearful tenseness. What responsibility had they taken upon themselves! But the child recovered and with that their trust in their own common sense. They weighed their child regularly, which was not common then and they read books on feeding and child raising. Alleb´e3 was extensively studied and for the more advanced education also Rousseau4, and dreams were dreamed as would any other young pair of parents do at the cradle of their first child. Mrs. Kapteyn was the first among her friends to push the baby-car herself rather then leaving that to a governess as the silly etiquette then dictated. And when they went out together, the young professor pushed the cart despite the scorn of the street-boys who were less tame then they are now, which meant a lot. But the criticisms of the street youth affected them just as little as the admiration of the colleagues. To the contrary, Mrs. Kapteyn found a welcome challenge in going against the tide of the conventions as she knew that her independence from the judgment of the rest of the world would make her happier. When the child was a year old the young father decided that it was not advisable to let the child grow up without company. He was full of pedagogical principles that were crying out to be applied and he therefore was of the opinion that the child would not have a joyful youth without a playmate, so they decided to have another child. A neighbor had gone bankrupt and had disappeared to America. The poor women was left behind in misery and she complained about her misfortune. Kapteyn immediately proposed to take the child as his own to free her from her worries. Mrs. Kapteyn was less idealistic and more practical, and opposed the idea of taking someone else’s child in their family. She was busy enough taking care of her fine, healthy daughter and they had to live frugally already. But the serious conviction of the pedagogical necessity prevailed. The child was admitted to the house and was a major burden. It was unclean and poorly raised and surprised by everything that she saw in her new home. Fortunately, it remained only for three months until the mother came to take it back and move also to America. It

2Jacoba Cornelia Kapteyn on May 31, 1880. 3The book De ontwikkeling van het kind naar ligchaam en geest; eene handleiding voor moeders bij de eerste opvoeding (The development of the child in body and soul: companion for mothers raising their first child) from 1845 by Amsterdam medical doctor Gerardus Arnoldus Nicolaus Alleb´e(1810–1892) was the standard text for young parents.07.11.14 4Jean-Jacques Rousseau (1712–1778), philosopher, writer and musician. 26 J.C. KAPTEYN: HIS LIFE AND WORKS turned out it was part of a plan conceived beforehand by the parents in order to arrange some temporary care-taking. The Kapteyns were enormously relieved that this ordeal was not afflicted upon them for the rest of their lives. Another theory of Kapteyn was that a child would not have a natural affection for dolls, but that this was inflicted by the parents. He did not want to present his child with a doll. Poor Dody5 had to go through life without a doll. Once he noticed that the child had an ugly Japanese doll, one that could be moved up and down on a stick, that she was slowly rocking in her arms much like a mother would do with her child. He did not say anything, but when he came back in the afternoon from town he had a small wrapped-up package as a present for Dody, in which the delighted child found a beautiful doll. His theory was wrong and he was able to let it go immediately when life itself in its beauty showed him otherwise. Later in his grand life of scientific endeavors this proved to be an invaluable attribute. Since often a much loved theory. developed with much effort would have to be abandoned and its failure be openly announced to be wrong.6 How many times must a servant of science not make himself subordinate, which is not known to others who only see the outcome and not the tortuous roads that have to be traveled to arrive at them. In the house on the Winschoterkade, where they lived for six years, two more children were born, a girl and a boy.7 Kapteyn was an educator by nature, just like his forefathers had been. He learned them things in a playful manner, during meals, during walks; he told then about everything that was interesting and had the spark of life. He was interested in everything that concerned them and never send them off with an easy answer. He treated every child as a human being that was growing up, took them very seriously, respected them and gave them the necessary attention. The law of causality was the path he followed in his educational effort. He seldom inflicted punishments; the child should learn to understand the consequences of its actions and accept them. Life itself punished, not he, and this way he prepared them for the life that was ahead of them. His earnestness and dedication, his patience and love, his common sense and practical outlook made his children trust him like a solid rock. He embodied a never failing or disappointing trust for them, but he was also a cheerful companion and a loving friend. Our mother supported him in every sense and smoothed the more sharp and cutting edges of his character with her warmth and spontaneity and unselfish love. She was always busy and caring, to make ends meet, which sometimes was not easy in the early years. She was merry, but also illogical and inexact in a funny way, and could well endure a joke or a little teasing at her expense. Her joyful originality enlightened his intellectual and critical mind, although they did sometime have difficulty to grasp what the other meant. Their backgrounds, characters and upbringings were too different for that. But they grew into a better mutual understanding, since both of them had a lot to offer and they found out how to learn from each other to the benefit of both of them. Kapteyn suffered under the impossibility to bring his scientific plans to reality and was always looking for things to do with his hands or his head. He collaborated in those days

5Dody was the nickname of Jacoba Cornelia. 6HHK uses here the French expression ‘amende honorable’, which means a public announcement of being wrong or incorrect. 7HHK herself on07.11.14 November 16, 1881, and Gerrit Jacobus Kapteyn on December 14, 1883. It has not been mentioned in this biography that the Kapteyns much later had a male stillborn child on August 29, 1895. Groningen 27 with his brother Willem, the mathematician from Utrecht. His holidays were spent at the Observatory in Leiden, where he had requested and obtained the hospitality of Bakhuyzen to use the Universal Instrument8) for studies that appeared as his first astronomical pub- lication. ‘Uber¨ eine Methode, die Polh¨ohe m¨ogstlich frei von systematischen Fehlen zu bestimmen’.9 This method was developed in order to improve fundamental observations.10 Later he used the meridian circle for studies of stellar parallaxes.11 The results of this work were published in his paper ‘Bestimmung van Parallaxen durch Registrir-Beobachtungen am Meridian Kreise’ in 1891.12 He was also busy with meteorologic studies and he traveled to Worms and to Paris in order to do research on the growth of trees in relation to the climatic circumstances. He sent a request to the government to ask the French government for slices of two hundred year and older trees from the surroundings of Paris. That was where the weather station was located that had the longest historical records of meteorological data in particular of amounts of rain. He occupied himself for some time with research into growth rings of trees, but he never came to formulate a theory. Only much later did he publish the results. But all this did not satisfy him. It was only child’s play and he wanted to do something much greater and he knew he was able to do that. Then, suddenly the solution came and it gave a entirely new direction to his life.

8This instrument was used for the determination of time. From 1859 onward the clocks in the Netherlands were set according to determinations of the local civil time at Leiden Observatory (and communicated using the telegraphic network), in spite of the fact that many countries were changing to the 24 hourly zones on the planet. It took until 1909 for all of the Netherlands to use a uniform (Amsterdam) time. 9‘On a method to determine the height of the Pole as free as possible form systematic errors’, published in Copernicus, 3, 147–182 (1884). However there was an earlier publication ‘the Kepler problem‘, concerning solving Kepler’s equation in Copernicus, 3, 25–34 (1883). In addition there had been short publications in the Astronomische Nachrichten with the brothers van de Sande Bakhuyzen from Kapteyn’s Leiden days, such as ‘Ephemeris of comet b, 1877 (Winnecke’s)’ by van de Sande Bakhuyzen and Kapteyn, in Astronomische Nachrichten, 1, 64. 10This means observations of accurate, absolute positions of stars on the sky. For this a so-called funda- mental system is required as a reference. 11A meridian circle telescope is one that always looks in the direction of the –north-south– meridian and can only move up and down. It is used to derive positions of stars from their time of the crossing of the meridian and the angle of the telescope w.r.t. the horizon. A parallax is the annual change in the position of a star as a result of the motion of the earth around the sun. Here it is used as an equivalent to stellar distance. 12Originally published in the Annalen Leiden 7(3), 117-244 (1891), later discussed in English by Prof. Lewis Boss, director of Dudley07.11.14 Observatory, New York in Publications of the Astronomical Society 3, 346–353 (1891). Available at adsabs.harvard.edu/abs/1891PASP....3..346K. 28 J.C. KAPTEYN: HIS LIFE AND WORKS

Figure 7.1 J.C. Kapteyn at age 35. 07.11.14 The Cape Photographic Durchmusterung 29

Chapter 8

THE CAPE PHOTOGRAPHIC DURCHMUSTERUNG

During the Christmas holidays of 1885 he read in Leiden in an astronomical journal an article by David Gill, Her Majesty’s astronomer at Cape of Good Hope,1 about a major project he wanted to undertake. He was planning to produce a catalog of the stars in the southern hemisphere down to magnitude2 10 and using photographic methods. That represented a gigantic effort that had not been attempted before. For the northern hemisphere there existed the Bonner Durchmusterung, that was completed around 1855 by the astronomer Argelander3 It was a catalog of more than 300.000 stars up to magnitude 9 to 10, measured visually at the telescope over many years of work. The Bonner catalog, although with only poor positional accuracy, was of utmost importance to astronomers. It was about the only source for researches into the structure of the heavens, for the distribution and luminosities of stars, and the basis for many new studies and observations; the indispensable catalog for the astronomer interested in observing faint objects. This catalog had contributed more than any other work to the knowledge of variable stars and star with peculiar proper motions4; it had uncovered all important errors in earlier catalogs of stars, etc, etc. But what was more important than anything else was that this work constituted an accurate and complete representation of the northern hemisphere. around the year 1855, that could be compared with he real sky at all times to look for any changes that had taken place. However, for the southern sky almost nothing had been done. Instantly, the thought occurred to Kapteyn: ‘here is my chance!’. He immediately wrote to Gill and offered to collaborate with him. ‘If you will confide to me one or two of the

1David Gill (1843–1914 was Her Majesty’s astronomer at the Cape of Good Hope from 1879 to 1906. (Footnote by E.R. Paul) 2Stellar brightness is expressed on a logarithmic scale in which a difference of 5 magnitudes corresponds to a factor 100 in brightness. A star of magnitude 10 is therefore about a factor 10,000 fainter than the brightest stars in the sky. 3Friedrich Wilhelm August Argelander (1799–1875) was director of the Sternwarte Bonn. He measured (at least twice) between 1852 and 1859 the position and brightness of 324,198 stars between the North Pole (declination +90◦.0) and just below the Equator (declination -2◦.0) with his assistants Eduard Sch¨onfeld (1828-1891) and Adalbert Kruger¨ (1832-1896). Sch¨onfeld was Argelander’s successor and extended between 1875 and 1881 the Bonner Durchmusterung with 133.659 more southern stars visible from Bonn in the so-called Sudliche¨ Durchmusterung.07.11.14. 4The proper motion of a star is is annual change in position on the sky. 30 J.C. KAPTEYN: HIS LIFE AND WORKS negatives, I will try my hand at them and, if the result proves as I expect, I would gladly devote some years of my life to this work, which would disburden you a little, as I hope, and by which I would gain the honor of associating my name with one of the grandest undertakings of our time.’ Gill accepted: ‘It is not easy to tell you what I feel at receiving such a proposal. I recognize in it the true brotherhood of science and in you a true brother.’ And this way these two men found each other, who would work like brothers side by side. Two titans that were to assault the skies, but with nobler intentions than the old Titans of mythology.5 Prof. Bakhuyzen and his brother6 in Leiden were not very enthusiastic. Of course, the results expected were of tremendous value, but the slave work necessary would be almost unbearable. But Kapteyn wrote to Gill: ‘... However, I think my enthusiasm for the matter will be equal to (say) six or seven years of such work.’ Indeed, his part of the effort would take much enthusiasm and energy, since it concerned the measurement of negatives and excecution of calculations, and cataloging the stars on the photographic plates, which would certainly take more than twenty times the amount of time it had taken to expose the photographic plates. But here was an opportunity for him without an observatory or expensive equipment to do a useful and for the development of science essential piece of work, with which he hoped to earn a reputation and in this way honor the university. In the mean time it was impossible to accomplish an effort of this magnitude without support of an assistant, not even with the uncertain help of astronomy students. With more hope than before he contacted the government to request a modest subsidy of f 500, spread out over a period of seven years7, from which he expected to hire the necessary assistant to perform most of the written and computational work. This subsidy was awarded for six years. Still he needed more, since he did not have a ‘pied `aterre’ and no measuring machine. Then his friend Huizinga8 offered him two workrooms in his laboratory, which was gratefully accepted. About ten years later, when he gave a speech at the opening of his own laboratory, Kapteyn turned to Huizinga and expressed what he had felt: ‘When I came to you, I experienced the desperate feeling of being a useless person. Now that I leave you, I am free of that feeling, thanks to you. And who would ever rest his head unless having had the experience not to have been useless?’ The instrument remained a problem, because there was no funding for it. He exam- ined old instruments that he possessed, old useless things, and he succeeded in designing an instrument from these various components, following a new concept. A mechanic in Groningen put it together under his guidance. It worked excellently, in fact so well that later the committee of the Carte du Ciel9 in Paris commissioned him to have such a ‘parallactic instrument’ also built for their researches.

5This account is incorrect. Kapteyn had been corresponding with Gill on his paper to measure accurate polar heights and actually Gill had been luring Kapteyn in offering his services. 6Ernst Frederick van de Sande Bakhuyzen (1848–1918) was observator at Leiden Observatory and would later become the successor to his older brother as director in 1908. 7Currently a little over e20,000. 8Dirk Huizinga (1840–1903) was professor of physiology and was the father of famous historian Johan Huizinga. 9The Carte du Ciel, a collaboration of an international group of observatories was et up to produce an atlas of the entire07.11.14 sky down to fourteenth magnitude and a catalog of precisely measured star positions down to the twelfth magnitude. The Cape Photographic Durchmusterung 31

Figure 8.1 The parallactic instrument that Kapteyn used for the measurement of photo- graphic plates to derive the properties of stars for the Durchmusterung.

But he still needed to find an assistant. Since he could not afford to pay a high salary, he decided to look for a ‘unskilled’ worker. But where could such a person be found? Intelligent and able men working for little money were rare. But he had an idea; he went to the director of the ‘Ambachtschool’10 and asked him if he had a good student that he could recommend for the work. The director knew one and mentioned the name of a 19 year old boy that he felt had the necessary qualities. The young de Vries11 came as a temporary apprentice and was directed to do the measurements with the machine, which he did excellently. He turned out to be very talented for performing very accurate measurements and was an superb observer; he also performed the calculations with such an accuracy and diligence that he soon became indispensable. Still Kapteyn was aware that de Vries with his capabilities deserved a better job with a higher salary than he could ever expect to offer him. So, in 1901 he recommended him to his colleague Nijland12, when a better paid job came available at his observatory. After having praised de Vries extensively, he wrote: ‘Heaven knows that I will see this man leave with much sorrow, but

10The Ambachtschool (ambacht is handicraft) was a secondary school for young men that prepared them for technical professions. 11This is Teunis Willem de Vries (1862–1937). 12Albertus Antonie Nijland07.11.14 (1868–1936) was professor of astronomy in Utrecht and director of Utrecht Observatory. 32 J.C. KAPTEYN: HIS LIFE AND WORKS

Figure 8.2 Mr. T.W. de Vries, amanuensis (technician and research assistant) at the As- tronomical Laboratory.

I don’t feel at ease to be in the way of his career. Such an opportunity will probably not be coming again soon.’ Fortunately07.11.14 for Kapteyn the position was filled by someone else and in 1911 Kapteyn was, after having sent a splendid testimonial to the government, able to offer him the The Cape Photographic Durchmusterung 33 position of ‘amanuensis’13 in his laboratory. De Vries remained his senior associate and dedicated assistant for 36 years until Kapteyn’s retirement. He worked another six years under Prof. van Rhijn14, after which he enjoyed his well-deserved retirement. The Labo- ratory owes very much to de Vries. Both of them started the work in the small rooms in the physiological laboratory with the self-designed measuring apparatus. They strove for the highest accuracy possible; each star was measured at least twice, the second time always by Kapteyn himself. And the work progressed beyond expectation. Gill wrote in 1886: ‘You have made an excellent beginning to the work, the results far exceeding in accuracy what I thought you could attain with your provisional apparatus.’ Indeed, the accuracy was remarkably better than had been obtained by the Bonner Durchmusterung. The small subsidy from the government, however, was not sufficient. He wrote in des- peration to Gill, who then wanted to try and secure the funds in England. But although he was grateful for the offer, Kapteyn felt it was difficult to accept it for the honor of the university and his country. He decided to find the money in another way. He contacted the various societies in the country, that provided financial support for scientific research and in this way he obtained f 10015 annually from the ‘Bataafs Genootschap’16 and the Utrecht Genootschap17 f 15018 for three years. And he tried to have the government change his subsidy of f 500 for six years into f 100019 during three years. ‘In the fourth year I will be sadly without means’, he wrote to Bakhuyzen, ‘but by then maybe something can be found.’ And his optimism, his wonderful quality in life, did not betray him, because after those years a lot had changed, he had done magnificent work and his name started to be known all over the world, so that he could obtain whatever he wished for. But in 1887 it look bleak for the two friends, as friends they had quickly become. Gill, who always was hitting the nail on the head, said ‘Surely if in this wicked world there ever were two men who should be good friends together, you and I should be so’. The correspondence between them was a source of joy for both of them, each letter being received with pleasure. And when in 1887 Gill was planning to visit Groningen the complete family Kapteyn lived in great expectation. Everybody prepared themselves, Mrs. Kapteyn did speak excellent English and in a hurry taught the daughters, who were by now 7 and 5 years old, some rudimentary English that could use as a greeting. Gill came, and everyone was instantly fond of him. He had the talent to quickly win all hearts through his open honest and warm enthusiasm. The first evening, Gill and the Kapteyns were sitting in front of the fireplace and Gill told his life’s story, which left a strong impression because of his modesty and dignity. The two young people learned from them a beautiful lesson

13An amanuensis is a skilled assistant in a laboratory or educational institution. In this case it was the highest position that could be created for a skilled worker as de Vries. 14Pieter Johannes van Rhijn (1886–1960) was a student of Kapteyn. He succeeded Kapteyn as professor of astronomy in Groningen and as director of the Astronomical Laboratory. 15Equivalent to a current roughly e2500. 16The Bataafs Genootschap voor Proefondervindelijke Wijsbegeerte (Batavian Society for Experimental Philosophy) was founded in 1769 when Rotterdam watch and instrument maker Steven Hoogendijk left his fortune to found a society dedicated to experimental science. 17The Provinciaal Utrechtsch Genootschap van Kunsten en Wetenschappen (Provincial Utrecht Society for Arts and Sciences) has been founded in 1776, also from private donations. 18Currently somewhat less07.11.14 than e4000. 19Currently about e25,000. All amounts on this page are in terms of purchasing power. 34 J.C. KAPTEYN: HIS LIFE AND WORKS for life that not many will understand: Trust and you will be trusted and your life will be enriched. Mother looked at them with joy, as she was standing in front of the house when they left arm in arm to the Laboratory (the two small rooms had this grandiose name in the Kapteyn family). Gill talking loudly and gesticulating, stared at by the Groningen citizens, Kapteyn, short and modest, quietly happy next to him. This first visit was a great success, and also the children were delighted with this big playmate, who had won their hearts even with his incomprehensible language. From then on he was ‘Oom Gill’ and remained that forever onwards. They talked and discussed much, since as was indicated already, the future for their work was not bright and they had to solve many difficulties. Christie, the Astronomer Royal from Greenwich20, saw with apprehension that Gill would overshadow him and did all he could to make the work of making catalogs impossible in those circles on which Gill was dependent. Articles appeared in scientific journals. There was opposition against the C.P.D. because there were so few stars on the test plates, that were sent to the Royal Society, much fewer stars then on the plates of the Milky Way by M.M. Henry in Paris.21 If it were not so sad, one should laugh at the fact that the work was rejected for these reasons. Gill wrote to Newcomb22, the American astronomer: ‘I told you that I had heard of babies crying for the moon, but I never dreamed of anything so funny as a row of Fellows of the Royal Society insisting on having more 9.5 magnitude stars in the heavens, else they would stop supplies.’ There were also articles in the astronomical journal the Observatory to inflict damage on Gill and to plant seeds of distrust between the friends. Typical is the comment Gill sent to Kapteyn: ‘I wish that the stupid fellows who are writing silly abuse of me in the Observatory would take a lesson from you and instead of doing that would really work at improving existing methods of measurement or inventing new or better ones.’ But the two stood firm and nothing could hinder their friendship and determination. Gill proposed to provide funds from his own pocket, his wife was prepared to sell her carriage and ... ‘We will somehow manage it together, even if we have to give up our tobacco –which God forbid!’ Auwers23, the astronomer from Berlin and friend of Gill, offered his help. The Humboldt Foundation of the Berlin Academy24 promised financial support if required. But Gill and his wife, who made all sorts of savings, could contribute that much to the work as was necessary for it to be completed. In June of 1892 Kapteyn wrote to Gill: ‘Finished! – The job of measuring the plates done, done at last. The work has been to me a source of no end of good things, but still its being done at last is one among the best ... the number of observations we got, must be upwards

20William Henry Mahonie Christie, (1854–1922) was director of the Royal Greenwich Observatory and Astronomer Royal in England from 1881 until his death. 21The brothers Paul (1848–1905) and Prosper Henry (1849–1903) built unsurpassed photographic tele- scopes, one of which in 1887 became the prototype for the international Carte de Ciel project. 22Simon Newcomb (1835–1909). See chapter 10 23Arthur Julius Georg Friedrich von Auwers (1838–1915) was an astronomer of the Berlin Academy and was a key participant in the founding of the Astrophysikalisches Observatorium at Potsdam. (adapted from footnote by E.R. Paul 24The Alexander von Humboldt-Stiftung fur¨ Naturforschung und Reisen (Alexander von Humboldt Foun- dation for Nature Research and Travel) was established in Berlin 18 months after the death of Alexander von Humboldt (1769–1859).07.11.14 Friedrich Wilhelm Heinrich Alexander Freiherr von Humboldt was a German naturalist and explorer. The Cape Photographic Durchmusterung 35 of a million – and the truth is that I find my patience nearly exhausted.’ Gill’s reply was a short note, that could just make it in the mail: ‘This is only a jubilant shout –a hurrah– a God bless you, my boy, –and long may you continue to prosper!’ The same year Kapteyn was elected Foreign Member of the Royal Astronomical Soci- ety, a high honorable distinction, that gave him much joy and satisfaction, and an extra stimulus to continue the hard work, that lay ahead of the reduction of the data and the production of the catalog of the stars. Indeed it was a demanding effort; his family could testify to that. The otherwise homely father and comrade who always knew a new game or joke, gave exercises and problems to solve and told exciting stories, especially during meals, now never had any time for them. The meals were eaten hastily, since he was always in a hurry to go back to work. He kept him occupied and busy. For a while he got up at 4 o’clock in the morning in order to be at the laboratory at 5. He then worked until 9 o’clock, rested till noon so that he could pick up with new strengths. He liked this regime very much, as it gave him enormous energy and make him highly productive. Mrs. Kapteyn supported him all the way. She also got up at 4 o’clock, organized her work in the house according to his schedule, invented with female ingenuity again and again new, delicious dishes, took all problems on her shoulders and optimized his energy and efficiency. She never asked anything for herself, since she knew she could not turn the tide and that after completion of this work the close, intimate life of the earlier days would return. To be a faithful supporter of her husband, in the true sense of the word, without complaining and looking for the limits of her strengths. But the tension was too high. He became easily irritable and would then be unreasonable to others in the house. It was no wonder that his temper would suffer under the heavy demands he put on his energy, but the children did not understand the reasons behind this and felt unfairly treated and unhappy. Mrs. Kapteyn talked to her husband about the fact that his children were complaining. She herself did not complain and never asked anything for her own, and precisely because of that the seriousness of her complaint on behalf of her children made a deep impression on him. He called the children to come to him and told them he had done them injustice. A happy atmosphere in the house was after all the most important thing in the world and he promised to try his utmost to control himself. He spoke long and in earnest, humbling himself for his children as only a great man would be able to do. In this solemn moment he not only won back what he had lost, but also he fostered a deeper understanding and admiration. They did not say much, since young people do not easily express their emotions, but they never forgot this moment. Physically the effort was also too great, he started to complain of nervous problems with his stomach and pain in his eyes. There was no thinking of resting. Sometimes he was lying on his stomach on the floor, working with his books spread around him as there was no other comfortable manner. He lost weight and slept poorly, work was haunting him and he often complained about that in his letters. To Donner25, the Finnish astronomer at Helsingfors, whom he had met during a congress in Paris and with whom he became good friends, he wrote among others: ‘I am being consumed by the work of the Photographic Durchmusterung, and that makes me ignore and neglect all other matters...’ 26

25The Finnish astronomer Anders Severin Donner (1854–1938) was professor of astronomy and director of the observatory at Helsingfors (1883–1915). (Footnote by E.R. Paul). 26Ich werde fast erdruckt¨ 07.11.14 von den Arbeit der Photographischen Durchmusterung, und das macht, dass ich h¨aufig alle andere Sachen afschieben muss. 36 J.C. KAPTEYN: HIS LIFE AND WORKS

The Durchmusterung consumed him completely and would not let go until in 1899 he had the three impressive quarto27 volumes lying in front of him. The work had taken 12 years, twice what he had anticipated at the start, but it was invaluable to astronomy. Gill wrote on April 6, 1899: ‘ A thousand heartiest congratulations on the completion of the Durchm. Catalogue. What a load off your weary shoulders! How splendidly you have redeemed the promise you made me in 1885 and thoroughly you have done your great work! It will ever remain a standing memorial of your devotion to science, your earnestness of purpose and your wonderful working capacity...’ The work was reward in itself, as it had shown Kapteyn new ways to pursue and it brought him recognition and admiration in all of the astronomical world. Newcomb wrote: ‘This work of Kapteyn offers a remarkable example of the spirit which animates the born inves- tigator of the heavens. Although the work was officially that of the British Gouvernment, the years of toil devoted to it were expended without other compensation than the consciousness of making a noble contribution to knowledge, and the appreciation of his fellow astronomers of this and future generations.’ And last but not least, it had given him a friend for life, whose rich experience and wisdom had had a large influence on him and whose honesty and warmth had won his heart. Gill wrote: ‘I congratulate myself that the material furnished to you –however many its imperfections– has enabled you to establish for yourself a reputation and position amongst astronomers of your time such as few men of your age enjoy. Above all I rejoice in the true friend I have found in you – may that friendship ever grow with our years!’ The work had not brought him any financial profits, but the harvest had been so rich that he felt a fortunate man. The feeling of being a ‘useless person’ was gone forever. He had not lived in vain.

07.11.14 27The book size quarto measures 30.5 × 24.15 cm. Gill 37

Chapter 9

GILL

Luck had been on Kapteyn’s side when, early in his astronomical career, he had found Gill, who would have such a great and important influence on his life and works. A noble person, a great astronomer, a dedicated husband and a trusted friend. ‘Such is the salt of the Earth’, exclaimed general Gordon, the famous hero of Khartoum1, when he had met Gill in Capetown. And everybody, who had a chance to get to know him, felt the same. David Gill was the son of a clock-maker in Aberdeen, and destined to take over his father’s prosperous business. But astronomy got a grip on him and whoever has been captured by science will not easily be released. That is how it went in his case, he relin- quished the business and a secure income and dedicated himself to science. In 1972 he became in charge of a small private observatory in Dun Echt (Scotland). He organized a few astronomical expeditions to Mauritius and Ascension, and made a name for himself through his talents as a practical astronomer. When the post of Her Majesty’s astronomer at the Cape Observatory became vacant in 1879, he was appointed and remained there for 27 years, during which time he brought the observatory to full prosperity. In 1906 he resigned, because after the enormous efforts of his professional life he was in need of some quieter life, and he settled in London where he again became a center of the astronomical works and astronomers. He had been knighted Sir David Gill, as England knows how to honor its great men and distinguish them for their accomplishments. He received many other distinctions, among which an honorary membership of the Amsterdam Academy of Science2. His advise was sought by many, since his extensive experience and deep insight were un- rivaled by others. From all around the world people came to him in his beautiful apartment in London, on 34 de Vere Gardens, Kensington. With much enthusiasm and dedication he looked into each new problem that was brought to his attention, and he always knew how to help and give advice. No more trusted friend and impartial admirer existed. He was in the possession of a beaming enthusiasm, a youthful delight in what was important and right, a deep religious feeling and a irresistible humor. Numerous anecdotes about him were being told. Although he was modest and humble, he still was in the possession of an appropriate sense of self-consciousness. That was evident at the Astronomical Congress

1Charles George Gordon (1833–1885) was the British general who became a national hero for his adventures in China and his defense of Khartoum (1884–1885) against Sudanese rebels during which he lost his life. (Footnote by E.R. Paul) 07.11.14 2The current Royal Academy of Arts and Sciences. 38 J.C. KAPTEYN: HIS LIFE AND WORKS

Figure 9.1 David Gill in 1884 (Aet. 41). From the portrait by Sir George Reid P.R.S.A.

in Paris in 1890. A vote had to be taken who would become the president of the Carte du Ciel undertaking. When the votes were counted, it turned out that all votes were for Gill. Obviously he had voted for himself. ‘Yes’, he said without restraint, ‘I thought I was the most qualified person’, which he was and proved07.11.14 to be without doubt. It is a sign of greatness to do a thing like that without giving rise to offense. Gill was one who could do and say anything without harming Gill 39 others.3 People appreciated him even more for it. When he met Hale4, the brilliant director of Mount Wilson Observatory, he asked him: ‘What are you going to do with that five foot reflector?’. When Hale explained his plans, he said: ‘All wrong! You should do nothing but radial velocity work. Now go ahead and defend yourself.’ Hale wrote later: ‘The twinkle in his eye overcame any fear of aggressive intent, and the cordial interest he showed was characteristic of the man.’ His marriage was as happy as ideally it should be. Mrs. (later Lady) Gill was definitely worthy of him. She was a intelligent woman with high standards, who shared in his worries and his triumphs. During their engagement she had unknown to him studied astronomical books. When she suddenly showed her knowledge, thinking she would surprise him, he exclaimed; ‘All exploded notions’. Being the sensible woman she was, she immediately gave up her efforts, understanding that a life-long study was necessary to fully understand the subject. When another lady once asked Gill: ‘I suppose your wife knows all about astronomy’, he replied from the bottom of his heart: ‘Not a word, thank God!’. Their relationship has probably been expressed best in a letter to Hale, thanking him for his invitation to come to the America and visit Mount Wilson: ‘.. The sad state of my wife’s health which Kapteyn will tell you all about, is such that I dare not go to the Solar Congress. It is a very bitter disappointment to me, but there are things dearer to a man than any congress, any gratification of friendship or the desire to see and know...’. Lady Gill wrote after his death to his biographer George Forbes: ‘Twenty six North Silver Street [their first home] was a comfortable, but rather ugly little house, and the furniture which I thought beautiful, and David did not think about it at all, atrocious. But to us both a very heaven of happiness lay between its four walls, at it always did between every four walls which held us two to the end of his life.’ Kapteyn himself offered the final words on this beautiful life: ‘In many a human heart his image will last as long as life itself.’.

3This is not correct. HHK refers here to a vote taken for the membership of the 1Permanent Committee’ during the first meeting of the Carte du Ciel in 1887. Gill was the only person collecting as many votes as voting persons present. 4George Ellery Hale (1868–1938) was the founder and first director of the Mount Wilson Observato- ry, which, in addition to becoming an international laboratory for solar research, Hale’s own specialty, dominated astronomy in07.11.14 the first half of the twentieth century with its 60- and 100-inch giant reflecting telescopes. (after a footnote by E.R. Paul) 40 J.C. KAPTEYN: HIS LIFE AND WORKS

Chapter 10

THE LABORATORY

The work of cataloging, how grand and useful it may be, could not satisfy Kapteyn’s mind. He envisaged a bigger goal that kept his thoughts occupied and that was the structure of the universe1. From his earliest to his his last investigations this kept his mind captive. Whatever different subjects his publications may seem to address, they were parts of this larger whole, the inquiry into the structure of the universe. In whatever detail his researches took him, he never lost sight of the great problem itself. That was the secret of his success. There is a remarkable pairing between the main picture and simultaneously the attention to the details. And also ‘the faculty to neglect the negligible’. The combination of these three factors is unusual and surely an element in his success in science. While he worked on the catalog with Gill, he already made plans in his thoughts for future investigations. In the mean time much was happening. At the Astronomical Congress in Paris in 1889 and 1890 it became clear that the excellent work Kapteyn was performing, was being no- ticed. He was chosen in the Comit´ePermanent de la Carte du Ciel, a gigantic, international undertaking to determine the positions of one and a half million stars, distributed over the whole sky. Kapteyn was charged with the construction of a parallactic instrument2, for which his access would be guaranteed. Gill and he had a major influence on the proceedings of the congress, which did not please the Astronomer Royal Christie3. He did whatever he could to oppose Gill’s plans and succeeded in obstructing his plan to found a central office for the Carte du Ciel, which would have benefited the undertaking enormously, by threatening to withdraw the British from the project. Consequently, Gill had to withdraw the proposal. This surely is one of the main reasons why this enterprise, in spite of its grand conception, is languishing up to the present day and will do so for many years. Kapteyn wrote after Gill’s death: ‘How

1Herschel, who initiated this work of determining the distribution of stars in space, referred to it as ‘the Construction of the Heavens’ 2A parallactic instrument is the measuring device in order to derive positions from the photographic plate, such as the one developed and used by Kapteyn for his Durchmusterung work, as illustrated in Figure 8.1. The trick Kapteyn devised was to observe the photographic plate with a small telescope from a distance equal to the focal distance in the telescope. This way all the offsets in both coordinates from the plate center could directly be read from the axes of the telescope mount. 3William Henry07.11.14 Mahoney Christie (1854–1922) worked most of his life at the Royal Observatory at Green- wich, was appointed director and Astronomer Royal in 1881. The Laboratory 41 different would be the outlook now, if he could have carried through his plan for a central bureau, perhaps the only important measure which he failed to see brought about.’ Gill wrote after the failure: ‘They said that Gill and Kapteyn would run the whole show. There was not the slightest foundation for this beyond a horrible jealousy ... to some men the working for science and truth for their own sake is a thing they cannot understand.’. So you see: even in astronomy, the science of the immeasurably large, narrowness of mind is not impossible. Both friends worked hard for the good case, they ignored as much as possible the obstacles that were put in their way, and defended themselves valiantly. They did obtain many good results and the work was in full swing in a short period of time. Not long after the congress, Kapteyn received the L´egiond’Honneur, his first distinction, that brought considerable happiness to the Kapteyn family. They had every reason to be proud; it was the first official recognition of his great accomplishments. Kapteyn now felt the time had come again to put a new proposal before the government. In May of 1892 he wrote to the curators4: ‘The Groningen astronomical collection already is in possession of 1. The measuring instrument with which the almost completed Photographic Survey of the Southern Hemisphere has been measured. 2. The measuring apparatus what has been allocated to me by the Comit´ePermanent de la Carte du Ciel for me to use and which has been almost completed, built according to the principles of the previous model, but designed to obtain the highest possible now. Furthermore, Prof. Haga5 has offered me for the execution of the instruments a room in his laboratory that is in all respects suitable for this purpose. If in addition a Repsold6 measuring instrument is purchased for the price of about f 24007, the astronomical institution here would have in its possession a unique set of photographic measuring instruments ...., and we will have in Groningen the possibility to study photographic plates of star fields by a variety of different methods and measurements that would be better than elsewhere, even abroad.’ The instrument request was approved and immediately ordered. However, it was not set up in the physics laboratory, since he was promised his own building, namely the unoccupied house for the Queen’s Commissioner8. The laboratory was officially opened on January 16, 1896. Finally astronomy in Gronin- gen had its own temple where the holy fire would burn and where young people could be initiated in the mysteries of science. In his address at the opening Kapteyn felt he had to give a justification of why it was a laboratory, which was a unique thing and its purpose therefore not clear to most people. They would never have heard of an astronomical laboratory, an observatory without a telescope or dome. So he explained in his opening address: ‘ Photographic observatories are counted by the dozens. In each of these observatories the amount of material produced is a multitude of what can be analyzed, since the manpower that

4The ‘College van Curatoren’ or Board of Governors of the university was responsible for all issues, in- cluding in particular the financial ones concerning the university. 5Hermannus Haga (1852–1936) was professor of physics and meteorology in Groningen from 1886 to 1922. 6Johann Georg Repsold (1770–1830) was a German astronomer and instrument maker, who also was director of Hamburg Observatory. 7Currently about e65,000. 8The Commissioner is the07.11.14 representative of the Queen (Emma at the time) and head of the province, Groningen in this case. 42 J.C. KAPTEYN: HIS LIFE AND WORKS is required for the measurement of the plates and the further reduction of these measurements, is not only larger than what one needs for making the photographic exposures, it is usually not even present in an amount equal to it and in many observatories, it seems, even completely absent... The curse of the majority of observatories, at most of which people work as hard as they can, is still the growing amount of work that is overdue. Many priceless sets of observations are still lying in their archives and since they are not or only partly analyzed, have not born the fruits for science they could have produced. No wonder that observatories start to be feeling the pressure to remedy this and isn’t it time that we start looking around to find means to restore, at least to some extent, so this balance between the collecting of observational material and its analysis, between the photographic work and that of the astronomer? To devote oneself to what in one of his letters Darwin described as ‘The grinding of the huge masses of fact into law’? I think that everyone would answer this question affirmatively. But with that at the same time the question is answered whether or not an institution as this one is justified. The second question is: ‘Where does the material come from?’ But the answer to this question is obvious after what I said. Of course from the observatories that are over-producing. Many will be inclined to make this material available in the expectation that it will be put to good use, of that there can be hardly any doubt... We will not be short of useful work to do.’ Kapteyn was grateful for this first step, but he was not yet one of the ‘Professores Contenti’, as he expressed it himself. There was still much to be wished for. The laboratory had only a temporary housing and he was not allowed to erect the pillars necessary to mount some instruments on. Many other instruments were necessary, there was no facility for observations9 and no library. ‘An astronomical institute without a library is impossible’, he said, ‘especially in an institute as this one where the work program concerns theoretical investigations based on observational data obtained elsewhere.’ But as said, the first step had been taken, the laboratory was elevated to an official establishment with its own housing and annual budget, and with his beloved motto: ‘The perfect is the enemy of the good’10, he accepted his astronomical laboratory. He received appreciation from all sides. Oudemans, the astronomer from Utrecht, who had failed to support him in realizing an observatory, wrote later: ‘In hindsight it is possi- ble that things worked out better (not having an observatory). When I was an observator, it must have been 1853 or ’54, I had to write to Airy11 (who then was England’s most famous astronomer). I told him that there was a plan to build a new observatory in Leiden and what was his reaction? ‘Your message, that there will be a new observatory in Leiden, does not make me particularly happy. There are more than enough observatories, but it is a computa- tional institution that we need.’ The computational institutions at Berlin and Groningen have shown that this judgment was not that bad after all. But for real progress the tenacity and perseverance like yours is essential.’ Now there is no justification necessary for the Astronomical Laboratory anymore. Great things have been performed there and it has a special, honorary place in the scientific

9For teaching purposes, I suppose. 10Le mieux est l’ennemi du bien. Quoted from Voltaire. 11George Biddell Airy (1801–1892), who from 1835 to 1881 was England’s Astronomer Royal, had a major influence on the07.11.14 development of British astronomy beginning with his appointment at the University of Cambridge as the Plumian professor in 1828. (Footnote by E.R. Paul) The Laboratory 43 world. In 1922, shortly before Kapteyn’s death, Baillaud12, the French astronomer, noted in his address at the opening of the astronomical congress in Rome13: ‘The three things”, he said, “that have shaped the appearance of the science of astronomy in the past half century, are photography, giant telescopes and the Groningen Laboratory.’ It may seem strange that an astronomical laboratory, not withstanding its great need, has remained an exception. Only Prof. Pannekoek14 founded a similar institution in 1921, so from a more recent date. The rarity has a deeper cause. The collaboration and the coordination of the work result in major difficulties and require utmost altruism and helpfulness of the collaborators. The laboratory depends on an observatory for its material. This requires a constant understanding and good will. But Kapteyn was the right man for this. As a result of his generous and noble character, his lack of pettiness, his modest friendliness and his ability to give others credit where they deserved it, made him and his collaborators friends. Some may have described him as diplomatic, but I would say diplomacy was not one of his characteristics. Diplomacy is after all a conscious use of tact and intellect to reach certain goals, setting up rivals against each other and making use of weaknesses in someone else’s position. He always had an honest interest in the work of others, his charming tact and admirable modesty, that silently worked for him. He fully understood each situation immediately and his sharp intellect perceived without hesitation what was the best combination, which persons and forces were required and how to win them for the grand cause. He never gave up and he was never discouraged by obstacles on his path. In short, his genius and personal charm were irresistible. This is in my view something much more profound and honorable than can be achieved with diplomacy. Still he felt the problems associated with his laboratory of being necessarily dependent upon others. For example, he wrote among other to Dyson15, the director of Greenwich Observatory: ‘I sometimes cannot help thinking that you must take me for the most trou- blesome man alive. The man who has always something to ask and has never something to offer in return. I hope you will consider that it is not all my fault. It is the one point that makes such an institution as an astronomical laboratory objectionable, which must live on the materials furnished by other institutions. For several of the observatories which have helped us, the laboratory has been able to render small contra-services and we hope that some day will come, that we can do this for Greenwich...’

12Edouard-Benjamin Baillaud (1848–1934) was a French astronomer, who played an important role in establishing the Carte du Ciel project and who was the first president of the International Astronomical Union. In his footnote E.R. Paul identifies him here incorrectly as his son Jules Baillaud, who features in chapter 19. 13This was the first meeting of the International Astronomical Union, held May 2–10, 1922. 14Anton Pannekoek (1873–1960) was a founder of modern astrophysics in the Netherlands, contributing to statistical astronomy, Kapteyn’s specialty, but today is best known as having written his well-known A History of Astronomy, George Allen & Unwin Ltd., London, 1961. (Footnote by E.R. Paul) However, the role of Hertzsprung was certainly as important or even more so. De Sitter was just as vital for Dutch astronomy, but when we take the term astrophysics literary (physics of stars) his work on the satellites of Jupiter and on General Relativity is not part of it. Pannekoek founded also the astronomical institute in Amsterdam which carries his name. 15Frank Watson Dyson (1868–1939) was the eleventh Astronomer Royal at Greenwich from 1910 to 1933; he is known today mostly07.11.14 for work in fundamental astronomical measurements and for his unusual ability for collaborative research. (Footnote by E.R.Paul) 44 J.C. KAPTEYN: HIS LIFE AND WORKS

The works at the Groningen Laboratory have been twofold. In the first place, the theoretical investigations and calculations concerning the structure of the Universe, the average parallax, the luminosity law of the stars and the number of stars as a function of their magnitude, which appeared in the Publications of the Astronomical Laboratory at Groningen. In the second place the practical research, which appeared in the same series of publications, concerning the measurement of photographic plates to extract information from them necessary for further general research into the structure of the Stellar System. In 1889 Kapteyn developed the plan to use photographic exposures to determine paral- laxes of stars in large numbers. Such exposures were extremely laborious for the photographer- astronomer; on three occasions, always half a year apart, plates had to be exposed and then stored away, before finally they could be developed. The same was the case for the subsequent plan that Kapteyn initiated to determine the motions of the stars from sets of plates on which exposures with 5 to 10 year intervals were taken. When Kapteyn discussed this plan in Paris with the Finnish astronomer Donner, the latter was immediately avail- able to go through all this trouble for the important scientific goal of developing a new approach in order to arrive at more accurate and complete results that had been possible before. Kapteyn took the task of measurement and analysis upon himself. Donner visited Groningen a few times and their collaboration developed into a warm friendship, although their correspondence was infrequent, as they both were poor writers. Their letters usually started with an earnest lamentation and excuses for the long interval it had taken. Both had busy, hard-working lives from which their correspondence suffered.

07.11.14 Newcomb 45

Chapter 11

NEWCOMB

In 1899 Newcomb, the famous American astronomer, visited Groningen. Kapteyn had been corresponding with him and had the highest admiration and respect for him. Simon Newcomb occupied a unique place in science. He was, what we here in Europe regard as very common for Americans, a self-made man. His father, a traveling schoolmas- ter, had intended him to become a carpenter, but he wrote in his autobiography; ‘I had gradually formed from reading a vague conception of a different kind of world, a world of light, where dwelt men who wrote books and people who knew the men who wrote books. I longed much to get into this world, but no possibility of doing so presented itself.’ By chance he came in contact with a doctor, who wanted to teach him his profession. But he turned out to be a humbug hiding behind a mask if scholarship and affability, and the boy had not found the world of illumination that he dreamed of. To the contrary, he found a dark world of disappointment and unsatisfied desire, as he did not learn anything useful and which only served to give him hints that did not lead him anywhere. He left without knowing where to go and was uncertain whether what he did was right or wrong. ‘Am I doing the right or wrong, am I going forward to success in life or failure and degradation? Vainly, vainly I tried to peer into the thick darkness of the future. No definite idea of what success might mean could find a place in my mind. I had sometimes indulged in daydreams, but these came not to a mind occupied as mine on that day. And if that had, and if fancy had been allowed its wildest flights of an honorary Academician of France, seated in the chair of Newton and Franklin in the Institute would not have been in the picture.’ But it happen that way. After a few trips back to his father, who managed to find a place for him as an assistant teacher in Maryland and where he proved to be good in mathematics, he was appointed as computer at the American Nautical Almanac offices. He was in a position to continue mathematics studies at Harvard, Cambridge, and dis- tinguished himself to the extent that in 1861 at the age of 26 he was appointed professor of mathematics of the United States’ Navy and astronomer at the Naval Observatory. In that way he became a ‘denizen of the world of light’. Later he was offered the directorship of Harvard Observatory, which he declined. ‘Per- haps unwisely for myself, though nobody knows what the Cambridge Observatory has become under Prof. Pickering1 can feel that Harvard has any cause to regret my decision.’ At about the same time he was appointed Superintendent of the Nautical Almanac and he held that

1Edward Charles Pickering07.11.14 (1649–1919) became director of the Harvard College Observatory in 1876 where he remained until he died in 1919. (From a footnote by E.R. Paul). 46 J.C. KAPTEYN: HIS LIFE AND WORKS

Figure 11.1 Simon Newcomb.

position for twenty years until his retirement. In 1884 he became professor of mathematics and astronomy at Johns Hopkins University in Baltimore. He received many high distinc- tions, among which07.11.14 the Huygens medal of Leiden University, a medal that is awarded only once every 20 years. Newcomb 47

He produced many astronomical works, but contrary to most astronomers, who con- centrate on a single subject, he was unusually versatile. As Robert Ball2, an English astronomer, expressed it; ‘Newcomb is as versatile as he is profound.’ He was president of many scientific organizations for which he had an interest. He occupied himself much with economic problems, on which he wrote several books. Also he had an interest in psycho- logical matters. As president of the American Society of Psychical Research he paid much attention to spiritualism, that however according to his experience had to be considered fraudulent. He even made a name as writer of novels. His visit as it was announced caused Kapteyn some concern, as he feared he did not have much of importance to show. When it seemed for a moment that the visit was going to be canceled, Kapteyn wrote him: ‘You will not see my instruments in this case, but this will only save you so much disappointment, as there is certainly no observatory in Europe, not to speak of America, so scantly equipped.’ Newcomb came to Groningen and turned out to be a remarkably stern man, who im- pressed with his appearance and through his economy of speech. Kapteyn was not much at ease with him and was sometimes upset with the visit, even though he appreciated it and felt it was informative and interesting for him. Not so Mrs. Kapteyn. She had an easy, open manner of going along with dignitaries, that was both very amusing and grand. She was not easily impressed by others, since she never thought much about the impression she would make, but was always busy to make her guest feel at ease and comfortable. She liked him immediately, his quietness did not bother her since she had enough conversation herself and had a good command of English. ‘He is a king among people’, she used to say and full of surprise her children noted that he was allowed to put his tired, dusty feet up on the beautiful golden chairs that were the centerpieces of their mother’s salon. She baked buckwheat cakes for him in the morning, because he liked those so much, and was the nicest hostess the stern American could imagine. The admiration came from both sides. For example, Newcomb wrote to Gill around that time: ‘I have read a letter of you to Mrs. Kapteyn’, apologizing, ‘It is hard to keep anything from so delightful a woman.’ Gill wrote about this remark to Kapteyn, whom it pleased very much. Newcomb did posses a special dry sense of humor, that surprised the children in this so imposing man. He was tall and robust with thick wavy white hair that was remarkable beautiful. He mentioned that his hair was his wife’s pride. Later his name will be entered in the lexicon as ‘the man who had the most beautiful head of hair; seems to have been an astronomer’. As a result of some ailment in a nerve in his leg he temporarily had to use crutches, which made him even more interesting in the eyes of the children. He left them alone, which they only felt to be natural, and they behaved quietly and restrained in his commanding presence. Still he was shy and had difficulty to express himself. His appreciation and interest was expressed in a strangely dry and still poignant manner. In 1907 he wrote to Kapteyn: ‘Next year I pass through the Hague on my return from Rome. If I do this, I hope you will be able to place yourself near my line of movement’. And later: ‘I will spend a week or two at the Hague. Perhaps you and Mrs. Kapteyn can also come to that region which I believe is very pleasant in the early autumn, when I shall probably be there.’ Once he even wrote: ‘I think it is nearly a year since I have heard from you personally. And I now write rather from a general

2Robert S. Ball (1840–1913)07.11.14 had studied astronomy and became well-known for his popular books on the history of astronomy. 48 J.C. KAPTEYN: HIS LIFE AND WORKS desire to hear, how you are doing than from having anything important to say.’ Which from this very withdrawn man might be called remarkable. This first visit, that was followed by others, was of much importance to Kapteyn and a stimulus to continue his studies about the distribution of stars. The subject, that occupied him for so long, but up till now had to assume a subsidiary place. It started to consume him more and more and when the C.P.D. was finished, he finally was able to devote his full attention to this grand problem.

07.11.14 Two perfect gentlemen 49

Chapter 12

TWO PERFECT GENTLEMEN

In 1900 Gill visited Groningen for the third time. He was welcomed as an old trusted friend. The Boer War1 that had outraged many among the Dutch, was in its last phase, and the British were not very popular in those days. Their greedy, unjust policies made them much hated by the Dutch, who choose enthusiastically the side of their own descendants in this far-away country. The family Kapteyn decided not to bring up this delicate subject, as it would be uncomfortable to the English guest and so the subject was not discussed. Gill’s visit was as nice and pleasant as always. His warm interest and spontaneous, lively spirit had a major influence of Kapteyn’s work. Vigorous and to the point it all was, his wisdom was a great help and comfort in difficult circumstances, his warm friendship amicable and his humor irresistible. He had himself photographed with the two girls, who he called his ‘lassies’, teased and spoiled them and was the most admirable uncle they could imagine. One day Gill and Kapteyn visited the Physics laboratory in Groningen, where over each door a wise quote had been painted. One said: ‘Wisdom is more valuable than rubies.’2 ‘I know what that means’, Gill called out. ‘Whiskey is better than red wine.’ He enjoyed his jokes as such as others did. Typical for Gill was his joking translation of the Latin saying ‘Experienta docet’. ‘You know: Experience does it.’ ‘I have obtained all my wisdom from Gill’, Kapteyn used to say. In his letter to Gill at the occasion of his 70th birthday he wrote: ‘I think I picked up something of your great ‘Lebensweisheit’, of your capacity of making life a joy to yourself and to others.’ There certainly is a lot of truth in this, since many were the prudent pieces of advice that he received from this knowledgeable and wise man. Kapteyn’s own wisdom certainly was not any less; his inner life was a serene confidence of spirit. He radiated a still strength, that could not be explained, since he was so modest and calm without pretension, contrary to what we see around us nowadays. One of his clerks, who had worked for twenty years at his laboratory, said of him: ‘He had a very deep wisdom in life, that however was explained often in such simple terms, that many failed to appreciate it. But I always was aware of the wisdom in it. He gave stability in my life, and in difficult moments, when I thought of him, he was a great source of inspiration.’ He told me much more about him and suddenly said: ‘Maybe you feel it is disrespectful that I always speak about ‘him’. But

1Also called the South African War (October 1899–May 1902) was fought between Great Britain and two Boer republics –South African republic (Transvaal) and the Orange Free State. The Boers are of Dutch and Huguenot descent. (Footnote07.11.14 by E.R. Paul) 2Wijsheid is beter dan robijnen. 50 J.C. KAPTEYN: HIS LIFE AND WORKS it isn’t, he always was ‘He’ with a capital ‘H’. The old saying: modesty is the trademark of what is right, is used nowadays without thinking and has become commonplace, and we hardly feel its real value. More so than ever, Kapteyn’s personality illustrates this in its full beauty. When Gill had returned to the Cape the correspondence was continued. Kapteyn always had difficulty forcing himself to writing letters, his life being arduous, busy and full. In response to an urgent question of Gill, he answered: ‘The fact is that I am and have always been a bad correspondent, wanting some direct stimulus for writing a letter. Add to this the feeling that I had better avoid writing on the subject of the disastrous war, a subject which as soon as we Dutchmen wrote to an Englishman will come uppermost in our mind, and you will see, how it is that I don’t let you hear for ever so long.’ Gill was very much moved by this and replied: ‘You certainly are a bad correspondent, but there is this about you, when you do write a letter, there is always something or rather there are many things well worth reading in it. Now first of all I really did not realize fully before, what a perfect gentleman you are –I did not realize till now that you feel so strongly about this miserable war. I only wonder, how you had the power to keep so completely away from the subject during the time I was with you in Holland. As your guest of course I did not open the subject,’ –was he not just as well a perfect gentleman?– ‘but I had felt as keenly as you do about it, I do not think, I could have refrained so perfectly as you did. I only feel about the war that we had to fight or to make up our minds to submit to a Boer Republic throughout South-Africa. But I have no feeling of animosity against Boers. Blood is thicker than water, so if you really want to let off steam about the war, you will find me an excellent safety valve, and don’t let in the future the fear of speaking your mind interfere with your writing to me on that or any other subject. Indeed it would interest me greatly, if you do so.’ So this friendship became stronger and more complete.

07.11.14 Vries 51

Chapter 13

VRIES

In the mean time the children were growing up. The girls attended the ‘Hoogere-Burger- school’ 1 for boys and the Gymnasium2, which was unusual in those days. Kapteyn did not even think about making a difference between his daughters and his son, who also went to the HBS. The same possibilities and the same rights for all, for him that was the way it was and the way it should be. He had an equal interest in the problems of males and females, thought deeply about the future of his children and wanted to provide all the materials to attain what they felt was their mission in life. No course that he deemed useful for them to prepare them for later life was too much or too expensive for him. He and his wife gave up all the luxury if that was required for this. He had a great interest in the schoolwork of the children, but he also made sure that it would not play too large a role in their lives. He was very much opposed to unbounded ambition and striving for preferential treatment, which he considered fatal in a young person’s life, both in a moral sense as in that it would be a waste of physical and mental strength. When his oldest daughter came home one day from school and told him that she was number one in the class, for which she had worked very hard, his reply was: ‘Don’t do that to me again!’ With this response he showed wise and true insight in what is really of importance in life and his daughter was taught an important lesson, which she never forgot. He was also opposed to all sorts of competition and did not want his children to participate in them, pointing at the lack of depth and true value in applause. As an example of the beauty of wise fatherly love, that goes well above usual opinions of upbringing, the following anecdote shines as a light between all things forgotten from the past. One day a large bunch of grapes was on the table, which in this simple family was a rare event. The youngest daughter saw this with delight and sighed: ‘Oh, if only I could have whole bunch for myself; then I would be perfectly happy!’ The righteous pedagogue would have pointed out that this would be egoistic and that everything had to be shared between them. However, he said: ‘Well, in that case, my little child, then for once you

1The Hogere-Burgerschool or HBS was a type of secondary school in the Netherlands, that was instituted in 1863 as a school for boys from the upper-middle class or higher to prepare them for advanced professions and positions in trade and commerce. It encompassed a broad curriculum with much attention for science and modern languages, but no Greek or Latin was being taught. Eventually it became a school preparing young persons for university studies, except in languages and literature. Teachers usually had university degrees (PhD’s often). The school type was opened for girls in 1871 and its diploma was recognized for entrance into university studies07.11.14 in 1917. 2In the gymnasium much of the curriculum was devoted to Greek and Latin. 52 J.C. KAPTEYN: HIS LIFE AND WORKS shall experience the perfect happiness that is so rare.’ And he gave her the whole bunch, which amazingly the others did not envy her. That sensation of perfect happiness has long disappeared, but what remained is the memory of the love of a father, who wanted to bring happiness wherever possible. But he did not only have an interest in his own children, as many parents do, other children also enjoyed talking to him and discussing their things of interest. For example, there was a small girl, who had learned in school that the earth is moving around the sun, which she found difficult to believe or understand. When she asked her father for an explanation, he said: ‘Why don’t you ask Prof. Kapteyn,; he knows these better than I do’. She indeed went to him and he explained everything in such a clear way that she returned home satisfied and convinced. Many parents came to Kapteyn to ask advice about how to raise their sons, what profession they should choose, etc., and always they left him with the right advice and a warm consolation when there were difficulties. His interest concerned all that lived and strove, equally for all. His children he taught to respect opinions of other people and not to be immediately give criticism. He himself was the clearest and purest example of this and he lived for this every day. We were very surprised when one day we heard him strongly criticize a colleague. We were so used to his humane, understanding judgment that this made a very deep impression. Such criticism is valuable and has its effect. For a small cost he bought a second house with a large garden in Vries, in the northern part of the province of Drenthe, a few hours from Groningen and the family spend their summers there. Mrs. Kapteyn worked all day in the vegetable garden, which was a joy for her, and the daughters did the housekeeping. Many guests came and went and everyone was welcome in this family where a simple and pleasant hospitality was exercised. There was a big study room, where anyone who had the inclination for it, could work in the mornings, and Kapteyn himself worked there also in the morning as his work never left him alone. In the afternoon he took his bicycle and wandered around and learned to know the surroundings well. He was always watching and listening with an open ear for what nature was presenting him. He listened to the birds and knew their particular songs. It became a close love and his face would lighten up when he heard an unexpected sound and silently he moved to observe it from nearby. Bird-trips, a full day with binoculars and an inexhaustible enthusiasm were a never-ending joy for him. He went over ditches and trenches and right through thick weeds he went, as if he had some goal in mind. Prof. Boissevain3, who often accompanied him on his trips, at one occasion felt it became a bit too much, ‘but’, he told later, ‘I was running behind him and I thought; where he can pass, I should also be able to pass’. and with such enthusiastic guidance the impossible became possible. And then he saw the gamecocks dancing, which was the most delightful and joyful thing for Kapteyn that nature presents. Thijsse4, our foremost authority on birds, described it as follows: ‘Every bird has its own colors like pages on a medieval court. Here is one with a white collar that blinds the eye, and pearl-gray feathers with white borders on his back, there we see one that is totally orange, and there again another one that seems to delight in possessing steel-blue sparkling feather as from a magpie. Again another one has red and blue stripes and

3Ursul Philip Boussevain (1855-1930) was a professor of history at the University of Groningen. 4Jacobus Pieter07.11.14 Thijsse (1865–1945) was a well-known botanist, ornithologist and conservationist, who published a set of very widely used books on birds. Vries 53 opposite from him a knight bows with an ermine collar full of black spots. And all the time we observe changes in the collection of colors. It seems as if the wind goes through these feathers all the time, but if so then if would have had to come from all directions; and yonder we see the oxtails standing motionless in the morning sun. It is the birds themselves that spread their feathers, turn them up, fold them together again and move them around as a dancer does with her dress. And the dance itself also has commenced. See them turn around on their thin legs! Three steps around, then bowing towards the ground, stand very still, now head in the neck, beak up in the air, the neck feathers stretched out as far as possible, tripping backwards with the wings tight against the body, then a normal position and all feathers shaken as a poodle does that comes out of the water...’ Kapteyn would give anything to be allowed to watch this feast of color and motion, with full abandon: getting up early in the morning, making long trips over unpaved roads, with endless patience, – but then he came home with the lights of a very special glow in his eyes and with growing amazement over and respect for nature. Also stones started to tell them their stories. His long geology hammer became a trusted companion during his wanderings over heather and sand. Cairns and grooves, that previ- ously were dead and meaningless to him, now came to life and told him stories of old times and wonders of nature. The children sometimes became very impatient when during their bicycle trips together their father got off at every heap of stones, since they were not yet enchanted by science that gives shine and inspiration, but his enthusiasm was getting a hold on them also, and after some time they searched and hammered along, also shouting out when they found an interesting stone that could be added to the collection. Geology books were read and discussed, bringing up new ideas. Kapteyn discovered a few hills near Vries, that in their regularity made the impression of having been made by men. From the owner of the land he purchased for a few guilders5 the plot with one of the mounts, and the family Kapteyn, armed with pickaxes and shovels, and their friends went to try and uncover the secrets of the hill. They dug and worked, but nothing of relevance was discovered. The diligence started to wane and in the end the work was given up, but the study of geology continued. Scientific interest was aroused; another part of nature grabbed him and he wanted to penetrate deeper into the secret. He asked Prof. Molengraaff6, the geologist from Delft, who every year undertook an excursion with his students, if he could join in these trips, and this was gladly granted. So, a few times he went along to far away places as if he were the youngest and most enthusiastic student. ‘Through his modest friendliness he immediately pulled all young men to him, was untiring, and during long marches, his spirit, with only a few of the others, remained high.’ This is what Prof. Brouwer7 wrote after participating in one of those trips. And: ‘He often asked the leader of the trip for further explanation, which showed his interest for the general findings of geology, and since we would because of all the details forget the major trends it was also very instructive to listen to those exchanges. I remember vividly at the dinner on the last day of the trip, when as a matter of habit various speeches were made, that Kapteyn spoke and turned to the students and emphasized that they should never be too serious about things. He said it just like that, but everyone felt that here someone was presenting himself exactly

5A guilder (f) is about e25 nowadays. 6Gustaaf Adolf Frederik Molengraaff (1860–1942) was professor of geology at the Technical University Delft. (Footnote by E.R. Paul) 7Hendrik Albertus Brouwer07.11.14 (1896–1973) was professor of geology and paleontology at the Technical Uni- versity Delft (1918-1928) and at the University of Amsterdam (1928–1957). (Footnote by E.R. Paul) 54 J.C. KAPTEYN: HIS LIFE AND WORKS as he was and it made a lasting impression.’ It is long ago now and the student from then is now a man of significance in the scientific world, but the memory of Kapteyn remains indelible, as happened to all those who in life came into contact with him. A great man, who felt for everyone, grand in the true simplicity of the word.

07.11.14 The professor 55

Chapter 14

THE PROFESSOR

Van Rhijn, his assistant and successor, has described his lectures as follows: ‘When students explained their ideas with some boasting of their erudition, Kapteyn used to say: ‘That is fine, but I would like to see some more fundamental understanding from you gentlemen, not to approach the matters first as a mathematician, but as a physicist.’ And then he embarked on an explanation so transparent and understandable and so fantastically graphic... That is’, van Rhijn said, ‘what your students are grateful for above anything else, namely that you rescued them from being pedantic learned men. He did not demand too much actual knowledge from his students and in his lectures he used little factual matters; he rather showed his students how the questions that we today consider to have been answered indisputably, earlier had been unsolved scientific problems and how earlier scientists had attacked and solved them. That was the secret of his fascinating lectures: one had the impression that the student and the professor worked together to solve a scientific problem rather then having the professor explain and the student listening.’ His lectures were clear through and through and well-organized. The aims of studies and theories became crystal clear for the students. ‘I never met anyone who was so able to do this and I believe that this is the best way to do scientific research’, wrote one of his former students, J. Oort1, the current observator at Leiden Observatory. And he continued: ‘Being in his company, during lectures or a colloquium, with something unusually stimulating and most of the time I came away from this with more beautiful and happier thoughts then when I arrived. This is because he saw the beauty of nature and of science, which he more closely connected than anyone else... While other lectures as a first-year student would make one loose confidence in oneself, the astronomy course restored and enhanced that.’ A more beautiful testimony for a teacher is not possible, and it is to a large extent characteristic of Kapteyn’s personality. He did not agree much with the methods of teaching in his time. The aim of just preparing to pass exams and collect learned facts did not appear to him to be a way of creating independent scientists. The most learned men often were unfamiliar and awkward when it concerned reality, and were not able to solve even the simplest of problems. To substantiate his opinion he used to take examples from his own experience. Many years

1Jan Hendrik Oort (1900–1992) studied with Kapteyn as an undergraduate and Kapteyn turned him to astronomy rather than physics, which he came to major in Groningen. Oort later wrote a PhD thesis under van Rhijn. Eventually07.11.14 he became director of Leiden Observatory and one of the most prominent astronomers of the twentieth century. 56 J.C. KAPTEYN: HIS LIFE AND WORKS he was ‘gecommitteerde’2 at final exams for the gymnasium. He usually had a different opinion on the mathematical skills of the candidates then the teachers had. At one time there was a boy, who had solved a problem concerning the calculation of an interest, but as a result of a small error in his calculations had ended up with an enormous number, so large that he had to turn the paper horizontally to find enough space for all the figures. The teacher felt that the calculation was acceptable except for the small error. But Kapteyn felt the boy should not pass the exam as he had shown clearly not to have any idea what it was all about, revealing a complete lack of mathematical understanding. In another specialty he took the same position. Once a candidate had to make a French translation about a tired pilgrim in a desert. ‘Comme son coeur rit, quand il s’approche d’un gˆıte.’ Apparently the student did not know the word ‘gˆıte’ and translated it as: ‘How his heart laughs when he sees a wild animal’, probably thinking of ‘gibier’ (the animal ‘game’). A strange pilgrim indeed! His opinion was that in such a case one can better leave that word untranslated rather than provide illogical absurdities that show a lack of common sense. ‘Give me the candidates that fail’, he said, ‘you may keep the ones that pass’, with the usual overstatement of a sound theory. He was a professor at the University of Groningen for more than 40 years. He remained true to her until he became an emeritus. An appointment at Utrecht was declined. And also one in Leiden, after the retirement of van de Sande Bakhuyzen3, although this was a difficult decision for him as the Observatory in Leiden was close to his heart. But he would not leave his Laboratory, this beautiful child of his imagination.

2This is a person who at an exam has been appointed by the government to oversee the proceedings. 3At the retirement of H.G. Bakhuyzen, Kapteyn was considered his most natural successor as professor of astronomy and director of the Observatory, but Kapteyn declined and proposed Willem de Sitter instead. Indeed Leiden appointed07.11.14 de Sitter as professor of astronomy, but van de Sande Bakhuyzen’s younger brother Ernst as director. After the latter’s death in 1918 de Sitter did become director. A versatile life 57

Figure 14.1 Kapteyn at age 45. 07.11.14 58 J.C. KAPTEYN: HIS LIFE AND WORKS

Chapter 15

A VERSATILE LIFE

Groningers could see Kapteyn walking in the streets every morning at 9 o’clock, on his walk to the laboratory. Everyone recognized his characteristic, boyish walk, with small shuffling steps, which he himself called a ‘heath-step’, and that he explained as due to his many walking trips while he was young through the heath. Between nine and noon no-one was allowed to disturb him and in those daily three hours with concentrated work behind his desk, his mind wandered far and his major scientific problems were addressed. He was very good at concentrating whatever was at hand. Nothing would distract him. At home he worked preferably in the living room, where the warm atmosphere was a stimulus. ‘I learned concentration as a youth’, he said laughingly. ‘Once when the chatter of the birds hindered my working and I closed the window, my mother became so cross with me about this sensitivity, that she slapped me and threw the window open again. That slap was more useful to me than all well-intended words.’ Also to have to work surrounded with all the boys had taught him to close himself off from the rest of the world whenever necessary. The other side of the coin of his ability of intense concentration was his real professorial absent mindedness. Fortunately there were always loving and caring persons around to help him. ‘Professor, you forgot to put on your hat’, his clerks often reminded him, when he was about to leave the laboratory, or ‘the leg of your trouser is folded up’. Or they called him on his phone to remind him that he was expected at a thesis defense ceremony. The old housekeeper, who had a tremendous memory, was of immeasurable worth in reminding him of exams and other important matters. It did happen for example, that he was on his way to America one day and got to Rotterdam when he noted he had forgotten his wallet with his money. He was only just able to return to Groningen and come back in time to catch his ship. Another time in America he has lost his wallet in a train; after a frantic search the black attendant was consulted. ‘Just look under your pillow, Sir’, was his advice. And indeed, that was where it was found. The clerks at the laboratory, who increased in number all the time, were much dedicated to him. He possessed a respectful love and an unshakable faith in them. No wonder, since he shared in their joys and personal troubles, supporting them and giving advice where life brought them ill luck, had an infinite trust and was, like all felt, like a father for them. In his laboratory there was an optimistic, serene atmosphere, brought about by Kapteyn wherever he was and that made the work which was often endlessly mechanical and boring, lighter for all. His radiating optimism helped him to overcome many difficulties. ‘Come, let us do first07.11.14 things first and then we will find out what comes next. We then will get A versatile life 59 it done.’ And when a problem was tackled with determination, the difficulties resolved themselves automatically. The afternoons were for the contacts with the rest of the world: his lectures, discussions, exams, his walks, among which his usual walks on Monday afternoon with his trusted friends Heymans1 and Boissevain. The three of them could be seen every Monday walking along the Harenschen weg. Heymans in the middle, the tall imposing figure in a pelarine coat with distant view in his eyes and his mind rising above humanity. Boissevain, who was small, but always moving and full of a lively interest. Kapteyn at the other side, slim and entertaining, enjoying the open air and the interesting things that each of them told about their work, while at the same time they noted the songs of the birds and recognizing them. They walked to Haren, a little village at an hour’s walk from, Groningen, and they never failed to continue this habit. They used to do this for twenty years, and it was remarkable, they noted themselves, how infrequently they had to terminate their trip because of the notorious weather in the Netherlands. It did happen, especially in later years that they first went to the Caf´ede Passage and kept talking there, being just as satisfied when they returned home. It was a beautiful friendship, full of mutual admiration and respect. Kapteyn had major problems of nervous pain in his eyes, for which he was unable for many years to find a solution, until in the ends the problem solved itself and disappeared. But for a long time he was forced to stop reading at night. That was a big sacrifice, but good friends came to help regularly and read out to him at night. Heymans did this for many years and both enjoyed these evenings. In the beginning these were articles from Kapteyn’s area of research, but later it concerned things of mutual interest, mostly articles or brochures of contemporary scientific problems. They discussed these problems, and since they were very different in character and temperament, there were always many things to discuss, which gave rise to new insights and new questions. Both had a great respect for science and held a firm belief in its sovereignty. It was typical for Kapteyn to have a strong sense of honesty. When in 1907 Frederick Cook2 said he had reached the North Pole and when later his scientific statements of how he climbed Mount McKinley proved to be false, many made fun of the gullibility with which the scientific world had accepted Cook’s story. Kapteyn opposed that attitude fiercely; he felt that it was only natural to a priori believe a scientist on his word of honor and not to consider the possibility of deception. The fraud itself was for him reason for deep indignation. After many years he would spark into anger about a physicist, who had proposed a theory, but when Kapteyn recommended him to read certain books, replied: ‘But you cannot expect me to collect data disproving my own theory!’ And of a well- known English biologist, whose articles had been found to be fraudulent, he said; ‘This is the only man I hate’. Twice in his life he personally came into contact with men who had been dishonest in science. He was upset about this, as it was totally impossible for him to comprehend how one may come to this. They never wanted to have anything to do with them, since they had failed the elementary rules of the code of honor and had lost the right to carry the torch of scientific research.

1Gerard Heymans (1857-1930) was a professor of philosophy and psychology in Groningen, founder of the first psychological laboratory in the Netherlands. 2The American physician and explorer Frederick Albert Cook’s (1865–1940) claims were challenged by Robert Edwin Peary (1855–1920),07.11.14 who is credited with being the first person to reach the North Pole in 1909. (adapted from a footnote of E.R. Paul) 60 J.C. KAPTEYN: HIS LIFE AND WORKS

Heymans and Kapteyn studied other books as well. They read the Novum Organum by Bacon3 and even Boswell’s4 Life of Johnson; this bulky volume that everyone knows but almost nobody has read. They read about Einstein’s5 relativity theory, that interested them much, but for which he felt he did not ‘become fully clear to him’. Prof. Ehrenfest6, the physicist from Leiden, offered to give a few lectures to them, for which he came to Groningen. Also philosophical articles were discussed, which Kapteyn did not appreciate much as a result of his strong taste for graphical realism, which prevented him from ‘coming into this’. An exception on this was exact psychological research, for which he held a lively interest. He followed Heymans’ own research in this area with a large interest and often Hey- mans could make use of his help in formulating his problems in a mathematical way. His writings about ‘Skew frequency curves’ and about ‘Correlation coefficients’ were written as a helping tool for the biologists in Groningen. This work, took him too much of his precious time according to Gill, who was an astronomer throughout. So he wrote in 1907; ‘Why a man of special astronomical gifts like yourself could waste his days in abstract math- ematical work which so many men are capable of working at – whilst they are so free to do what you can so well do – I don’t know. After all what is the value or interest in a frequency curve compared with the structure of the universe? I am glad to hear that you confess to a temporary possession by an evil spirit7 – Some form of exorcism is necessary – and I wish to administer it, if I can. I do think that in astronomy at the present time there is nothing comparable in interest with your work and the place you propose for its accomplishment.’ He also always was keen to have correspondence with teachers and others, who had imagined to have found errors in Newton’s theories or to have done important discoveries, although this always led to nothing but never made him impatient. Although Kapteyn did not have a real interest in pure philosophy as his nature was not one of deep reflection, he did whatever was needed to keep himself informed. For a year he followed a course by Prof. Bolland8, the famous philosopher from Leiden, not because his ideas appealed to him –to the contrary he rejected them since he felt they were unbalanced and wild in his opinion– but he wanted to know and try to understand why so many got carried away with it and admired it. For a while he was captured by Bolland’s eloquent speech, but too soon they became unmotivated outbursts and illogical turns, and his reckless dislike of those who had different convictions resulted in Kapteyn’s indignation. Level-headed as he was, this hot-headed and unconstrained temperament, this absolute subjectivity did strike him as unsympathetic. Scientific research should according to him, being used to think as a natural scientist from his early days on, be in the first place objective; without objectivity, he felt, science would became a ship without helm, from which nothing good and lasting would result, but could only confuse and blur. Lessing’s philosophy and classic tranquility appealed to him more and more. His wisdom resonated with him and he resolved that after his retirement, when he hoped to have more time to

3Francis Bacon (1561–1626) was a British philosopher, scientist and politician. 4James Boswell (1740–1795) was a Scottish lawyer and writer, especially known for his biography of the British author Samuel Johnson (1709–1784). 5Albert Einstein (1879-1955). 6Paul Ehrenfest (1880-1933) was an Austrian-Dutch professor of theoretical physics at the University of Leiden. 7The printed book here has ‘are coil spirit’; the text however seems to be ‘an evil spirit’ (as E.R. Paul also inferred. 07.11.14 8Gerardus Johannes Petrus Josephus Bolland (1854–1922) was a professor of philosophy in Leiden. A versatile life 61 devote to literature, he would study his work in detail. A few days before his death he ordered Lessing;s complete works. He still did see the beautiful volumes and he looked forward to reading them, but they were left unopened. In this way many other things that he wanted to do after retirement remained undone. Also Hudig9, a young neighbor who quickly became a friend, read to him regularly from Geological and historical publications and those evening were a great pleasure to both of them. Mrs. Kapteyn read him daily from novels that the had chosen together, and stories in bright and colorful magazines that amused them both. They also worked together often, since in the course of time it had become a normal routine for her to write his letters as he dictated them. And since his correspondence was very extensive that was a help of enormous value. And in this way the difficult years passed, until his eye problems disappeared and he himself became again master of his life. For many years he was president of the Scientific Chapter of the ‘Natuurkundig Genootschap te Groningen’10, that had as its primary aim to organize lectures and other presentations to keep its members up-to-date with developments in the rapidly changing scientific re- sults and views. To this end they invited prominent scientists to explain the results of their investigations. Kapteyn and his energetic secretary Hudig11, were able to find many important speakers to come and give a presentation. For example, they were able to or- ganize lectures by a.o. Eug`eneDubois12, the discoverer of the skull of Pithecanthropus erectus, and Dr. Tienemann13, the director of the Vogelwarte of Rossiten in the Kurische Nehrung, whose interesting studies were of much interest to the bird watcher Kapteyn. During his tenure of office the Society came to full prosperity. He held many presen- tations himself and never tired to keep scientific life in Groningen at a high level. After the lectures came the most important part: the gatherings at the Restaurant Willems14, where they talked further about the things they had learned and in this way exchange ideas. It was around his 60-th birthday that he started to attend a course about music in order to better appreciate this art. He was not gifted as a musician, did not play an instrument and did not have a good singing voice. He had his own manner of expressing himself using music, which at home was referred to as ‘trumpeting’. As soon as he started producing this sound, Mrs. Kapteyn could not resist going to the piano and accompanying him, which

9This was Joost Hudig, HHK’s later husband; see Preface. 10This is the Natural Sciences Society, now with the designation ‘Royal’ the ‘Koninklijk Natuurkundig Genootschap te Groningen’. It had already been founded in 1801. It has a website in Dutch (www.kng- groningen.nl). I am since 1998 chairman of the Board of this ‘KNG’. In Kapteyn’s days it had three departments, of which the Scientific Chapter was the most prominent one and is the only part surviving. Kapteyn was member of the Board of this Society from 1886 to 1904 and from the Scientific Chapter from 1904 to 1921. 11Again Joost Hudig, Kapteyn’s friend and later HHK’s second husband. He was not a member of the Board of the KNG. 12French anthropologist E. Dubois (1858–1941) was professor at the University of Amsterdam. (adaption of footnote by E.R. Paul) 13Johannes Tienemann (1863–1938) was a German ornithologist and a pioneer bird bander who established the Rossitten Bird Observatory, the world’s first, in Eastern Prussia. 14This restaurant was located in the center of Groningen at the address Heerestraat 52; this is still one of the busiest shopping areas of Groningen. In current times these gatherings are being held in building owned by the KNG at nearby07.11.14 Poelestraat 30, the ‘Concerthuis’, where a special room for this purpose exists, which is called the ‘Kapteyn Room’. See also ‘the Legacy’, p.30. 62 J.C. KAPTEYN: HIS LIFE AND WORKS was a very original effect. You could hear him coming home while singing and sometimes while working he would suddenly start singing, usually parts of sonatas or symphonies that he was familiar with. He ‘¡wanted to hear the same old pieces again and again, the new unknown ones having little appeal to him. But the art in itself, the depth of it, was a mystery for him; it filled him with a quiet, respectful awe and for an artist he felt the deepest admiration. He followed a course, given by Peter van Anrooy15, at that time the conductor of the Groningen orchestra, which interested him enormously. Every Wednesday evening he attended the concert of the orchestra in the ‘Harmonie’16, concentrating on the beauty of music and always felt enriched by it. His acquaintance with van Anrooy, which soon became a close friendship, took him closer to art. He found many parallels between science and art. Isn’t it true that both are in their ideal form unselfish and striving towards truth and purest expression? Oblivious to earthly fame and prosperity in order to give the highest that a person has to give? In that way he regarded art as the sister of science. One piece of beauty actually remained a mystery for him and that is poetry. Not that he was looking down on it with pity as so many intellectuals do, but he did not have the ability to appreciate poems. Make up your own mind about his only poem as his present to the world. It was the poem he wrote when initiated in the student society upon entering university. Now it has to be admitted that this period really is not the right time for high-level poetic expression, but his poem was undoubtedly the most unpoetical among those of his fellow students that went through initiation:17

‘The initiation poem is obligatory for students to be initiated, I notice. So it seems to be the wisest thing To simply accept this custom.’

Is anything less poetic possible? Still it is interesting, since it shows characteristically his logical philosophical mind! Epic poems did appeal to him, however. His favorite one was the poem of Waltharius18 with delightful primitiveness, of fighting and blood, of primitive people with primitive instincts. It moved him and roused his enthusiasm. Many found this incomprehensible about him, but it was his simplicity, this little piece of primitive character that he certainly possessed, that found satisfaction and expression in this thundering song of the superman with unrestrained passion. The beauty of life revealed itself to him in many ways: he had an unlimited ability to learn and enjoy. The first flowers and birds of the spring were a source of great happiness for him. When he had made a walk he always brought some daisies with short stems, that he put on the table, partly shyly and partly overjoyed. Every bird that he heard for the first time exalted him and the appearance of the first swallows was noted in the diary. ‘In my next incarnation I would like to be a swallow’, was his often heard wish. They seemed

15P. Van Anrooy (1879–1954) was appointed director of the orchestra at Groningen (1905), Arnhem (1910) and the ‘Residentie-orkest’ in the Hague (1917). (Footnote by E.R. Paul) 16The ‘Harmonie’ was a sorority in Groningen; it was housed in a major building in the center, which contained a well-known concert hall. 17Het groenversch is een eerstvereischte// voor groenen, zo merk ik/// Het is dus zeker wel het wijste// dat ik mij naar die gewoonte schik. 18A Latin poem07.11.14 of the ninth or tenth century dealing with a Germanic hero written by a Bavarian named Geraldus. (Footnote by E.R. Paul) A versatile life 63 to be to him the personification of carefree joy. Some other time he wished to be a dandy in a new life. The admiration for a neat appearance remained part of him. ‘There is a lot of self-confidence in an elegant appearance’, he often said. He never reached that ideal; the best he managed was to put on a clean collar every day. He was able to enjoy and laugh unrestrained and irresistibly, especially when he was telling an anecdote –it made no difference whether he told it for the first time or for the twentieth– he laughed so that tears came in his yes and even the most gloomy among them forgot his dark thoughts and laughed along, taken over by his talent and his genuine joy. A wise Frenchman once said: ‘The prudent one does not laugh, but only smiles’19. Kapteyn knew better. Dr. Easton20, the journalist-astronomer expressed this eloquently in his ‘Personal Rec- ollections of J.C. Kapteyn’ as: ‘This extraordinary man was even able to neutralize the oppressive influence of a new Rotterdam street under a dark and wet summer-sky by his not overly enthusiastic, but perfectly natural cheerfulness.’ He also did not disapprove of movie theaters, as was the norm in intellectual circles. He liked to go there with an open mind, looking for relaxation without looking for deeper values. Why always this destructive criticism of hopelessly superficiality and inferiority in life? Why did one have to cling so desperately to a life of superior values? Much more harmful is the deadly criticism that withers everything that it touches. One can after all leave the unbearable and sensational to the side, like one does with theater or literature. And then enjoy the unlimited technical and artistic possibilities that modern art offer. At a time there was funfair in Groningen where a tent had been erected on the Os- semarkt, close to our home.21 Every evening we heard the most interesting noises: pistol shots, joyful yelling, outrageous clapping and we could not resist to go and have a look and join the excitement. Seated in the first row my father and I watched a funny show of stupid detectives that were always making fools of themselves, creepy bandits that started to sing a long song ‘the birds of the night’ at the moment of supreme tension, and went through the most ridiculous and unlikely situations. And we laughed and clapped our hands over so much silliness and did sing this song of the birds at night for a long time afterward. Also in this we were shown the divine rhythm of the grand, versatile life for whoever was open to it. Life in Groningen went on calmly and evenly. Someone uninitiated and unfamiliar would not suspect that behind these quiet going-ons of things a slowly developing discovery was hiding, as had not occurred in its area since the days of Herschel. Young people that look at their own development as the only and most important thing, fail to notice the great things that are happening in their immediate surroundings. ‘Oh, for a life of emotions rather than of thought!’, I once cited Keats22 when I was still a young and enthusiastic child that has little experience. ‘My child, do you think that a life of thought does not

19Le sage ne rit pas, il sourit. 20Cornelis Easton (1864–1929) was a newspaper journalist and amateur astronomer, who published a number of papers in the Astrophysical Journal, most of which are studies of the Milky Way Galaxy. On the recommendation of Kapteyn, the University of Groningen in 1903 awarded Easton an honorary doctorate in physical sciences. 21The Kapteyns lived between 1910 and 1920 at Ossemarkt 6, just outside the central part of Groningen. See ‘the Legacy’, p.xiv-xvii for pictures and a tablet that was put there and revealed by some of his great- great-grandchildren. The name Ossemarkt is now spelled Ossemarkt, but I will keep the older spelling here. 07.11.14 22John Keats (1795–1821) was an English poet from the Romantic period. 64 J.C. KAPTEYN: HIS LIFE AND WORKS know any emotions?’ was his grave answer. No argumentation or presenting other points of view, just this calm assuredness, which made a lasting impression. That was his great strength and I realized for the first time that behind this world of numbers and instruments there existed a world of greatness and a deep emotion of a creator of everlasting values.

07.11.14 Star Streams and Selected Areas 65

Chapter 16

STAR STREAMS AND SELECTED AREAS

His research was directed towards the structure of the stellar system. For this he needed to know the density of stars at various positions in the universe. And that could only be determined after he had derived the distance that they were away from us for all kinds of groups of stars. He felt that this would be possible by using the motions of stars on the sky, which would be on average less if the distances of those stars would be larger. But this is only true when these motions are distributed at random, without any preference for any directions; his method was based on this assumption. In the course of his research it became clear that this assumption was incorrect. Therefore his method could not be used and his studies would become a failure. But this changed into a success when he decided to study the reason why this hypothesis of random motions was wrong and he came to his most important discovery of his career: the two Star Streams.1 The distribution of the stellar motions can be seen best when one imagines that all stars in a certain area on the sky are grouped together in one point in the middle and then indicate their proper motions with arrows, that point away from this point in all directions. If the peculiar motions would be distributed completely at random and would one add only the influence of the motion of the solar system, then all the arrows that point away from the apex would be largest in number and largest in size and those in the direction towards the apex more rare and smaller, but would be distributed symmetric to the left and right with respect to this direction.2 In reality the distribution of the motions in most parts of the sky showed a much more irregular distribution; there was not a single preferred direction (away from the apex), but the arrows clustered in two different, not opposite directions. When one would draw on a celestial globe for each point of the sky those directions in which the motions were most

1HHK here refers in a footnote to a paper in Dutch by A. Pannekoek, ‘J.C. Kapteyn and his astronomical work’, published in Wetenschappelijke Bladen 1922. She probably has taken parts of the text that follows from this publication. 2This explanation is not very clearly presented. What she tries to say is the following. If stars have only random motions their motions projected on the sky, represented by arrows that indicate their direction and velocity, these would be distributed randomly. If the stars are stationary but have a systematic motion with respect to the solar system then this reflects in a systematic pattern in the arrows, such that they in general terms point away from the direction we move in with respect to the stars and towards the opposite direction (the apex), and07.11.14 stars further away show less motion on the sky then nearer ones. The actual situation would be a superposition of the two. 66 J.C. KAPTEYN: HIS LIFE AND WORKS numerous, then one found that these pointed towards two points of convergence, one in the constellation Orion, the other in Sagittarius, roughly 140 degrees apart. So, while it had been assumed there was only one (apparent) stream of stars away from the apex, it now appeared that in reality there were two such streams. When the stars are actually organized in two streams, both need to be in opposite directions; this is so because the only stable point that can act as a point of reference for both streams must be the center of gravity of both, since after all they encompass the whole universe. That the directions of the two streams are not opposite, but are at an angle of 140 degrees from each other, is the result of the motion of the solar system with respect to that same center of gravity. From the apparent points of convergence one can then find both the motion of the sun as well as the in reality opposite directions of streaming and the positions on the sky, where they are pointing towards.3 For the latter Kapteyn found the points on the sky Right Ascension 91◦, Declination 13◦ north, and Right Ascension 271◦, Declination 13◦ south4. With this discovery, that Eddington recently listed as one of the six most important astronomical discoveries since the turn of the century, all difficulties and contradictions, that until then appeared in the study of proper motions5 and the solar motion, disappeared. Kapteyn worked out the hypothesis that the system of stars would have been formed when two previously separate and remote systems of stars merged. Since both the directions of these streams as well as the direction of motion for a few smaller systems (as the group in Ursa Major, the Hyades and the Perseus Group) all were lying in the plane of the Milky Way, he supposed that the wide extent of our stellar system in the Milky Way plane was a result of the merging of these streams. It is no surprise that such a discovery would give rise to a revolution in the ideas concerning the structure of the sidereal system. But more than ever did Kapteyn feel the urge to collect more material, facts and measurements, that would be the basis for further research. Herschel had built a giant telescope and had collected with this more material than had ever been available. Kapteyn however had his mind set on other sources and his aim was to obtain the cooperation of the large observatories with their enormous instruments and resources. In order to accomplish this he drew up a plan of work that he would submit to some leading astronomers and then put before the whole astronomical world. That was his famous ‘Plan of Selected Areas’. The main points in it were: 1. Accurate photometric determination of the magnitude6 for all stars. 2. Accurate counts of the numbers of stars per square degree, separately for stars of magnitude 1, 2 to 14. 3. Rough determination of the positions of the stars. 4. Proper motions. 5. Parallaxes.7

3This explanation is dated and inconsistent. The correct interpretation of the two star streams has proved to be the fact that motions in space are not random in all directions (isotropic), but are larger in some directions than in others. A full understanding of this phenomenon has appeared as part of the discovery of the rotation of the Galaxy. 4Right Ascension and Declination are astronomical coordinates on the celestial globe, similar to respectively geographical longitude and latitude. 5The motion of a star on the sky, expressed usually in seconds of arc per year, are called proper motions. 6This is the astronomer’s term indicating apparent brightness. magnitude 1 being the brightest and each magnitude a factor about 2.5 fainter. Five steps or magnitudes corresponds to a factor 100. 7Due to the motion of the earth around the sun the position of each star on the sky describes an ellipse on the sky; its major07.11.14 axis being a measure for the star’s distance. The maximum displacement is called the parallax. Star Streams and Selected Areas 67

6. The type of spectrum. 7. The radial velocity. 8. Brightness of the background sky for various parts of the celestial sphere. Since this research over the full sky would be enormous, he decided to distribute areas over the sky in a regular way, from which the name Selected Areas came. They numbered 206, each the size of a photographic exposure, for which all then possible information would be collected from the brightest to the faintest stars possible. The discovery of the two star streams was first presented at St. Louis in September 1904. There the great World Exhibition was held and Kapteyn had been invited to make a presentation, which he gladly accepted. The chance to meet his brothers on the other side of the ocean and their observatories where such beautiful work was being done, was very attractive to him. In addition he hoped to find support for cooperation in his grand plan. Mrs. Kapteyn accompanied him. Their departure was characteristic for these unassum- ing people. They left towards the end of August, together traveling by bike from their second home in Vries to the train station in Groningen to take the long-distance train to Rotterdam. The children were sitting on the fence and waved them goodbye. That way their first major trip to America started. Kapteyn to conquer the astronomical world, Mrs. Kapteyn to make new friends in these remote places in whom she would find people of equal hearts. Upon arrival in St. Louis they were welcomed by the consul of the Nether- lands, who had to put them up in primitive wooden buildings, since nothing else was available. They had arrived just in time to be present at the opening of the astronomical conference and they sat quietly in the back of the auditorium. Newcomb, the president, had noted them and came immediately to Kapteyn to take him to the table with the Board members where the important astronomers were gathered. He was welcomed warmly and respectfully by everyone and felt immediately admitted as a welcome and honored guest. Newcomb, who did not know yet about his new discovery, introduced him as follows: ‘Prof. Kapteyn will tell us something about his interesting Durchmusterung work’. So it was a surprise when it turned out that his lecture was about a completely different subject. They were very impressed and he was happy with the interest in his discovery, but the most important part of his trip to America was the fact that he made the acquaintance of Prof. George Ellery Hale, at that time the director of Yerkes Observatory. At the same time there was an international conference in St. Louis ‘on solar research’ at which Kapteyn and Prof. Julius8 from Utrecht, were the delegates from the Nether- lands. Hale presided over the conference. Kapteyn showed him his Plan of Selected Areas. The meeting took place on the grounds of the exhibition in the Tyrolean Alps, a artifi- cial scenery set up on grandiose American scale. They talked all afternoon, absorbed by their deliberations and unconcerned with the natural beauty(!) around them. Hale was enthusiastic and would later prove to provide important support to make the plan reality. Kapteyn was deeply impressed by the wide views and the sharp mind of this American, who was about to become the leader of the biggest and best equipped observatory in the world, the Mount Wilson Observatory in California, that was nearing completion. Support from this man was of utmost importance to him. Hale wrote later about this first acquaintance in a letter with ‘Reflections of Kapteyn’: ‘As chairman of the Academy’s Committee international cooperation, and convinced that much

8The Dutch solar physicist07.11.14 W.H. Julius (1860–1925) was professor of physics at the University of Utrecht. (Footnote by E.R. Paul) 68 J.C. KAPTEYN: HIS LIFE AND WORKS

Figure 16.1 George E. Hale.

might be accomplished through joint effort. I was therefore greatly impressed by Kapteyn’s scheme of Selected Areas, which he presented at the International Scientific Congress, held in conjunction with the St. Louis Exposition. My own experience had been in the field of astrophysical research,07.11.14 and my plans for the then nascent Mount Wilson observatory were chiefly confined to an attack upon the physical problems, involved in the study of the sun as a Star Streams and Selected Areas 69 typical star. Researches on the distribution of stars in space did not then enter into the scheme. However as I listened to Kapteyn’s masterly paper and realized the wide scope of his plans and the skill with which he availed himself of international cooperation in assuring their execution, I was deeply impressed by his appeal. Could we not help him to secure the data, needed for the fainter stars and at the same time broaden and strengthen the attack in our own problem of stellar evolution? The answer is obvious to-day, but at that time, approaching the subject along the path marked out by Huggins9, Lockyer10, and other pioneers of solar and stellar physics, and seriously hampered by lack of funds, the case was not so clear. Nevertheless the genius of Kapteyn and the personal charm which brought him to the unqualified support of astronomers the world over, convinced me at once that the Mount Wilson Observatory ought to profit by his cooperation as soon as circumstances might permit.’ And further: ‘A powerfully creative imagination, glowing with optimism and enthusiasm is prone to set itself too vast a task. But Kapteyn, though he would gladly have measured all the stars of heaven, recognized the necessity of limiting his endeavor. Hence the Plan of Selected Areas, and the successful appeal for international cooperation.’ On their way back they visited Newcomb in Washington, where they also attended the big reception at the White House. They arrived in Newcomb’s carriage with two horses and a black groom, and were introduced to the President Roosevelt11 together with Hugo de Vries12, who happened to be in Washington at the same time. The President had a suitable word and a handshake for everybody. ‘Ah Mr. de Vries, I suppose we are cousins, because the name de Vries is in my family.’ This of course with pride as Dutch names meant distinction in America. Kapteyn was greeted with a friendly remark about astronomy, and this remarkable moment was history. In February of the year 1905 the plan was sent to 25 astronomers who were ‘best in a position to cooperate in its execution’. It was well received. Wit a few exceptions they were willing to give their cooperation to it. In the summer of that year Kapteyn traveled to South Africa, at the invitation of the British Association that held its large international meeting there. De Sitter, who at the time was his assistant, accompanied him. A few other astronomers also traveled with him: Backlund13, the director of the Pulkova Observatory (Russia), Donner, Hinks14 and Cook- son15, both English astronomers. They discussed a lot during the traveling and established for the duration of the trip on the sea the ‘Astronomical Society of the Atlantic’. A ma- jor subject of their discussions was the Plan of Selected Areas, of which all of them had brought a copy. Hinks produced a protocol of what was proposed, and Kapteyn could be

9W. Huggins (1824–1910) was a pioneer astrophysicist who made spectroscopic observations of stars, comets and novae. (Footnote by E.R. Paul) He was the first to realize that some nebulae like the Orion Nebula were gaseous. 10J.N. Lockyer (1836–1920) was a pioneer spectroscopist who discovered how to make spectroscopic ob- servations of solar prominences in full sunlight. (Footnote by E.R. Paul) 11Theodore Roosevelt (1858–1919) was President of the United States from 1901 to 1909. 12H. de Vries (1848–1935) was a plant physiologist and geneticist at the University of Amsterdam from 1878 to 1918. During the 1890s he rediscover Mendel’s law of inheritance, which he, along with C.F.J.E Correns (1864–1933) and E. Tschermak (1871–1962), announced independently in 1900. (Footnote by E.R. Paul) 13J¨ons (or Johan) Oskar Backlund (1846–1916) was a Swedish-Russian astronomer, who specialized in celestial mechanics. He was director of the Pulkova Observatory in St. Petersburg. 14Arthur R. Hinks (1873–1945) was a lecturer at the University of London. (Footnote by E.R. Paul) 15Before his untimely death07.11.14 B. (Bryan) Cookson (1974–1909) had been trained as an astronomer. (Footnote by E.R. Paul) 70 J.C. KAPTEYN: HIS LIFE AND WORKS satisfied with the outcome of their talks. All of them wanted to have a part in the work. In South Africa he would win even more participants for his plans. It had been an old wish for him to see Gill in his own observatory, since you only get to know a man completely when you see him in his working quarters. With admiration he saw the observatory that Gill had brought to fruition, and he saw his friend as the central person, as President of the British Association and host. He was the radiant center of attention, flexible and sociable, full of attention and untiring. Gill did not know how to fulfill his high position with dignity. One of his friends said to him, that he should not forget he was the president and ‘be on his dignity’. ‘That is just what my brother said to me’, was his reply. ‘Davie’, he said, ‘you’ve no more dignity than a duck.’ And he remained his amiable and jovial self. Kapteyn returned home happy and satisfied. He had seen and admired Gill’s observa- tory, had seen his friend in full glory and had made major progress with his own plans. He had seen the land with which the Dutch felt so related to in all its beauty, since the Association had organized a few excursions that aroused his interest. He was delighted by the abundant flora. Fields full of arums had made a strong impression. The Zambezi Falls he felt were less impressive, but more mysterious than Niagara Falls. The Southern Cross, the much praised constellation, had disappointed him and did not live up to his expectations. In general he felt that the starry sky of the northern hemisphere was more beautiful, although seeing Scorpius in the zenith was magnificent. He was most touched by the grave of Rhodes16, that as a result of its greatness made a deep impression. Only a large, flat stone with the name Rodes chiseled in it, among the majestic emptiness of the Matoppo hills in Matabeleland, an impressive reminder of his victory over the Matabele king Lobengula. He met a few South Afrikaners of distinction and heard many interesting facts about the country after the war. So in all respects the trip had been interesting and successful. After he returned home he immediately set to work to define he plan in more detail and distribute it to a wider audience. In February of 1906 the Plan of Selected Areas was send out into the world with an appendix containing the proposals of astronomers that had already confirmed their participation. When in February 1907 Hale had definitely confirmed his participation, the success of the project was assured. Kapteyn wrote to Prof. Rambaud17 in Oxford shortly thereafter: ‘The plan is making its way splendidly. Quite recently I got proposals from Hale at Mount Wilson, which, if we get to some agreement, will nearly bring us to the point that everything that can be done with the present resources of astronomy, will be done.’ Many had their remarks or suggestions. For example Pickering, the director of Harvard Observatory, proposed to investigate 46 additional special areas, that showed peculiarities, such as unusual clumpiness or emptiness, mostly in the Milky Way. These were added to the Plan. Others had other proposals. Kapteyn was able to accept criticisms with tact and modesty, leaving the proposers in their right and worth, while still quietly urging them to accept his own ideas. When a human possesses genius, character and charm he can move mountains. He was able to get things accepted the way he wanted and soon the work was in full progress.

16Sir Cecil Rhodes (1853–1902), businessman and politician, founded the state of Rhodesia, the present Zimbabwe. 17Arthur Alcock07.11.14 Rambaud (1859–1923) was Irish Astronomer Royal and later director of the observatory at Oxford University. (Footnote by E.R. Paul) Star Streams and Selected Areas 71

The undertaking was however so gigantic, that it was impossible for Kapteyn to take the leadership and coordination by himself. He proposed to set up a committee and wrote about this among others on July 22 1907 to Prof. Kustner¨ 18, the director of the Obser- vatory at Bonn: ‘The coordination in my Plan of Selected Areas is now so much that the responsibility threatens to become so heavy for me that it is no longer possible for a single human being to bear the full oversight of the whole project and the coordination of the ar- rangements. Gill has insisted for a long time that a conference be organized. I have declared myself strongly against that. Almost all matters, yes, really all of them, are in good hands. Maybe a conference will be too unwieldy, and will in the end not result in better plans. We have extensive experience with this.’ 19 So he asked Sir David Gill, Prof. Edw. C. Pickering, Prof. George E. Hale, Prof. K, Kustner,¨ Prof. Schwarzschild, Sir Frank Dyson and W. S. Adams20, all men that together represented the astronomical world of those days, to serve on a committee, which each of them accepted. The last twenty years there has been much work in all countries in the world on this great project and already significant results have been obtained. Kapteyn became rather well-known in America. Honor upon honor was bestowed upon him. But the crown on this was, that in June 1907 Hale proposed that he come and work at Mount Wilson for a few months every year to lend his cooperation and provide leadership. In the year 1905 the Solar Observatory at Mount Wilson, the 6000 feet high mountain near Pasadena (California) had been opened, which was the most grandiose and best equipped observatory in the world. Carnegie21, the well-known American millionaire and maecenas, had provided the funds, under the condition that everything would be first class. So it was a wonder of perfection. During two years tests were performed to find the best suitable climate, during the construction money and effort were not spared, and a 100 inch telescope –that is to say a telescope with a mirror22 with a diameter of 100 inch (254 cm)– would be the crown jewel. The best of the American astronomers became the director. No wonder that Kapteyn accepted the proposal with joy. To a letter of Hale, that expressed some reservations about the collaboration, he replied without a trace of scruple: As to such questions as the credit which each of the cooperators will get for the work, I have not the slightest fear of any disagreement and think we need lose no words about it. I never had the slightest disagreement with Gill, with whom I have worked for so

18Karl Friedrich Kustner¨ (1856–1936) was professor of astronomy at the University of Bonn. (Footnote by E.R. Paul) 19Die Mitwirkung an meinem Plan of Selected Areas ist schon jetzt eine so grosse, dass die Verant- wortlichkeit fur mich eine druckende¨ zu werden droht und es jedenfalls nicht gut ist, dass die Ubersicht¨ uber¨ das Ganze und das Arrangement desselben ausschliesslich bei einem einzigen Sterblichen beruht, Gill hat schon lange darauf gedrungen, dass eine Conferenz beisammen gerufen wurde. Ich habe mich aber mit alle Kraft dagegen erkl¨art. Fast alle Gegenst¨ande, ja eigentlich alle sind jetzt in den besten H¨anden. M¨oglich wurde¨ eine Conferenz zu grossartigen, aber auch nie zu Ende fuhrenden¨ Pl¨anen leiten. Wir haben mannigfache Erfahrung auf diesem Gebiete. 20The American astronomer Walter Sydney Adams (1876–1956) was an Assistant to George Ellery Hale at the Yerkes Observatory (1901–1905) and at Mount Wilson Observatory (1905–1946) where he became director from 1923 to 1946 following Hale’s retirement. (Footnote by E.R. Paul) 21A Scotsman by birth and an American by choice, Andrew Carnegie (1835–1919) made his fortune in the American iron and steel industry. A philanthropist he used his fortune to establish educational and research facilities, including the Carnegie Institution of Washington (1902). (Footnote by E.R. Paul) 22HHK here erroneously used07.11.14 the word ‘lens’; the telescope concerned was really a reflector with a primary mirror. 72 J.C. KAPTEYN: HIS LIFE AND WORKS many years. I felt proud to work with him for a great end. I feel proud that now again I may work with a man as you for still greater ends.” Hale now assured him of his cooperation, in the form that the Carnegie Institution appointed him as Research Assistant, for which he would have to be at Mount Wilson for three months per year. It may be seen as strange that in his own country there was so little interest in him and awareness of Kapteyn’s importance, while he had a high reputation worldwide. When he requested during this period a stereo-comparator from the government, the Parliament took this as an opportunity to ask the question ‘whether the Groningen Laboratory really is not too much in addition to the two observatories in the country’. He was asked to explain the raison d’etre of his laboratory. This seems ridiculous, considering the major leading role that the laboratory was already then playing in the astronomical world. Really great and unselfish as Kapteyn was, he was not offended by this indifference and lack of interest. He was not looking for success or fame, just for the financial means to continue his work. So he provided a long explanation. The year after that the stereo-comparator was allocated; probably they were informed of his worldwide fame.

07.11.14 At home 73

Chapter 17

AT HOME

The children had grown up by now and all three of them were doing academic studies. The two girls were among the first female students. The oldest had chosen for a medicine and I law as subjects, which gave rise to much criticism in these turbulent days of the fight for woman rights, but Kapteyn felt that females studying at universities was so natural and unquestionable, that one did not get far with counter arguments. The son went to Freiburg in Saxony to study mining engineering. It was comfortable and colorful in the large upper level house they occupied in the Heerenstraat, where Mr. and Mrs. Kapteyn were a prime example of hospitality. The atmosphere was pleasant and free, people were received simple but well; they were sur- rounded by youth, music and singing. Plans were made for the future, problems youngsters encountered were discussed, council and comfort were sought and given. The father was ever present when there was a need to solve a problem or a judgment required. He was the councilor and friend, versatile and familiar with all matters. He was not like other great scientists, whose special studies became in the way of their interests in other per- sons. ‘What an unusual man’, a simple woman, who had visited the Kapteyn family with some timidity, remarked, ‘He was so normal and had real interest in me, while I am not anything special.’ Every human being was special for him and nobody ever was given this onerous feeling of inferiority that make many great people so unpleasant. And in the end this does most harm to themselves. In 1905 a revolution took place in the Kapteyn household. When the oldest daughter had married, the second one studied in Amsterdam and the son studied in Freiburg, the house became too large and they decided to look for a smaller place to live. Mrs. Kapteyn, who had developed a lot of sympathy for her American ‘sisters’ and their regiment of self- help during her short visit to America, proposed to her husband to break with the Dutch convention and stop hiring a maid and having a working woman for help during the morning hours. She was the first in her circles to take this bold step, and her husband fully agreed with it, as long as the work would not be too much for her. They rented a small first-floor house on the Eemskanaal on the outer edge of town, the maids were sent away and a working woman was hired. Mrs. Kapteyn did the cooking herself and joined in with the household work. The Groningers found this not-done, but she enjoyed her ability to do the work and the financial help this provided for her husband. What would they care about the opinion of others? In all respects they were ahead of their times, what is common today was then still out of the question. And their lives flourished in fullness and happiness. 07.11.14 74 J.C. KAPTEYN: HIS LIFE AND WORKS

They lived in this small place for 5 years, very happy with their simple but active lives. Mrs. Kapteyn hated washing the dishes most of all, a necessary evil, until she found a magic formula that made it a pleasant activity. Every evening she put the problem before her to find a way to do it in the quickest way possible, and every day she became more skillful at it and quicker and happier. Great minds think alike1, since it were Kapteyn’s own words: ‘Fortunately, nature has been designed in such a manner, that every job, even the most unpleasant one will eventually develop its own charm. The most boring routine calculations (and nothing in the world is more boring than routine computing) will have its appeal when one has developed a certain aptitude in it.’ In this manner everyone did his boring, routine computing with a happy cheerfulness. After 5 years they moved to a larger, old upper house at the Ossemarkt. This was a house ‘with a soul’. People who visited it did never forget it. Less beautiful, but very special was the entrance; a narrow alley like a cleft between two tall houses and a very humble door with an old-fashioned doorbell, that however was seldom used, since friends and acquaintances could open the door themselves and were always able to let themselves in. The beautiful, old oak staircase led to a lobby unto which large rooms opened, with a serenity and peacefulness that felt beneficent. The spacious living room with its old carpentry, antique wallpaper and deep windowsills had a view through the high windows with the orthodox Dutch half-drawn curtains towards the quiet, grand square. Kapteyn’s second laboratory, the former meteorological institute, that had been given to him when the old Commissioner’s house had been put back to its real use, could be seen lying on the other side of the bridge with its friendly old style and bricked-up archway. They lived in this house, that was so fitting for them and was adored by everyone, for 8 happy years.

07.11.14 1Les grand esprits se rencontrent. America 75

Chapter 18

AMERICA

In October 1908 Kapteyn traveled to Mount Wilson for the first time with a three months leave-of-absence from the government. Not long before that his son, who had finished his studies, had left for America to test his luck as a mining engineer. Kapteyn saw him in Denver on his way out and on the way back. On many later travels too the parents would see their son in distant America, a piece of luck not many parents had. This first trip, however, did not give him satisfaction in all respects. Without his wife he felt a half person. She was the radiant and easy one, who took advantage of every opportunity with her carefree self-confidence and her playful openness. He was timid and like when he was a youth, admired the easy going manners of others. Characteristically he wrote his daughter about this in 1913, when he once more made a trip by himself: ‘On board ship. How do I go about getting to know people? I don’t have that ability. Mother is much better at that and many times she was the one that broke the ice. Now that I am by myself, I feel doubly this weakness. I just had a hard lesson. I was walking on the deck with the safety vessels. I saw a man that had drawn my attention because of his pleasant appearance. When he saw me, I came straight at me and said: ‘Mister Kapteyn, may I introduce myself, my name is Biesterman’ (or something like that), a grain merchant from Rotterdam. See, simple as good morning1. I think I always worry about a ‘rebuff’. ‘It won’t do to be too sensitive in life.’ We people people of the wisdom from books always run that risk. People that are always among other people, on the otherhand, at their office or traveling, loose that unnecessarily deep sensitivity and obtain a expansive readiness and become easy going, which I always envy. Although it seems to me that in order to get far in arts, like van Dijk2 says, one has to take position at the window of the soul. He that lives much in himself and who always keeps himself occupied with things that are far from every-day life, it seems to me, would never feel at home in this every-day life then others. Now I would miss this inner life, but still... instinctively my heart is longing for the other way. If, later, we would travel the world for a full year, daily meet other people and see other things, every day need to adapt to a new environment, then we will become real ‘people of the world’3, but then would be 70 years old.’ The basis of course goes deeper and he was certainly aware of that. One does not become a ‘Weltmensch’ simply on the basis of external circumstances. After all, Mrs. Kapteyn was the easiest of persons and did not have other chances in her quiet, simple life. Now they

1Simple comme bonjour. 2A professor of theology07.11.14 in Groningen at that time (Footnote by HHK.) 3‘Weltmenschen’. 76 J.C. KAPTEYN: HIS LIFE AND WORKS

Figure 18.1 On board the ship on the way to the United States.

complemented each other in a beautiful way: together they experienced the fullness of life. She used to clear obstacles out of the way for him, so that he could do the first step introducing themselves and in this way they made good friends everywhere. Further steps were taken by others, who got to know him better and immediately started to like him. In that way their travels were full of interesting experiences and lasting friendships. To his disappointment during this first year of his stay at Mount Wilson the large instrument was07.11.14 not completed, and Hale was too much preoccupied with his research on the sun to spend much time on that of the stars. ‘That way I have little to keep myself busy. America 77

I do more or less the same work I would have done at home. Not very satisfying, but that will be better next time.’ But then it was very nice on the mountain. Sunshine every day during November, which is the most chilly and unpleasant month in the Dutch year. He loved the beauty of the landscape with his constant interest in nature. The 2000 meters high mountain was like a enormous castle high above the Pacific, the clouds being a sea of fog at his feet. ‘The view in the evening over the valley and behind that the great Pacific is often idyllic in its beauty. All features in the wrinkled valley become softer in a indescribable manner in the evening fog, the colors above that being in magnificent warm harmony with that.’, he wrote full of marvel. He enjoyed much more of the life up there when the following year Mrs. Kapteyn accompanied him to America. On their way out and back they visited Gill in London, who would then organize an astronomical gathering, so that Kapteyn kept in touch with his friend and his English colleagues. ‘We rejoiced to hear again the familiar guttural exclamations and quaint expressions, as with youthful mind and enthusiasm he unfolded his latest ideas’, Eddington wrote about these meetings with Kapteyn. Everywhere on his travels through America he was greeted as a very welcome guest. The visited most observatories: Harvard, Yerkes, Princeton, Albany, Newhaven(Yale), Alleghany, etc. and their trip was a real triumph. For Kapteyn this annual meeting with American astronomers was of utmost importance. America had been for some time the leading nation in astronomy, as a result of the extent of her resources, of the enormous work spirit and energy of its astronomers and also of its openness towards problems, free of the limitations imposed by traditions. This young nation developed an energy and fresh strength that was unknown to the older Europe and which captivated the ever young and enthusiastic Kapteyn. Every time they stayed for a few days with Prof. Pickering at Harvard Observatory. Kapteyn admired his enormous working capabilities and his bringing together of an im- mense amount of data, that, although of limited accuracy, earned him a great name in the astronomical world because of its overwhelming amount. In 1912 Pickering proposed to Kapteyn that they should produce jointly a northern Durchmusterung. This would be a catalog of all stars in the northern sky from declination 20 to the North Pole, down to about 14th magnitude. Kapteyn was not enthusiastic, but Pickering insisted; he would do three quarters of the work and provide all costs. After much careful consideration Kapteyn decided to agree to the collaboration. He knew the work would be useful and that with the drive and resources of Pickering it would not take long to be successful. Hale was not very happy with this collaboration. ‘The value of he work is of course obvious enough, but a man of your ability ought not to be compelled to devote time and attention to such a piece of routine. The more opportunity you have for thought on the larger phases of astronomical work, the more will astronomy benefit through the extra-ordinary range of your imaginative power.... It seems such a pity that you should be merely collecting material for somebody else, incalculably less able than yourself.’ Kapteyn’s reply was: ‘There is a sort of fate which makes me do all my life long just what I want to do least of all. – The making of the Durchmusterung has no attraction whatever for me, but in what I have tried to do in the direction that has a true attraction for me, I have always been hindered by want of suitable material. So if –after I hope not too small a number of years– I come07.11.14 to die, I will probably leave behind me more Durchmusterung work 78 J.C. KAPTEYN: HIS LIFE AND WORKS

Figure 18.2 Mount Wilson Observatory, California. and bringing together of material than almost anybody – leaving it to the next generation to do the real work that I hoped and longed to do. Well, when you don’t have what you love, you have to love what you have4.’ He hoped that apart form the benefit that the work would have for science, there was another potential gain. For this large amount of work he would have to hire more working force, and he hoped that these would be permanently employed at the laboratory when the project was done. ‘By that time I will be too old to profit very much by such an extension, but of course I hold the future success of the laboratory much at heart.’ The collaboration proved to be very difficult. After the first few weeks which had to be spent on organizing the project, Kapteyn did not have to spend much time on it, since it could be done by his assistants, but Pickering was not like Gill or Hale, and after a year he regretted that he had agreed to the undertaking. The problems resulting from difference of opinion in ‘matters astronomical;’ became after a while so great they they had to abandon the project before it led to any conclusion.

Now that the Kapteyns went to America every summer the house in Vries had to be given up and an end came to the wonderful summers near the heather fields of the province

4Quand on n’a pas07.11.14 ce qu’on aime, it faut aimer ce qu’on a. This has been attibuted to Roger de Rabutin, Comte de Bussy (1618-1693) America 79 of Drenthe. The family said goodbye with much sorrow to have to leave such a beautiful thing behind. The mayor and his wife has come to say farewell and had expressed the hope that they would see them many more times. ‘My wife and I have discussed and concluded’, he said, ‘that we do not know anyone else who puts a modest income to such good use and who gets so much happiness for it as you.’ This remark made them very happy, since they had talked much with him about their lives and aspirations. Kapteyn indeed did know how to live in such a manner; he did not know how to increase his financial wealth, but he did know how to put it to use for things that had real value and those fruits were more important than increasing their wealth. The first summer at Mount Wilson they stayed in a tent, as the observatory did not provide any lodging facilities for couples. The house where astronomers stayed was called the ‘Monastery’. This was so, because it only had small rooms for astronomers that were working at the telescopes, which they did in turns, while those not observing worked in Pasadena at the Solar Office. Their families lived in Pasadena. This was done to avoid the difficulties that arise in a community of people living to close together, which many found unpleasant. The arrangement worked excellently and provided a peaceful atmosphere that is necessary for serious and concentrated work. For the Kapteyns however it was not very practical. Living in a tent had many disadvantages, although they accepted these with a wise optimism. It was small and not very comfortable. They spent the days in the open air, which was easily possible with the warm and reliable climate. Tables, chairs, books, everything was outside. And there they worked, although they were bothered by small flies that sometimes disturbed the idyllic setting; these two human beings, being relocated from a comfortable upper house in a city to live in pure natural circumstances, quickly adapted to be happy. Next year a major surprise was awaiting them. Upon arrival they found a small wooden house that Hale had let decorated with all sorts of comfort. Everyone at Mount Wilson had contributed. And they had asked themselves: ‘Now, if it were to be yours, how would you like this or that to be done?’. In so with the help of all this became a small jewel in all its simplicity. The Kapteyn Cottagee as it is known up to the present day, became their American home for many years and it became as dear to them as their home in the Netherlands. It was erected at a beautiful spot, in the shadows of knotted oaks and centuries old pine trees, and large yuccas were growing around the house as enormous bouquets. The view as magnificent, stretching far over the mountains and the canyon, the deepest ravines of Western America, over the valley with its many towns that enhanced the feeling of peace high up there at night like brilliant star clusters, but which also gave the lonely feeling that the richness of human life was close at hand. And in the distance they could see the majestic waters of the Pacific Ocean. The peaceful quiet, that reigned there far from the hectic world, was doubly dear to them after the exhausting and emotional trip. Often deer came to look at them with curiosity, squirrels ran silently from tree to tree, and the chickadees, the tits of the West, were singing all day. The cool nights were wonderful and Mrs. Kapteyn put her bed on the porch that stretched the length of the house. One night she saw something dark on the bed, which turned out to be a nest of young squirrels for which the mother had chosen a soft place to give birth to. Idyllic but also a little bit creepy. She took up another place to sleep that night.

Kapteyn missed the07.11.14 heather fields that he loved so much and he decided to import these. The next year he traveled with a small box filled with heather as if it were his 80 J.C. KAPTEYN: HIS LIFE AND WORKS

Figure 18.3 The Kapteyn Cottage at Mount Wilson.

most valuable piece of luggage, which held his attention most closely. One time it was left behind in an compartment that unexpectedly was being uncoupled from the train and he telegraphed everywhere and it was returned to him. It reached the mountain unharmed and was planted near the house, but the heather did not catch on in spite of all love and care. So this wish had to be given up. Many came to visit the Kapteyn Cottage and were welcomed heartily. Kapteyn had a good advice for everybody and had an interesting conversation with them, and Mrs. K. had already won the trust of many, and so it was a rich life for them. The mountain air was for him like a elixir of life that gave him double the strength and desire to work. He had no constitution for the climate of the Low Countries that made him tired and depressive. It was a suffering for him that he fought by the use of quinine. In the mountain air however, he felt a different person with a much larger vitality and unlimited power. ‘I feel a youthful desire awakening to clamber around in the mountains, descent into the canyons, and since I cannot give in to that as pater familias5, I have recently tried my strength on a trip well-known as strenuous, following steep, poorly kept mountain paths.’, he wrote from there. With the Americans he felt at home. The lack of conventions and the simplicity of the Kapteyns fitted well with the open-minded, uncritical people of the far West, that were not burdened with centuries old commitments, conventions and sentimentalities. So they were always received with much joy when they arrived. ‘We are glad to see you folks again!’, all said07.11.14 with happiness in their eyes and warmth in their handshake. 5Head of the family. America 81

In 1913 Kapteyn wrote to his friend Boussevain: ‘I have had a wonderful, very moving time in America. Parties and parties and parties. Speeches and trumpet sounds. Vanitas vanitatum.6 And, yes, the American is much more unprejudiced and cheerful than we are. And I have found here much cordiality.’ No wonder everyone was impressed by his personality. Barnard7, astronomer at Yerkes, wrote him in September 1913: ‘I simply wish to tell you, how much we enjoyed the short stay you made at the Yerkes Observatory, It is a great pleasure to see you here, and it always leaves behind a recollection that makes me feel good for a long time afterward.’ At all observatories that he visited he was asked for his advice and explanations, help with the definition of work programs, judgment on the use of instruments, and choice of possible hires. He enjoyed the expeditiousness and efficiency of the Americans, which were almost unbelievable by European standards. If early in the morning on Mount Wilson he had an idea, he would send a list to the Solar Office with details of stars for which he needed exposures, calculations on data. The next day the result were brought to him up on the mountain. That was the way of working that he liked. The Mount Wilson Observatory was situated high above the mountains like a temple of science. Her servants worked there with religious seriousness and dedication. And what makes science different from religion in that it is searching for the truth, the eternal power that governs the world and keeps it in its tracks? In America the support for scientific research has the highest priority, not just financially but also socially. With the enormous material and intellectual resources everything is possible as long as energy and dedication is applied to it. His visits to Mount Wilson were however not satisfactory in all respects to Kapteyn. He wrote about this in 1913: ‘I don’t think I can continue with these Californian trips very much longer. It would be the nicest and most beautiful position one can think of – as long as it were not paid for. Now that I am being paid for it, I sometimes feel: do I deliver my money’s worth? I would probably answer that with a yes when I could just put these people here to work on the things that are necessary to accomplish my plans and ideas. But of course I only have an advisory position –a voice that is listened to, but still... the institution is in reality mostly a ‘Sonnenwarte’, an observatory for solar physics – and I represent the other part. The employees are all persons from the other part. But then they have done very very much according to my ideas. That is the ‘illness of the doubt’8 that I have deep in my soul.’ That was it, the ‘maladie du doute’, since after all his presence was an inspiration, his example and inspiration, his enlightenment a guide, that cannot be expressed in gold? See how Hale wrote him, congratulating him with this 40 years as a professor. January 22, 1918: ‘You have given a marvelous illustration of the possibilities of work by an observatory without a telescope. You have also stimulated astronomical research throughout the world in an extraordinary way and initiated an undertaking which will be continued far into the future. Under such circumstances you must surely find pleasure in reviewing the years of your world- wide work of cooperation, which has meant so much to astronomers every-where. Most of all

6From Vanitas vanitatum omnia vanitas; Vanity of vanities; all is vanity. (Ecclesiastes 1:2) 7Edward Emerson Barnard (1857–1923) was the foremost observational astronomer of his time. He was observer at Lick Observatory in California (1888-1895); in 1895 he was appointed professor of astronomy at the newly founded University of Chicago and observer at the Yerkes Observatory, where he remained until his death. He is best known for his discoveries of comets, nebulae, and particularly Jupiter’s fifth satellite. He was the first07.11.14 astronomer to photograph the Milky Way. (Footnote by E.R. Paul) 8Maladie du doute. 82 J.C. KAPTEYN: HIS LIFE AND WORKS the Mt, Wilson Observatory is deeply indebted to you for the greatly broadened conception of its possibilities, which you have awakened. Every member of the staff would thus join with me in the assurance of their cordial appreciation of the aid and inspiration which we owe to you.’ But modest as he was, Kapteyn could not fathom the extent of what he gave, but instead just thought of what he received. Sometimes, but that was rare, he would feel exhausted under the pressure of the prob- lems that gathered around him, and that took up so much of his attention to leave any time for other things, for which he longed. He wrote from Mount Wilson to his daughter: ‘My life is almost a constant struggle with scientific problems – and so seldom do I bring any of these to a satisfactory conclusion, and then I look out for something better. I sometimes think of Cauchy9:10

‘O, what sad occupation What a humiliating weakness To have to calculate all day and to integrate without pause.’

But things don’t change and that is it, even though I have such a strong desire for the time of my retirement, when I shall put down the pencil to perform calculations and will enjoy more like a human being all that is human. Will I ever be able to do that?’ But this was only a temporary melancholy mood. Quickly he cast that aside to devote himself with new drive and energy to the problems without which he could not live. And he found again a happy optimism and fresh drive, and the next letters were nothing but hope and radiating vitality. Mrs. Kapteyn did all she could to make the stays up there homely and cozy. She did all the work on the house herself, which was according to her character. But that work was not all that easy. Everything had to be ordered from Pasadena by telephone and all the time the wrong things were brought up the mountain, which then had to be sent back and exchanged. Everything had to be negotiated over a distance of a dozen kilometers and a kilometer or so in height. But she enjoyed overcoming all obstacles and remained cheerful. Already on one of the first days of the stay she would travel to Los Angeles, the large business and harbor city that bordered the villa town Pasadena, with a long list of items to buy everything she wanted to take back. The shops offered a range of interesting but unknown household items and elegant new things that irresistibly attracted a real house- wife and loving mother. And when our mother returned home, she did bring a suitcase full of surprises, those great American wonders, ingenious, practical and graceful – in one word tempting. Together they had selected and bought the more monumental presents, beautiful books, a typical American swinging chair, an enormous swing for the garden that would be used by their grandchildren, that had been confiscated at Liverpool for it looked like a camouflaged airplane and that gave them much trouble during transport. But nothing was too much for them, as long as it would bring joy.

9A mathematician of enormous ability, the Frenchman Augustin Louis Cauchy (1789–1857) worked on the calculus placing analysis on a rigorous footing, made fundamental contributions to complex function theory, contributed significantly to error theory, algebra and differential equations, and made numerous contributions to celestial mechanics. (Footnote by E.R. Paul) 10Oh mais quel07.11.14 triste emploi// Quelle humiliante faiblesse// Que de chiffrer toujours// Que d’integrer sans cesse. America 83

Kapteyn came back from America with his head full of new plans and ideas, with renewed health and work drive, longing for his laboratory and his peaceful life of concen- trated work. It was always a joy to go to America and a happy returning back home. Both valued this unusual, happy life to the full.

07.11.14 84 J.C. KAPTEYN: HIS LIFE AND WORKS

Chapter 19

THE WORLD WAR

In the beginning of 1914 Kapteyn was hit with a severe blow. After a short, not very severe illness Gill passed away. Deeply saddened he and his wife went to London to see their friend for the last time. They found Lady Gill in deep mourning. She was lying on her bed, completely exhausted, waiting for Kapteyn as the most trusted friend of her husband. He knelt next to her bed, and solemnly putting her hand on his head, she blessed him and thanked him for all he had been for her husband. All the love and gratitude, all the grief and mourning of a great human heart were expressed in this impressive gesture. Gill was buried in his place of birth, Aberdeen, where he was taken by his wife and a few friends. In London at the same time a memorial service was held at St. Mary Abbot Kensington, where the Kapteyns and many sad friends attended. On the organ the beautiful song of Tennyson1, ‘Crossing the Bar’ was played, and a pure boy’s voice as if heavenly music sang the comforting words. Silently they returned to Gill’s house, packed their suitcases and left the house on 34 de Vere Gardens, this hospitable house that they would never return to, as Lady Gill would leave it as well. They returned home poorer. And every Sunday morning Mrs. Kapteyn would play this sad death song, of which we heard the sounds rustling through our home in the quiet morning. We were quiet for a while and remembered this trusted friend. That was the sad start of the disastrous year 1914, that would leave its ominous mark on the world and would bring death and destruction. Even science, the invincible, that many had believed was above those developments of the world, would have to succumb to this god of war, which was a major disappointment for Kapteyn. At the outbreak of the war Kapteyn and his wife were at Mount Wilson. In one instant the world had changed completely. Everybody lived in fear and worry about distant friends and family, and who was able to return home did so as quickly as possible. For the Kapteyns the journey across the ocean was however impossible due to the threat of mines; they stayed until January and then came home safely, expected with much trepidation. It was their last journey across the ocean; they would never return to America. In July of 1914, not long before the outbreak of the war, the Emperor of Germany bestowed the ‘Ordre pour le Merit´e’upon Kapteyn, which he accepted with much joy. ‘I don’t understand, why this honor is bestowed on me and not someone else.’, he said to his assistant van Rhijn. After all, this honor has been awarded to no more than 30 foreigners,

1Alfred Tennyson,07.11.14 1st Baron Tennyson, FRS (1809–1892) was a well-known and very popular British poet. He was poet laureate (the official poet of the King or Queen) during much of Queen Victoria’s reign. The World War 85 and was seen in the scientific community as one of the highest distinctions. Gill wrote in 1892 when he himself received this award2: ‘The Ordre pour le M´eriteI regard as the highest distinction open to a literary or scientific man.’ When, however, in early August rumors started to go around in America about German violations of the neutrality of the Netherlands, Kapteyn felt obliged to refuse the award after all and he wrote to the German Consulate in Groningen: ‘6 Aug. 1914. Now that the German army has violated Dutch neutrality, I feel obliged to reconsider my decision of July 18 of this year. In the present circumstances I cannot accept the distinction designated to me by the Emperor of Germany and King of Prussia, the nomination of ‘Ausl¨andische Ritter3 des Ordre pour le M´erite’.I sincerely hope this letter will reach you in time.’ The Consul wrote him back that the rumors were not true and that therefore there was no reason for him to refuse the award.4 In England, however, there was a great deal of indignation about the acceptance of Kapteyn of the German award; in particular a few hotheaded individuals among astro- nomers were reproaching him, who like so many in those days could only appreciate their own point of view in spite of being educated as scientists. This was what Kapteyn, then and later, was not able to comprehend. Maybe it is impossible for people being neutral to understand the mentality of nations at war. After all, the physical and mental suffering, the apprehension and insecurity, the terrible tension, all this gave rise to an almost insane bitterness and hate that made all objectivity impossible. A scientific person was a human being in the first place like anybody else. So, who was able to be objective? Karl Schwarzschild, the eminent German astronomer, expressed it in a very humane way as: ‘When everybody is feral, why would I not be feral?’ 5 One was part of a war faring, suffering and hating society and nothing else. Hubrecht6, the son of Kapteyn’s friend, who was astronomer in Cambridge at the time, wrote on August 27, 1914: ‘Allow me to extend my sincerest congratulations to you at the occasion of this unique distinction, bestowed upon you by the German emperor. My brother wrote me about this. The news has not yet reached the English newspapers. I think the censoring authorities feel the emperor is being given too much positive exposure! At least, the few English friends that I have been able to tell the news were all very surprised and had not imagined him being able to do such a thing that testifies of him being able to appreciate real culture.’ The terrible years of war brought about a clear division in the scientific world, that would heal only slowly long after the hostilities had come to an end. Kapteyn wanted to help, to bring people together, but his idealism failed to appreciate the cruel reality, and for the first time in his life his tact and power failed. Since it was impossible for him to travel to America he had a guilty conscience with respect to the Carnegie Institution, and he wrote Hale that he wished to resign as Research Associate. Woodward7, the president of the Carnegie Institution, was able with Hale’s help to convince him not to resign, since

2Actually, HHK did not get this right; Gill received the Ordre pour le Merit´ein 1919. 3Foreign Knight. 4HHK has the year wrong. Gill received the medalm in 1910. 5Wenn alle wild sind, warum sollte ich dann nicht wild sein? 6Jan Bastiaan Hubrecht (1883–1978) was the son of botanist Ambrosius Hubrecht, friend of Kapteyn in Utrecht during his student days. Jan Hubrecht was astronomer. 7The American Robert S.07.11.14 Woodward (1849–1924) was trained as a geologist and became president of the Carnegie Institution of Washington in 1904 serving until 1920. (Footnote by E.R. Paul) 86 J.C. KAPTEYN: HIS LIFE AND WORKS his services were appreciated very much by the institute, regardless of whether or not he was able to come to America or not. Notwithstanding the difficulties in exchanging letters, he remained in regular contact by mail with America and was able to continue collaborating with Hale. The latter however became more and more under the spell of the war, and after the sinking of the Lusitania8 he wrote in 1915; ‘I have lost all patience with the Germans, since they turned pirates, and would not blame England for the most drastic action, whether it injures or not.’ He offered his services to the American government for ‘Preparedness for War’9. Until the end of the war this was his major occupation. Kapteyn saw with great worry the continuously increasing hatred of the allied and America against Germany. His feeling of justice objected to the condemnation of the German people, who were not guiltier than others. His clear mental eye saw how through the ages history repeated itself, how each country in turn had behaved badly when it was in power. Was the Boer War, that dark page in England’s history, not fresh in our minds, could France go freely after what it did in Morocco or the Dutch in the East Indies? Was the blockade that brought Germany on the brink of starvation, that much more humane? A hard-working nation with many virtues, was being blinded in its too sudden revival and poisoned by a system of military slogans and patriotic phrases and ruined by a uncritical belief in a government that was not up to its task. That is how he saw the tragedy of this nation: he had no choice than to back the repressed who were not really more barbaric than others. There was hope at the end of 1918. Woodrow Wilson, the president of the United States, whose peaceful and constructive ideas were well-known, the prophet of a new generation, would come to Europe to help formulate the terms of the new peace. He was expected like the dawn of a more beautiful society, and with Kapteyn many looked forward to his arrival with hopeful expectation. Kapteyn wrote to Frost10at Yerkes: November 10, 1918. ‘But for your admirable president I for one feel but little hope of an issue which to humanity would be worth the awful misery it has gone through. Again, I for one have no fear of Wilson. After Christ he may become the greatest benefactor of the world, I feel sure that he will fight for his ideals or perish in the attempt. Better forgo rightful vengeance, rightful punishment, anything than the hope for some enduring settlement of human affairs, that will make for the real happiness and progress of mankind. There never was such a chance. There would not be now, but for America.’ Unfortunately, the appearance and quick disappearance of President Wilson became a tragedy, this not very strong leader and idealistic professor who from his viewpoint in distant America understood little of European relations and who was no match for the cunning diplomats of the old world. ‘Even though strength may fail, the intention should be praised’.11 Since this intention, how much it failed to accomplish at Versailles, still

8RMS Lusitania was a British ocean liner that was torpedoed by a German U-boat on 7 May 1915 near Ireland. More than one thousand of the almost two thousand people aboard lost their lives. 9The Preparedness Movement, also referred to as the ‘Preparedness Controversy’, was a campaign led by Leonard Wood and Theodore Roosevelt to strengthen the military of the United States after the outbreak of World War I. The movement was originally opposed by President Woodrow Wilson, who believed the United States should be in a position of unarmed neutrality. 10The American Edwin Briant Frost (1866–1935) succeeded George Ellery Hale as director of Yerkes in 1905, serving until 1932. He became a specialist in stellar spectroscopy, specifically the determination of the radial velocities07.11.14 of stars. (Footnote by E.R.Paul) 11Ut desint vires tamen est laudanda voluntas. The World War 87 planted the seeds for a slowly emerging principle of peace and brotherhood that became the League of Nations. Instead of being conciliatory the Peace of Versailles made reconciliation more difficult, and long after the end of the war the world was possessed by this feeling among both victors and the defeated. Also in science one chose sides, and in July 1919 the ‘Interal- lied Association of Academies’ was founded, excluding Germany. Kapteyn discussed with Heymans what they could do about this. He had an unwavering trust in science and its ultimate victory over bitter subjectivity. ‘It is my conviction that science must in the long run directly and indirectly become a mighty factor in bringing peace and goodwill among men. If the men of science do give an example of hate and narrow-mindedness, who is going to lead the way?’, he wrote in January 1917 to Eddington. In this great English astronomer, Quaker and strict scientific man, he found a humane and impartial judge, who was not blinded by hate. But that was an exception in those days. Kapteyn and Heymans composed an open letter in 1919 after much deliberation: ‘To the Members of the Academies of the Allied Nations and of the United States of America,’, in which they adjured that science should be ‘the great conciliator and benefactor of mankind.’ And they ended: ‘We understand how your attention of late has been monopolized by what is temporal and transitory. But now you more than all others are called upon to find again the way to what is eternal. You possess the inclination for objective thought, the wide range of vision, the discretion, the habit of self criticism. Of you we had expected the first step for the restoration of lacerated Europa. We call on you for cooperation in order to prevent Science from becoming divided, for the first time and for an indefinite period, into hostile. political camps.’ To Prof. Korteweg12 Kapteyn wrote: 14 August 1919. ‘I do not expect that this open letter to allied members of academies will end the division between them or the association of academies13. I have however not given up all hope that a letter like this, signed by a large number of prominent neutral scientists, will turn many of the allied scientists to a more objective appraisal of the matters concerned, with the result that less haste is being made in founding the interallied societies. In my opinion when time is won, all is won. If this would however not work either, then I will not regret this step that Heymans and I took.’ However, the time was not ripe; their plea had no effect. To the contrary, the interference by scientist from neutral countries, who after all had distanced themselves from the terrible fighting with all its suffering and desperation, irritated them more than causing them to reflect, now that the emotions were still high and the wounds fresh. When at the end of 1919 a proposition was sent to all neutral academies to join the International Research Council (or Conseil International de Recherches) with the exclusion of Germany, he opposed this with all his strength, and tried to keep the Amsterdam Academy of Sciences14 from making this step. In the decisive meeting he and Heymans used all their influence to keep members from voting for it, but they failed. Here it was opportunism against idealism, and also in science idealism lost. He had not expected that and he was so shocked that he and Heymans resigned immediately from the Academy that according to them had proved to be unable to act just and in a scientific manner. It was

12The Dutchman Daniel J. Korteweg (1848–1941) was professor of mathematics at the University of Amsterdam from 1881 until his retirement in 1918. (Footnote by E.R. Paul) 13Kapteyn refers to the ‘Interallied07.11.14 Association of Academies’. 14The current Royal Netherlands Academy of Arts and Sciences. 88 J.C. KAPTEYN: HIS LIFE AND WORKS more than a passing shock; until his death this damaged trust in justice and objectivity remained a painful wound. There was someone else that like him was fighting for reconciliation. That was the Swede Str¨omgren, the director of the Copenhagen Observatory15. Like most Swedes he was on the side of the Germans, had an unlimited admiration for German science and also his personal sympathies were with that country. He was upset by the exclusion of Germany, and when in 1919 the French refused to circulate the data on a comet among Germans, which Harvard (America) did, he became indignant. ‘I have two boys of 11 and 9 that would never do such a thing.’16 Through his arbitration he was able to obtain many results favorable for the Germans in these times of isolation and when it was decided in 1920 to organize again a congress of the Astronomische Gesellschaft, he worked with all his powers as its president to make it a success. He proposed to appoint Kapteyn in the Board of the A.G.17 ‘... This decision, as you will understand, will bring you in temporary discord with Baillaud, Lecointe and Turner18, for which the entire neutral world will be grateful to you, and a large number of astronomers from allied countries will welcome this decision with sympathy – not to mention the feelings of the German scientific world.... You have given much in your life to the scientific world – I think here you again can give something of lasting importance.’ Kapteyn decided to accept. He wrote Seeliger19, the astronomer from Munchen:¨ 20 ‘I am fully aware that in general I lack the qualities necessary for a really good member of the Board. That is the reason that I have always declined such offers. Now the situation is in my view somewhat different, since it is the case that he who does not protest the in my opinion irresponsible division, should give all his strength to help to heal this objectionable separation.’. Eddington was the first from the camp of the enemies, who was prepared for recon- ciliation. He wrote to Str¨omgren in November 1919: ‘I hope to show my interest in the

15Svante Elis Str¨omgren (1870–1947) was a Danish astronomer. He first was professor of astronomy at Kiel and later professor of astronomy and director of the Copenhagen Observatory. He is the father of the famous astronomer Bengt Georg Daniel Str¨omgren (1908–1987). 16Ich have zwei Jungens von 11 und 9 Jahren, die wurden¨ so etwas nie machen. 17...Der Entschluss, wenn Sie ihn fassen, wird Ihnen die vorubergehende¨ Misstimmung bei Baillaud, Lecointe und Turner einbringen, dafur¨ wird Ihnen aber die ganze neutrale Welt dankbar sein, und eine grosse Anzahl der alliierten Astronomen wurde¨ diesen Entschluss mit Sympathie begrussen¨ – von den Gefuhlen¨ der deutschen Wissenschaft brauche ich wohl nicht zu sprechen... Viel haben Sie in Ihrem Leben der Wissenschaft geschenkt – ich glaube Sie k¨onnen ihr hier wieder einen dauernden Nutzen bringen. 18The French scientists Jules Baillaud, Lecointe and the English astronomer H.H. Turner were among the most militant in arguing for exclusion of German scientists after the war. (Footnote by E.R. Paul) Jules Baillaud (1876–1960) was the son of Edouard-Benjamin Baillaud, the first president of the International Astronomical Union. George Lecointe (1869–1930) was not French, but Belgian. He was director of the Royal Observatory of Belgium and played an important part in the creation of the International Research Council. Herbert Hall Turner (1861–1930) was professor of astronomy and director of the Observatory at Oxford University. 19Along with Kapteyn, Hugo R. van Seeliger (1849–1924) was most responsible for developing statistical astronomy into a major research branch of astronomy. He spent his most productive years as professor of astronomy and director of the observatory at the University of Munich (1882–1924); he was president of the Astronomische Gesellschaft for 24 years (1897–1921). (Footnote by E.R. Paul) 20Ich bin mir vollkommen bewusst, dass mir im Allgemeinen die Eigenschaften fehlen, die dem wirklich guten Vorstandsmitgliede zukommen. Das ist die Ursache, dass ich sonst immer abgewiesen habe, Jetzt liegt aber doch in meinem Gefuhl¨ die Sachs etwas anders, da es gilt, wo nicht gegen den nach meiner Meinung unverantwortlichen07.11.14 Bruch zu protestieren, so doch nach besten Kr¨aften dazu mitzuarbeiten, diesen verderblichen Bruch heilen zu helfen.” The World War 89

Astronomische Gesellschaft by attending the next meeting – an individual step which no one has any right to object to ... International science is bound to win and recent events –the verification of Einstein’s theory– has21 made a tremendous difference in the last month.’ In 1920 he was the only Englishman that attended the congress of the A.G. And now in 1928, when I write these pages, Germany has become a member of the League of Nations, Wilson’s idea is gaining increased acceptance, and time, that great benefactor and conciliator, is busy healing the wounds and the divisions, including those in science.

07.11.14 21..have? 90 J.C. KAPTEYN: HIS LIFE AND WORKS

Chapter 20

LEIDEN AGAIN

At the Observatory at Leiden after the retirement of van Bakhuyzen in 1897, his brother Ernst had succeeded him, after it had been unsuccessfully tried to convince Kapteyn to assume this position. When Ernst Bakhuyzen died in 1918, de Sitter had become director. He was thinking seriously about a general reorganization, to make the observatory more efficient. Much unreduced material had been accumulated and he wanted the work to take a different direction. After consultation with Kapteyn he decided on a new structure. Two adjunct directors would be appointed that would together with him direct the three lines of research. He would himself be director of the theoretical department, and he proposed that Dr. Pannekoek, who had been studying at Leiden Observatory and knew the situation quite well, would lead the department of astrometry, and Prof. Hertzsprung, the Danish astronomer from Potsdam that had married Kapteyn’s youngest daughter, the new astrophysics department. This proposal would provide success for all involved. Hertzsprung was appointed, but the ministry unfortunately opposed the appointment of Pannekoek, who for political reasons could not employed as civil servant.1 The position remained open since the right person could not be found, neither in the Netherlands nor abroad. Director and adjunct director set to work with much energy, but they did miss the third person sorely, since the astrometric work that needed leadership urgently remained neglected. One time when Kapteyn visited the Hertzsprungs in Leiden the matter was discussed over dinner. The daughter had a sudden thought and said: ‘Father, would you not take this position temporarily? You know this kind of work intimately. And because of your upcoming retirement you will have sufficient time available.’ Hertzsprung immediately supported this unusual idea. Kapteyn laughed and said that the idea had not even occurred to him. After some thought he said: ‘It is not such a strange idea anyway and I will give it some serious thought.’ He felt that it was urgently necessary that order was brought to the chaos in the material collected and its old fashioned state. Leiden Observatory had had a special place in his heart, he felt at home there, was acquainted with the staff and was highly regarded by all. Someone like that could only succeed. He wrote to Hertzsprung on 18 April 1920: ‘Now that you are serious about this, I have given it some thought. I am prepared to accept a position as ‘adviser’ for one day a week to work on this... This is inspired only because of the extensive07.11.14 plan of yours and only to help the Observatory. Personally there is in this 1Pannekoek was an active participant in and supporter of the socialist movement. Leiden again 91

Figure 20.1 Kapteyn and his daughter Henriette.

new obligation little that07.11.14 is really attractive to me...’ It did take away a major part of his freedom to do what he wanted, which he was looking forward to. ‘ It is exactly this complete 92 J.C. KAPTEYN: HIS LIFE AND WORKS freedom that makes the emeritus status for me so attractive.’ 2, he wrote to Prof. Eberhard3 in Potsdam, with whom he had developed a good friendship over the last years. The conclusion was that de Sitter was extraordinary pleased with the proposal that Kapteyn would temporarily take upon him the adjunct directorship, for which he would every two months spend a week in Leiden. He would stay with his children, which was a great joy on both sides. ‘Now you will be the assistant of your former assistant’, they teased him, but they were also proud for the greatness of his mind which when it came to matters that he felt were important, did not care about the form. There were some that felt that a special title had to be found for him, as they felt this modest position was not doing justice to his dignity, but such considerations were foreign to him and his unselfish wish to serve was more to him than whatever title. So this way he came back to Leiden Observatory, the place where his astronomical career had started half a century earlier. He made new research plans, that would scrutinize the adopted fundamental systems of star positions and at the same time oversaw the reduction of older observations at Leiden to final results. It was unfortunately only for a short period, since one year later the first signs appeared of the fatal illness that ended his labors. But in this one year he was able to organize, put things in order and set out new directions so that others could continue the work.

2Es ist gerade dieses v¨ollige Freiheit, welche fur¨ mich das Emeritat anziehend macht. 3Gustav E. Eberhard (1867–1940) received his doctorate under Hugo von Seeliger at Munich, and from 1918 until his retirement07.11.14 he was senior observer at the Astrophysical Observatory at Potsdam. (Footnote by E.R. Paul) Leiden again 93

Figure 20.207.11.14Kapteyn at age 70. From the painting by Jan Veth. 94 J.C. KAPTEYN: HIS LIFE AND WORKS

Chapter 21

THE LAST YEARS IN GRONINGEN

In 1918 the Kapteyns had to leave their beloved house on the Ossemarkt since the owner wanted to live in it himself. It was in the period of shortage of housing that was prevailing everywhere just after the war, and they decided to live the last two years until he became emeritus in a hotel, as they were planning to move to Hilversum after his retirement. They moved into the well-known hotel ‘de Doelen’ on the ‘Groote Markt’1, where two rooms were allocated to them and with their own furniture was turned into a comfortable home. The large living room at the front side of the hotel, where their grand piano and old-fashioned canopy were placed, looked out over the large market place with the Martini tower to the right and the massive City Hall to the left. Tuesdays and Fridays were market days and these were pleasant and busy happenings. One could spend hours sitting in the deep window sill looking at the busy movements without getting tired. But every year in May, which was a month bringing nice weather, all hell broke loose, then a big fair took place with loud and blaring noises from the merry-go-rounds and hippodromes, steam whistles yelled and the music droned and the spring air was filled with the smell of ‘poffertjes’2 and hot engines. Groningen was celebrating and it did not want this privilege to be taken away from them. The Kapteyns often fled and looked for peace that was not to be found at home. At the Martini-kerkhof, not even five minutes from their hotel, lived Prof. Bordewijk3, who had not yet become used to living in a strange place like Groningen, which was very familiar to Mrs. Kapteyn who forty years earlier had the same difficulties. He and his impulsive wife soon found the way to the hearts of the Kapteyns because of their spontaneous openness and their similar minds. At the Kapteyns they were welcomed so heartily that a close friendship developed between them. Much later people asked Mrs. Bordewijk how their close friendship with the Kapteyns had arisen, while they themselves after all were so much younger. ‘I don’t know, how it came about’, she answered, ‘but I do know that these were the most beautiful years I had in Groningen.’ For the Kapteyns these contacts with younger people were a great joy. They always remained young of mind, they could understand and sympathize with everything that

1The Groote (now spelled Grote) Markt is the central market square in the center of Groningen. 2Small pancakes that are being served with butter and sugar. 3Hugo Willem Constantijn07.11.14 Bordewijk (1879-1939) was a professor of constitutional law and economy at Groningen between 1918 and 1938. The last years in Groningen 95 happened and enjoyed the enthusiasm and flexibility of the young people that older ones often loose. They were able to follow the ways a younger generation had discovered with joy and ease and were always open to change. The younger ones learned from his experience and optimism, they enjoyed with relief the serene rest that was typical for the atmosphere in their home. They learned to see that things are really much simpler and more natural. This great, humble man taught them through his own example to see the greatness of great things and smallness of small ones. Was life not enjoyable and worth fighting for? All luck and doubt disappeared in a beautiful harmony and they felt new strength and determination to find this harmony in their own circumstances. With this blessing they returned home. Some of their closest young friends called them ‘Father and Mother Kap- teyn’. They had an international mind, because there were friends in America, in England and in Germany that also enjoyed the privilege of being allowed to address them like that and with this beautiful name came the fullest trust that can possible exist between humans. Many Royal distinctions and decorations, medals, honorary doctorates etc. were be- stowed upon him in his many years of scientific work. The first order, the Legion d’Honneur of France brought him much happiness; it was the first official recognition of his work, and he wore the red ribbon always in the buttonhole on his lapel. He regarded his further distinctions that he thought highly of, in the first place as means to get what his scientific work required and also to provide a better income for his clerks. This fame and these public honors gave a kind of awareness from which his case would profit and that could make the means available to do his work on an ever expanding scale. That then indeed happened: in 1903 he was given the previous physiological laboratory of his friend the late Huizinga, the same laboratory where as a young man he started his scientific endeavors in two small rooms. An assistant and a large number of clerks did their daily work there and a great piece of work was being accomplished. Much more impressive to him then the worldly fame was the love of the people around him. It touched his deepest soul and made him happier than any worldly success could do. I have seen him deeply moved by a letter from the German astronomer, Prof. Eberhard from Potsdam, with whom he developed a deep friendship in the last years. He wrote him during his serious illness, how much this friendship meant to him and that it was the best thing that the later years had brought him. ‘And I thank destiny most that is has been possible for me to come closer to you in the last two years.’ 4 Very appropriate this sensitive German wrote later about Kapteyn’s work:5 ‘Every time when a new publication from you arrived, it was studied thoroughly and with joy.’ This admiration was also very well expressed in a letter at an earlier date from Dr. Innes6, the previous director of the observatory at Johannesburg in Transvaal, whom Kapteyn had proposed to address each other on a less formal basis: April 14, 1906 ‘It is very kind of you to admit me into the inner circle by addressing me as ‘my dear Innes’ and

4Und ich danke dem Geschicke ganz besonders, dass es mir vergonnt hat, Ihnen in den letzten zwei Jahren nailer treten zu durfen.¨ 5Jedesmal wenn eine neue Schrift von ihm kam, studierte man sie gewissermassen in einer feierlichen Stimmung. 6Robert Thorburn Ayton Innes (1861–1933) was the director of Transvaal Observatory in Johannesburg from 1903 until 1927. A talented07.11.14 observer, he is known as the discoverer of Proxima Centauri, the nearest star to the solar system. (Footnote by E.R. Paul) 96 J.C. KAPTEYN: HIS LIFE AND WORKS

Figure 21.1 Professor Kapteyn at work with his secretary. more kind to give me permission to address you in the same way. If I do not do so, it is not because I do not value the permission, but because your truly eminent talents place you on a higher sphere, I do hope you will not object to my hero-worship (which for my part I think a very good thing for a man to be capable of)...’ Love, which was most important for him, he gave as much as he could to others. He wanted to see people happy and wanted to help in their troubles. He was unable to send away poor merchant07.11.14 women that came to the laboratory with their wares to offer for sale, although he knew that their merchandise was of inferior quality. The closets were full of The last years in Groningen 97 poor quality paper and inferior pencils for which he however always found some good use. Since he, the blessed, was always willing to help whoever came to ask for help. He was inexhaustible in helping and advising, wherever his family, his friends, his clerks or anyone else needed him. The husband of a former housemaid, who had become an invalid after a fall and was unable to perform his work, was given appropriate help. He went around to all his friends and colleagues and asked them to have their bicycles cleaned regularly by this man. I felt he could in this manner create a way of supporting himself and he helped him to get it off the ground by providing the necessary materials. The business became a success and poor Ottens was again able to support his family. And this is just one example among many. It was moving to see him with his grandchildren. His interest in them and his patience with them, their plans for the future, it all was infinite. For some he started a fund to pay for their studies immediately after their births and he always knew the right advice in cases of difficulties with their upbringing. Even on his final sick bed not long before his death did he make plans: every child would pant a tree and see it grow from his house in the countryside, where he planned to live after his retirement. They would enjoy staying with him there away from the cities, he would learn them about the birds, collect flowers with him and he would read them stories. No wonder that his grandchildren loved him very much. The following happened after his death. One of his grandchildren, who was learning history in school, practiced her lessons which involved the remembering of years of historical happenings. The mother explained: ‘But Rigel, what does that year 1500 signify?’ The child did not know. Apparently the teachers had not explained the significance of the calendar. And the mother explained: ‘1500 years ago a man was born and his birth has now been adopted as the start of our calendar. That man was the best, noblest and wisest of all men who had ever lived. Can you guess who that man was?’ Without a moment of hesitation the child called out: ‘Grandfather!’7 For his students and for his science he also wanted to provide for the future. He es- tablished a fund for which he provided the financial basis himself, and wanted this to be increased by contributions from others. It would open the possibility for talented students with little financial means to study astronomy and to support astronomical research fi- nancially. But scientists are not the ones experienced in financial matters and do not have financial means available. The fund increased only very slowly. On one occasion he met a friend of his oldest daughter, W. Dekking, a merchant from Rotterdam. He was very im- pressed with Kapteyn’s personality and his ideals and proposed that he turn to the world of traders in Rotterdam and solicit their interest in his fund. Through Dekking’s enthusi- asm that was very infectious, the fund increased its capital significantly. After Kapteyn’s death and the efforts of friends and admirers, but especially through the energetic help of Dekking, it became a fund of standing, that under the name of Kapteyn Foundation8 became an official institution with significant means. It has provided much support and will continue to do so in the future. January 19, 1921 was Kapteyn’s 70-th birthday. Is was not a day of joy and festivities, but a day of stress and quiet worry. On that day he and his dearest awaited in Amsterdam the return of his daughter with her husband child, that came back from America. The

7The story as told by HHK is confusing. She seems to refer to Jesus Christ, but the 1500 years ago refer to that year and not to the07.11.14 present time. 8Now known as the ‘Studiefonds J.C. Kapteyn’; at this time I am its chairman. [But no longer in 2014] 98 J.C. KAPTEYN: HIS LIFE AND WORKS

Figure 21.2 The Astronomical Laboratory Kapteyn.

child had undergone surgery in Boston by Dr. Cushing9, the famous neurologist and was expected back that day. The future was uncertain; a message had been sent by wire that everything went well during the ocean trip, and everyone breathed in relief. To Kapteyn this first grief in his family had been a severe blow. The ‘luck of the Kapteyns’ had been proverbial in Groningen; everything had been prosperity in this happy family, no great disasters had happened to them, until this dear grandchild had suddenly shown signs of a severe brain illness. He supported the distressed parents with all his power and love, sent them hopeful and comforting letters during their absence, and now waited to welcome them again with love and help carry their burden. It became a day of quiet joy after all as a result of the happy reunion and the togetherness of all that loved each other so much and were so much attached to each other. He did not hear much of the world around him; everyone knew that this day had to be celebrated quietly and all respected his sorrow. So the day went by that brought him his retirement. Not long after that he received a letter from the government that had decided following a proposal by the curators of the university to attach Kapteyn’s name to the laboratory as a permanent tribute to its founder.

9H.W. Cushing (1869–1939) became the first American to devote full-time to neurological surgery devel- oping a world-wide07.11.14 reputation as professor of surgery while at Harvard. He was surgeon-in-chief at Peter Brigham Hospital from 1913 until his retirement in 1932. (Footnote by E.R. Paul) The last years in Groningen 99

In June he delivered his last lectures, and the Kapteyns said goodbye to Groningen. The good friends were invited in turn by them at their table at de Doelen. They felt this was a more intimate way to be together than a big dinner with a large group of people that would bring more glitter than intimacy. And when they said farewell to the hotel it was much sadness for all they left behind. The manager was very much touched when she said goodbye and said they had been the nicest and easiest guests they ever had had. The appreciation was mutual. The left at seven o’clock in the morning. None of the friends knew about it so nobody was present to bid them farewell. They had wanted it that way, and quietly they said goodbye to the city where they had lived and worked for 43 years. A long happy life with fruitful labors was behind them, but a new promising life was ahead of this pair, that in the latter part of their lives still had the energy and strengths to accept new values. But first they wished to enjoy a vacation in a high, clear climate where Kapteyn could regain his strengths after all his labors of the last few years. During the last years before his retirement he had worked with all the strengths that he had available. He had declined everything that would have taken precious time: lectures, meetings, even a visit to Mount Wilson, how much he would have liked to do this: ‘I have to give all my time, including my vacations to the laboratory’, he wrote to America. Everything had to be put in order and finished, so that he could leave without worries and his assistant van Rhijn could continue his work. At the same time a new, grand theory, that resulted from the analysis of the material he had collected over the last years, had been forming in his mind, which occupied him so much that his books and papers accompanied him to Switzerland. 10 ‘This theory is the first attempt to develop an dynamical explanation, based on and tested against observations, of the structure of the sidereal system. It was not a complete and definitive theory, but merely the result of a first exploratory investigation into the unknown, outside the bounds of current knowledge. He named it himself A first attempt at the theory of the arrangement and motions of the stellar system, and presented it for the first time in 1921 at the Dutch Astronomical Society11 and after his vacation at the congress of the Astronomische Gesellschaft at Potsdam, where he and Einstein were the center of attention, and not long after that in the meeting of the British Association in Edinburgh; in November 1921 he explained his theory at an informal meeting in Leiden, where in addition to a few Dutch scientists also Prof. Einstein and Dr. Jeans12, a well-known English physicist, were present. Briefly, the theory says that the circular motions of the stellar streams as we observe them are only a local phenomenon. In reality the motions of both ‘herds’ of stars are not along a straight line, but both move in circular orbits in opposite directions, so against one another. This contributes towards an explanation for the flattened form of the stellar system, that has been found from a study of the observed number of stars, and that without such motion would be in contradiction with the laws of mechanics. The reason that we do not see effects of a deviation from motions along a straight line but only observe it as such, is that we can only see such a small part of the circle that the difference is unobservable. Indeed, the stars whose

10The following paragraphs have been taken from the article ‘Jacobus Cornelius Kapteyn’ by Dr. W. de Sitter, Hemel and Dampkring, 1922. (Footnote by HHK). Hemel & Dampkring is a Dutch periodical about astronomy and meteorology for a wide audience. 11The ‘Nederlandse Astronomen Club’ is and was the official professional society of astronomers in the Netherlands 12James Hopwood Jeans07.11.14 (1877–1946) was a famous British physicist, astronomer and mathematician. He was a pioneer in physical cosmology, cosmic dynamics and stellar evolution. 100 J.C. KAPTEYN: HIS LIFE AND WORKS motions we can observe, are only our very nearest neighbors in the universe. We cannot go deeper into these considerations here, but refer the reader to the May issue of the Astrophysical Journal, his last contribution to a foreign journal.’ Kapteyn’s model for the structure of the universe –which has already been named the ‘Kapteyn Universe’– may be a first exploration of new territory – is also the final culmination in his life’s work, such as is given to only a few.” It was indeed the end of his life’s work, since the deadly illness, that would put an end to his works and dedications, had already taken a hold on him. Nobody suspected a fatal illness, he himself the least, since the doctors had found nothing alarming. The Kapteyns decided to spend some time with their daughter and son-in-law, Prof. Noordenbos, in Amsterdam, from where they could easily reach Hilversum to look for a house where they had dreamed to spend their last years. After some searching they did find a house that was to their liking in all respects. They had inspected the house in the morning and had it checked over and with his usual vigor Kapteyn in the afternoon presented his wife the keys of the house that they owned by then. And how happy they were with their new home, it was in their eyes a jewel and it promised luck and peace for many years to come. Unfortunately this was not to happen, as the illness got worse. Another half year the family lived between hope and fear for this dear life that slowly wasted away. Half a year of deep togetherness, so rich and happy in mutual love, that Kapteyn was able to say a few days before his death with his radiant tenderness: ‘Children, this is still the happiest time of my life.’ He had borne his sufferings as a hero, he never complained but only spoke cheerful and wise words. The large sunny room, where he was lying for months, looked after by his wife and two daughters, was named by him ‘the haven of delight’, since it was a true haven of peace and happiness for whoever was down or sad. His cheerful face and hopeful plans for the future, his interesting stories and cheerful humor –all this turned the room into a blessed and consecrated place, where one could only be strong and courageous and accept the blow that would be dealt soon, undauntedly. Many came to visit him, hoping against all hope that their good friend would be saved, since they loved him with a very special love that is rare in life. In April the big international astronomical congress was going to be held in Rome13. Since he was unable to be present, he wrote, supported by pillows, a program of activities for the coming period, for which he solicited the cooperation of all astronomers. His old fire blazed again and it ended up being a great piece of work, that made a big impression when de Sitter read it out at the congress. Everyone pledged to cooperate and it became a major success for Kapteyn. It was here, that the French President of the congress14, in his inaugural speech ascribed the progress of astronomy in the last half century next to two other factors to the Groningen Laboratory. And when a few days later a treatise was sent to him with the title ‘The Kapteyn Universe’, he smiled happily. His largest wish in life had come true: He had been able to serve science in an important way and he could put his head to rest. The concentrated efforts had made him tired and weak. His body was no longer able to do what his spirit wanted him to do. On the 24th of May he wrote his last letter; it was a beautiful greeting to the Royal Astronomical Society that celebrated its 100th anniversary.

13This was the founding07.11.14 congress of the International Astronomical Union (IAU). 14This was Edouard-Benjamin Baillaud. The last years in Groningen 101

The end came on June 18. It was a holy silence, a suffering so lofty that death was a blessing. And is death not a blessing when it can bring such noble peace? Many trusted friends followed him to the cemetery at Westerveld; no official speeches by dignitaries, according to the wishes of the family who felt that was in his spirit, only true love and friendship accompanied him. Van Anrooy played on the organ the moving last choir from Bach’s Saint Matthew Passion:15

‘We lay ourselves with weeping prostrate And cry to thee within the tomb: Rest thou gently, gently rest!’

And his friend Bordewijk spoke a few tender words with a voice full of emotion. That was all, but the beautiful music played by the friend, the deeply felt words, the quiet pain of all filled that moment with a holy ordination. In the evening in Groningen Kor Kuiler, the director of the orchestra of the Harmonie16, performed the Beethoven’s Death March as a tribute to this great Groninger. And all over the world, wherever he was known, people mourned. All realized that a great men had departed, great in spirit and mind that would be missed sorely; but they also knew that his spirit would remain alive and his works would be continued and extended.

15Wir setzen uns mit Tranen nieder// Und rufen dir im Grabe zu:// Ruhe sanfte, sanfte Ruh. English translation by Z. Philip Ambrose, www.uvm.edu/ classics/faculty/bach/. 16The sorority ‘Harmonie’07.11.14 was housed in a major building in the center, which contained a well-known concert hall, the home of this orchestra. 102 J.C. KAPTEYN: HIS LIFE AND WORKS

07.11.14 Index

Airy, George B., 42 Dyson, Frank W., 43, 71 Albany Observatory, 77 Alexander von Humboldt Foundation, 34 Easton, Cornelis, 63 Alleb´e,Gerardus A.N., 25 Eberhard, Gustav E., 92, 95 Alleghany Observatory, 77 Eddington, Arthur S., 20, 66, 77, 87, 88 Argelander, Friedrich W.M., 29 Ehrenfest, Paul, 60 Astronomische Gesellschaft, 88, 99 Einstein, Albert, xi, 60, 88, 99

Bach, Johann Sebastian, 101 Frost, Edwin B., 86 Backlund, J. Oskar, 69 Bacon, Francis, 60 Gill, David, 29–41, 47, 49–50, 60, 70, 71, 77, 84, Baillaud, Edouard-Benjamin, 43, 100 85 Baillaud, Jules, 43, 88 Grinwis, Cornelis H.C., 13, 14, 19 Ball, Robert S., 47 Barnard, Edward E., 81 Haga, Hermannus, 41 Bataafs Genootschap, 33 Hale, George E., xii, 39, 67–71, 76, 81, 85 Benno, 3 Hamburg Observatory, 41 Blaauw, Adriaan, ix, xv–xvi Hamerling, Robert, 13 Boissevain, Ursul Ph., 52, 59, 81 Harvard College Observatory, 45, 70, 77 Bolland, Gerardus J.P.J., 60 Helsingfors Observatory, 35 Bonn Observatory, 71 Henry, Paul, 34 Bonner Durchmusterung, 29, 33 Henry, Prosper, 34 Bordewijk, Hugo W.C., 94, 101 Herschel, William, 20, 63, 66 Boswell, James, 60 Hertzsprung, Ejnar, x–xiii, 90–92 British Association, 69, 70, 99 Hertzsprung, Rigel, xiii, 97 Brouwer, Hendrik A., 53 Hertzsprung–Kapteyn, Henriette M.A.A., siehe Burck, William, 20 Kapteyn, Henriette Mariette Augus- Buys Ballot, Christophorus H.D., 14 tine Albertine Hertzsprung–Russell Diagram, xi Cape Observatory, 37, 70 Heymans, Gerard, xiii, 59, 60, 87, 88 Cape Photographic Durchmusterung, 29–36 Hinks, Arthur R., 69 Capteyn, Paulus, 1 Hoek, Paulus P.C., 20 Carnegie, Andrew, 71 Hubrecht, Ambrosius A.W., 20, 85 Carte du Ciel, 30, 34, 38, 40, 41, 43 Hubrecht, Jan B., 85 Cauchy, Augustin L., 82 Hudig, Johanna Clementina, xiv Christie, William H.M., 34, 40 Hudig, Joost, xiii, xiv, 61 Cook, Frederick, 59 Hudig, Sophia Alida, xiii Cookson, Bryan, 69 Huggins, William, 69 Copenhagen Observatory, xi Huizinga, Dirk, ix, 30, 95 Corneille, Pierre, 4 Huizinga, Johan, ix, 30 Cushing, H.W., 98 Innes, Robert T.A., 95 Darwin, Charles, 23, 42 Interallied Association of Academies, 87 Daudet, Alphonse, 13 International Astronomical Union, 43, 100 de Harmonie, 62, 101 International Research Council, 87, 88 de Sitter, Aernout, ix de Sitter, Willem, ix, 30, 56, 69, 90, 92, 99, 100 Jeans, James H., 99 de Vries, Hugo, 69 Jones, Derek, xviii de Vries, Teunis Willem, 31–33 Julius, Willem H., 67 Dickens, Charles, 23 Donner, Anders, 44, 69 Kustner,¨ Karl F., 71 Donner, Anders S., 35 Kalshoven, Jacobus Wilhelmus, 17, 18 Draper, John W., 23 Kalshoven, Maria Gabrielle, 18 Dubois, Eug`ene,61 Kalshoven–Frieseman, Henri¨ette Mari¨ette Au- Dudley Observatory, 27 07.11.14gustine Albertine, 17–18 Dutch Astronomical Society, 99 Kapteyn 103 104 J.C. KAPTEYN: HIS LIFE AND WORKS

spelling of name, xi Newcomb, Simon, 34, 36, 45–48, 67, 69 Kapteyn Astronomical Institute, 10 Newton, Isaac, 60 Kapteyn Cottage, 79–80 Nijland, Albertus A., 31 Kapteyn homes Noordenbos, Willem, xiii, 100 Eemskanaal, 73 Noordhof-Hoorn, Annelies, x Heerestraat, 73 Hilversum, 94, 100 Oort, Jan H., 55 Hotel de Doelen, 94, 99 Ordre pour le M´erite,84 Ossemarkt, 74, 94 Oudemans, Jean A.C., 22, 42 Vries, 52 Oxford Observatory, 88 Winschoterkade, 24, 26 Kapteyn Laboratory, 98 Pannekoek, Anton, 43, 65, 90 Kapteyn Room, 10, 61 Peary, Robert E., 59 Kapteyn, Adriaan Pieter Marinus, 7 Pickering, Edward C., 45, 70, 71, 77 Kapteyn, Albertina Maria, 10 Plan of Selected Areas, 66–72 Kapteyn, Albertus Phillipus, 7 Potsdam Astrophysikalisches Observatorium, xi, Kapteyn, Frederik Willem Hendrik, x, xiii 34, 92 Kapteyn, Gerrit Jacobus, 1–8, 26 Preparedness for War, 86 Kapteyn, Henriette Mariette Augustine Alber- Princeton Observatory, 77 tine, x–xi Pulkova Observatory, 69 first name, x marriage to Hertzsprung, xii, xiii Rabutin, Roger de, 78 marriage to Hudig, xiii–xv Racine, Jean, 4 Kapteyn, Hubert, 10 Rambaud, Arthur A., 70 Kapteyn, Jacoba Cornelia, 8, 25 Repsold, Johann G., 41 Kapteyn, Jacobus Cornelius, ix–xviii, 1–101 Rhodes. Cecil, 70 love letters, ix Roosevelt, Theodore, 69 Kapteyn, Nicolaas Pieter, 19 Rousseau, Jean-Jacques, 4, 13, 25 Kapteyn, Willem, 19, 21, 27 Royal Astronomical Society, 35, 100 Kapteyn–Kalshoven, Catherina Elisabeth, ix, Royal Greenwich Observatory, 34, 43 17–101 Royal Natural Sciences Society, xiv, 61 Kapteyn–Koomans, Elisabeth Cornelia, 2–8 Royal Netherlands Academy of Arts and Sci- Keats, John, 63 ences, 87 Knipperdolling, B., 13 Royal Observatory of Belgium, 88 Koninklijk Natuurkundig Genootschap, xv, 61 Korteweg, Daniel J., 87 Schwarzschild, Karl, xi, 71, 85 Kuiler, Kor, 101 Star Streams, 65–67 Str¨omgren, Bengt G.D., 88 L´egiond’Honneur, 41, 95 Str¨omgren, S. Elis, 88 League of Nations, 87, 89 Sturm, Charles-Fran¸cois,13 Lecointe, George, 88 Leiden Observatory, ix, 19, 22, 27, 90–92 Tennyson, Alfred, 84 Lembang Observatory, ix Thijsse, Jacobus P., 52 Lessing, Gotthold E., 4, 13, 60 Tienemann, Johannes, 61 Lick Observatory, 81 Transvaal Observatory, 95 Lockyer, Joseph N., 69 Turner, Herbert H., 88 Lusitania, 86 U.S. Naval Observatory, 45 Munchen¨ Observatory, 88 Urania Observatory, xi Molengraaff, Adolf F.M., 53 Utrecht Observatory, 22, 31 Moli´ere(Jean-Baptiste Poquelin), 4 Utrechtsch Genootschap, 33 Mount Wilson Observatory, xii, 39, 67, 69, 71, 75–83, 99 van Anrooy, Peter, 62, 101 Multatuli, 23 van Berkel, Klaas, ix, xii, xv van de Sande Bakhuyzen, Ernst F., 30, 56, 90 Natuurkundig Genootschap,07.11.14 xiv, 61 van de Sande Bakhuyzen, Hendrikus G., 22, 27, Nederlandse Astronomen Club, 99 30, 33, 56, 90 INDEX 105 van der Kruit, Pieter C., ix, xv Walter, Adams S., 71 van Rhijn, Pieter J., 33, 55, 84, 99 White House, 69 Versailles, Peace of, 87 White, Andrew D., 23 Veth, Jan, 93 Wilson, Woodrow, 86, 89 Vollgraf, Carl, xiii Wiltens, Andree, 14 Voltaire, 42 Woodward, Robert S., 85 von Auwers, Arthur J.G.F., 34 World Exhibition St. Louis, 67 von Beethoven, Ludwig, 101 von Goethe, Johann W., 4, 13 von Schiller, Johann C.F., 4, 15 Yale Observatory, 77 von Seeliger, Hugo R., 88 Yerkes Observatory, 67, 71, 77, 81, 86

07.11.14