space facilities, where data arrive at your doorstep. As a PhD student in
Leiden in the 1960s I did not have to go to the Dwingeloo for radio
observations (although I did for the larger part of my observations) and only
had to fill in a form to ‘order’ observations. The same has been the normal
modus operandi in Westerbork from the start in 1970. Obviously, satellites and
observatories in space by definition are operated this way. Dedicated staff
ensures efficient use of the telescope or satellite.

The problem van Rhijn faced was the reliance on other observatories to provide
date without having the authority Kapteyn had. The concept failed if it relied
entirely on the generosity of directors of observatories with telescopes, but
with funding to arrange access to facilities it is a model working well in
many places. In fact, going to the telescope oneself is seen more and more as
a poor and inefficient use of manpower and budgets. Kapteyn could concentrate
on research without having the burden of running an observatory. We still
conduct our research that way.

What precisely caused the Kapteyn Laboratorium to loose its prominent place in
the international context?

There is no single cause. One simple answer is that van Rhijn was not in the
class of Kapteyn. Indeed, Kapteyn really was a though act to follow. It would
be difficult to rival the discovery of the Star Streams, the vision of the
Plan of Selected Areas, the two concepts of statistical astronomy and galactic
dynamics that Kapteyn had developed. But there must have been more to it. Part
of that was van Rhijn’s attitude to give data collection in the context of the
Plan of Selected Areas priority over analysis to develop an overall picture of
Galactic structure. And there of course was Leiden, which with no less than
three world class leaders in Willem de Sitter, Ejnar Hertzsprung and Jan Oort
was impossible to better by any single person. And then there is the lack of
funding, resulting among others from the remoteness to the seat of government.
This affected all of the University of Groningen but more so astronomy, where
in addition funding the Sterrewacht in Leiden was more opportune than the
small scale Laboratorium in distant Groningen.

On the other hand, one should not forget that the Kapteyn Laboratorium and
Pieter van Rhijn did not become insignificant in the global ranking of
astronomy.

How important was the Plan of Selected Areas for the progress of astronomy and
did it ever reach the goals Kapteyn had in mind for it?

Astronomy profited from the coordinated approach in various ways. First of
course the availability of a database in general. For photometry the existence
of photometric sequences in the Selected Areas, sometimes to very faint
magnitudes has been important, as discussed by Kinman in the Legacy Symposium
[55]. The existence of reasonably consistent and uniform catalogues of
spectral types, color indices, proper motions and radial velocities must have
been of value for many ongoing investigations.

Was this what Kapteyn had in mind? Not in the first place. His focus was an
analysis to determine what we now call the structure and dynamics of the
Galaxy, which he pioneered in his seminal ‘first attempt’ paper of 1922 [49],
in which he was the first to apply dynamics to observational information. The
dynamics was studied extensively by, in particular, James Jeans, Arthur
Eddington, Bertil Lindblad and Jan Oort, so not in van Rhijn’s research
establishment. But it seems faur to say that the ultimate goal of
understanding Galactic structure was brought within reach by the data
collecting of the Plan of Selected Areas. On the other hand, the Plan did not
fundamentally contributed to the discovery of extinction.

Kapteyn’s original goal had been to use the data collected in the Plan to
determine the distribution of the stars in space as a road-map for the future
of his Laboratorium. In his presentation of the Plan [46] he wrote in the
Preface:

As will be explained more fully in the introduction, the present plan was
originally meant only as a working plan for the astronomical laboratory of
Groningen. Having first determined on its main lines it was my task to try to
convince astronomers, who were in a situation of executing the necessary
plates and observations, of its urgency and thus win their cooperation.

And in the Introduction:

In the following pages a plan is outlined, wich may be realised by the
cooperation of few astronomers in a, relatively speaking, moderate time.

The aim of it is to bring together, as far as is possible with such an effort,
all the elements which at the present time must seem most necessary for a
successful attack on the sidereal problem, that is: the problem of the
structure of the sidereal world.

What he had in mind was a final attack on this problem when all the data would
be in along the lines of his ‘first attempt’. I have argued that Oort’s paper
in 1938 [72] was just that, except that much of the data used where not yet in
the final form.

This concerned the Systematical Plan. The Special Plan, I stress, adopted
under pressure from Pickering, had a few goals which can be appreciated from
his justification of the choice off the Special Areas in his presentation of
the Plan. As far as I am aware only the study of those few Areas that were
targeted to dark clouds was actually realized in the thesis by Broer Hiemstra.

Did van Rhijn get the support he deserved from his university and the
government?

The correspondence between van Rhijn and the Curators of Groningen University
show a healthy level of support for the Kapteyn Laboratorium. The problem is
the bias, at least in the 1930s, of the Ministry for observational facilities
in favor of Leiden and Utrecht and against Groningen. The statement that
computers in Groningen should not expect to be paid at the same level as in
Leiden is curious, to say the least. After all, the work is similar and the
salary should not depend on the success or prominence of an institution on an
international level or the fame of the professor in charge. It is much to van
Rhijn’s credit that he persevered in his attempts to obtain his own telescope.

A comment that should be added here is that van Rhijn’s idea in the 1930s to
address the important and urgent issue of the wavelength dependence of
interstellar extinction by comparing similar stars with strong effects of
reddening and insignificant amounts, was timely. The difficulties to obtain
the funding and the crippling effects of the economic depression and the War
delayed the project by about two decades, so that by that time photoelectric
techniques had made photographic approaches obsolete, while the results
obtained by van Rhijn and Borgman have shown the program to be viable and
capable of providing definitive answers. The lack of support from the Ministry
in funding the telescope and refusal to fund at least the dome, has been a
major factor in the delay and has therefore been a major cause of van Rhijn
missing this important opportunity.

How influential was van Rhijn and what was his role in the prominence of Dutch
astronomy that followed during the twentieth century?

Obviously, in the field of stellar and Galactic astronomy he was overshadowed
by de Sitter, Hertzsprung and Oort in Leiden, but probably not by Pannekoek in
Amsterdam. The latter’s legacy after all is in the area of astrophysics and
stellar atmospheres. Minnaert in Utrecht was prominent in solar research. Van
Rhijn kept Kapteyn’s legacy alive, but the dominance of Dutch galactic and
stellar astronomy in the twentieth century resulted from the work of the trio
from Leiden and constitutes a major part of Kapteyn’s legacy. Van Rhijn’s
influence on the development of Dutch astronomy was limited, but his
coordination of the work on the Selected Areas remained important for
astronomy as a whole.

That is not to say, van Rhijn did not contribute. He did produce — or at least
supervised in the final stages — a number of excellent PhD theses and
students, that later would occupy very prominent positions in the Netherlands
and abroad, such as Jan Schilt, Jan Oort, Peter van de Kamp, Bart Bok and
Adriaan Blaauw. In the same period there had been in Leiden Dirk Brouwer (with
de Sitter), and Pieter Oosterhoff, Gerrit (Gerard) Kuiper and Adriaan
Wesselink (with Hertzsprung). In this respect van Rhijn did extremely well,
also compared to Leiden. Of course there have been more outstanding
astronomers in the Dutch school, but these had their PhDs only after van
Rhijn’s active period: Oort’s most prominent students Maarten Schmidt and
Lodewijk Woltjer and van de Hulst’s Gart Westerhout followed in 1956 and 1958.
Utrecht with Marcel Minnaert contributed with Henk van de Hulst and Cees de
Jager, and Pannekoek in Amsterdam with Bruno van Albada, but only after WWII.
This selection is of course a subjective inventory and somewhat arbitrary, but
does show van Rhijn did contribute very significantly to the reputation of
Dutch astronomy in the first half and a bit of the twentieth century.

What made Adriaan Blaauw accept his appointment as van Rhijn’s successor?

I have argued that a significant factor was Blaauw’s appetite for adventure
and challenges. This made him accept Oort’s request to join the Kenya
expedition and give up leave his secure, tenured position in Leiden for an
uncertain, temporary one at Yerkes. But adventure or challenge is not enough.
The advent of radio astronomy and the allocation upon his request for a new
staff position and of funds for travel abroad to collect new observational
material together made an effort to effectuate a revival of Groningen
astronomy one with a reasonable chance of success. I suppose that had it
failed, Blaauw would easily have found some position elsewhere. What helped
him to pursue the kind of career he desired, was the attitude of his wife to
agree to let him go away from his family for often long periods and her
consent to move the family across the Atlantic twice in only a few years.
Adriaan Blaauw was exactly what Groningen needed after van Rhijn.

Of course, the University of Groningen had been keen as well on attracting
Blaauw and even provide opportunities for growth of the astronomy laboratory.
I have not been able to find any written statement on this in the Archives,
but to back this up I would think the university leaders and administrators
were still sufficiently aware of Kapteyn’s significance and his stature as one
of the greatest scientists in its history to keep his legacy intact, even
though the focus of Dutch astronomy had moved with Kapteyn’s protégés de
Sitter, Hertzsprung and Oort to Leiden.

In conclusion, Pieter van Rhijn may not have been of the same caliber as
Kapteyn (who could possibly blame him for that?), and his work and leadership
may have been not highly imaginative, but still an enormous amount of
important and useful work has been done at the Laboratorium under his
directorship to the great benefit of astronomy. And an admirable number of
extraordinary students wrote their theses under his supervision. He deserves
much credit for keeping the Plan of Selected Areas alive and coordinating the
enormous effort that was not likely to lead to much fame or praise for
himself. Van Rhijn disliked travel and attending large meetings, and it was
his misfortune to have much of his directorship coincide with the Great
Depression, World War II and his own protracted suffering of tuberculosis,
while the Minister denied him funding at the level he was prepared to supply
Leiden, yet under these unfavorable circumstances, in the end in his modest
and unassuming manner his efforts left the spirit and heritage of his
inspiring predecessor intact until new leadership took over.

Acknowledgments I thank my colleague astronomer Jan Willem Pel and historian
Klaas van Berkel, for a critical reading of a draft of this paper and making
many detailed and very useful comments and suggestions. David Baneke and Ton
Schoot Uiterkamp also read the manuscript and made a number of important
remarks. I am grateful to Kalevi Mattila of Helsinki Observatory for providing
the information on the correspondence of Kapteyn with Donner on the issue of
Kapteyn’s succession. I am especially grateful to Gert Jan van Rhijn, grandson
of van Rhijn’s brother Maarten, who maintains the van Rhijn Website [113], for
good quality photographs and much extensive biographical information on Pieter
van Rhijn. I thank the staff of the Kapteyn Astronomical Institute, in
particular the secretariat and computer group, for support and help and the
director, Prof. Léon Koopmans, for hospitality extended to an emeritus
professor as guest scientist.

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