Karl Schwarzschild, Annie J. Cannon

and Cornelis Easton:

PhDs honoris causa of Jacobus C. Kapteyn

Pieter C. van der Kruit,
Kapteyn Astronomical Institute, University of Groningen,
P.O. Box 800, 9700AV Groningen, the Netherlands.
email<EMAIL_ADDRESS>
###### Abstract

Honorary degrees and particularly doctorates are important instruments to
enhance the standing of universities and professors, in addition to receiving
these as a measure of a scientist’s recognition. Jacobus C. Kapteyn from the
University of Groningen in the Netherlands, one of the most prominent
astronomers of his times, received three of these and has persuaded his
university to award at least three, possibly five. I examine the background of
the selection of the latter in view of developments in Kapteyn’s times in his
career, international astronomy and political and cultural circumstances.

Key words: History: Honorary degrees – History: Statistical astronomy –
History: Galactic astronomy – History: spectral classification – History:
Selected Areas

##  1  Introduction

Universities boost their standing and prestige by appointing or keeping
important scientists on their staff that have performed groundbreaking
research, preferably while being employed by them, but if necessary when
performed elsewhere. Listing alumni that have become leading researchers
anywhere else but who started their careers by defending their thesis work at
the university involved is also used to justify claims concerning success and
status. Another important instrument is awarding honorary doctorates to
prominent scientists or cultural or political figures.

One aspect of a professor’s success may be taken to be who his or her students
are. This involves for professors at most universities supervising research of
students that leads to the defense of an important thesis, after which
conferring a doctorate on the student as ‘promotor’111In the Netherlands,
Belgium, Germany and a few other countries the professor who has supervised
the research is designated as the promotor and bestows the degree upon the
candidate on behalf of the university. When I use this word ‘promotor’ below
it may also refer to the equivalent in other countries. on behalf of the
Senate (or other body representing the university). But it is also possible to
nominate persons for the degree of doctor honoris causa (or for short h.c.)
and acting as a honorary promotor.

Jacobus C. Kapteyn was a professor of astronomy at the University of Groningen
in the Netherlands from 1878 to 1921. In his days he was one of the most
prominent astronomers in the world, which can be illustrated by the fact that
the most important observatories signed up to his Plan of Selected Areas in
which observations of stars in each of the 206 Areas were collected. Harvard
College Observatory, under the directorship of Edward C. Pickering,
photographed all these areas and sent large stacks of plates to Groningen for
measurement of positions and magnitudes. Even more telling, George E. Hale in
1908 adopted Kapteyn’s Plan as the primary program for his brand new, giant
60-inch telescope on Mount Wilson, the largest in the world, and had Kapteyn
appointed for life as research associate by the Carnegie Institution of
Washington, and between 1908 and 1914 Kapteyn annually visited Mount Wilson.

Kapteyn did not have many students defending their theses under him during the
tenure of his professorship. There were only eight of which Willem de Sitter
undoubtedly is the most prominent. Pieter J. van Rhijn, who was to become his
successor, has also gained some international standing. Actually Adriaan van
Maanen might also be counted as a student of Kapteyn, since he performed a
significant part of his thesis research in Groningen with Kapteyn; in spite of
graduating under Albertus A. Nijland in Utrecht he regarded Kapteyn as his
real mentor. Three more students started their thesis work while Kapteyn was
in office but completed their theses only when van Rhijn had taken over (and
in fact after Kapteyn had died). Of these Jan Schilt pursued a very successful
career in the USA, but without dispute the most prominent of Kapteyn’s
nachwuchs (i.e. including the ones that started their research under Kapteyn
even though van Rhijn eventually was their thesis supervisor) is Jan Hendrik
Oort. Much more on Kapteyn and Oort can be found in my academic [24][25] and
wider-audience [26][27] biographies.

It is not well documented that Kapteyn was promotor in at least three cases of
a doctorate honoris causa. The first one, awarded to journalist and amateur
astronomer Cornelis Easton, is usually noted, but those of very prominent
astronomers Karl Schwarzschild and Annie Jump Cannon are much less known.
Schwarzschild’s is rarely mentioned, Cannon’s is usually listed but
understandably eclipsed by the more prestigious one she obtained a few years
later from Oxford and sometimes only the latter one is mentioned.

The circumstances applying to the conferring of these degrees are the subject
of this paper. Questions addressed are the following. Was this unusual? Why
did Kapteyn select these people? Did Schwarzschild and Cannon actually come to
Groningen to receive the honors? Kapteyn himself had three doctorates h.c.,
Capetown, Harvard and Edinburgh. How does this compare with others? He had
none from English (distinct from British), German or French universities,
where after all many of the leading astronomers in Europe were working. Does
this tell us something or doesn’t it?

This article is aimed at an audience of historians and others interested, not
necessarily with an astronomical expertise. I therefore will describe in some
detail the required astronomical background to appreciate the significance of
matters discussed. The reader is assumed to be familiar with basic
astronomical concepts.

##  2  Background

###  2.1 Honorary degrees: General

According to Merriam-Webster an honorary degree is ‘a degree given by a
college or university to someone who is not a student but who has done
something important’ [16].

The first such degree seems to have been awarded by the University of Oxford,
UK. On their Website we read [18]:

> ‘The earliest honorary degree (in the sense which we would understand it
> today) appears to have been offered to Lionel Woodville in 1478 or 1479.
> Woodville, Dean of Exeter and the brother-in-law of Edward IV, appears to
> have already held the degree of Bachelor of Canon Law; the University
> offered to confer the degree of Doctor of Canon Law on him without the usual
> academic exercises. It was thus an offer to dispense with the usual
> requirements, but was apparently unsolicited and clearly an attempt to
> honour and obtain the favour of a man with great influence.’

In the USA the first honorary degree has been awarded by Harvard University in
1692 to Increase Mather, a minister and staunch Puritan, who between 1685 and
1791 was Acting-President, Rector and President of Harvard College. Harvard
University claims to have awarded over 2,300 honorary degrees by now [7]. In
the UK the universities of Oxford and Cambridge are prolific awarding
institutions; according to Heffernan & Jöns [8], these awarded respectively
1,487 and 1,111 doctorates during the twentieth century. Both universities now
award between eight and ten degrees per year. Harvard University nowadays
awards between 5 and 15 degrees. But these of course are it not all doctor
degrees.

The University of Groningen has awarded almost 300 honorary doctorates, the
first occurring in 1717 [23], which was for Abrahamus Trommius (Abraham Trom)
in theology for a concordance of both the Old and the New Testament. However,
already from 1618 onward there were doctorates awarded sine examine, so
without submitting and defending a thesis. An interesting example is one
awarded in 1634 to famous theologian Gisbertus Voetius, who was a preacher
that was being appointed professor of theology in Utrecht. It was felt
undesirable that he would award PhDs without having obtained this degree
himself and this is how that was solved. Astronomer Frederik Kaiser in Leiden
was a comparable case; he was promoted honoris causa in 1835 so that he could
be appointed lecturer in 1837 and eventually in 1840 professor of astronomy.

Honorary doctorates in Groningen are rare in most years but do come in a
larger numbers in years of an anniversary that is a multiple of fifty years.
The peak at the tricentennial in 1914 was enormous, 67 in total starting with
Queen Wilhelmina of the Netherlands. At other times the university has been
more modest, in 2014 at the fourth centennial (aptly named ‘for infinity’ or
‘4$\infty$’) there were only nine. In recent years that are not a lustrum
(quinquennium) it is mostly zero, sometimes one or exceptionally two.
Groningen doctorates honoris causa are much, much rarer than Cambridge, Oxford
or Harvard, but the latter ones are certainly more sought after.

For completeness I note that as a matter of principle some universities do not
confer degrees h.c. at all, such as Cornell University, the Massachusetts
Institute of Technology (MIT) or Stanford University. All Dutch universities
do award honorary degrees.

Like the first one in Oxford, doctorates h.c. are awarded often to royalty and
political figures. In case of the Netherlands, for example Queen Juliana of
the Netherlands has received honorary doctorates from both Leiden and
Groningen and in addition eight more from foreign universities. Some renowned
persons have tens of such honorary degrees; for example Nelson Mandela had
over fifty, in the Netherlands from Leiden university. Kapteyn’s University of
Groningen honored the Queens of the Netherlands, Wilhelmina in 1914 at its
300-th anniversary and Juliana at its 350-th, and more recently international
dignitaries Helmuth Kohl, Desmond Tutu and Ban Ki-Moon. This may sometimes be
controversial, e.g. for the case of Groningen when Kohl a few years later was
implicated in the CDU donations scandal, or when Oxford University’s governing
assembly, Congregation, by a large majority refused Margaret Thatcher an
honorary doctorate.

###  2.2 Honorary degrees: University of Groningen

For reference to the discussion in the remainder of this article, I give some
statistics on doctorates honoris causa in Groningen during Kapteyn’s tenure of
his professorship, actually between 1878 when he was appointed and 1922, one
year after his retirement and the time of his death. I already alluded to the
remarkable peak of 67 in the year of the university’s tricentennial, 1914.
There were 14 in Kapteyn’s day before that year and 5 after. The first one in
1884 did not set the scene; it was famous physician and microbiologist H.H.
Robert Koch, one of the founders of bacteriology and the concept of infectious
disease. He would receive the Nobel Prize in 1905 for identifying the
bacterium that caused tuberculosis. But after that there were only Dutch
persons that had important academic contributions on their record, but for one
reason or the other had never been in a position to present a PhD thesis; this
is of the kind of what we might call ‘corrective’ honorary doctorates. One of
these was amateur astronomer Cornelis Easton, to whom I will return below. One
other example was the first woman that received a honorary doctorate in
Groningen, Jantina Tammes, who studied for and obtained secondary teacher
qualifications in a number of natural sciences and came to work as assistant
of Willem Moll, professor of botany in Groningen. Her contributions led to the
award of a doctorate h.c. in 1911. She went on to become the first female
professor in Groningen and the second in the country (Johanna Westerdijk, also
botanist, in Amsterdam preceded her; it is telling for the slow progress of
women equality that both were extraordinary, that is professorships added to
the ordinary contingent for the individual involved). Except for Koch in 1884
all up to the 1914 peak are of this type of ‘corrective’ honorary doctorates.

Another good example is Maria H.J.P. Thomassen, who was promoted in 1905
honoris causa in the Faculty of Medicine. This is an amusing case for another
reason, namely that this person was incorrectly claimed on the basis of the
list on the University’s Website [23] to be a physician and the first female
doctor h.c. in Groningen [17]. However Maria Thomassen was definitely male
(but from Roman Catholic parents in the Dutch southern province of Limburg,
where boys named Maria were not uncommon) and not a physician, but a
veterinarian. He had studied at the prominent School for Veterinarians in
Utrecht, which could not award doctorates. Later he became a teacher at this
school and did significant scientific research, some of which together with
Hartog Hamburger, professor of physiology in Groningen, who acted eventually
as honorary promotor. This School in 1925 would become a Faculty within the
University of Utrecht and then doctorates could be awarded.

It should be noted that cases of ‘corrective’ honorary degrees were not
uncommon, because the occasion arose easily. An important illustration is
related to the admission to universities. This was at first restricted to boys
(no girls yet) from the Gymnasium (grammar school). But in 1863 a new school
type was introduced (Hoogere BurgerSchool HBS or Higher Civic School), in
which much attention was paid to mathematics and natural sciences, but where
no Greek and Latin was taught. This was a preferred route around the Fin de
Siècle (the end of the nineteenth century) for boys and girls attracted to
mathematics and science, where admission to the university was accomplished by
taking a special, additional exam. It is the route many Dutch Nobel Prize
winners of that period had followed. Also astronomer Jan Hendrik Oort did
this. Now persons having gone through the HBS were like others admitted to
defend a thesis, except for a long time in Medicine. Oort’s father was a
physician specializing in psychiatry and, having attended the gymnasium, had
defended a thesis in Medicine in Leiden under the first professor of
psychiatry in the Netherlands, Gerbrandus Jelgersma. Jelgersma had attended
the HBS and then studied medicine, and for this reason was not allowed to
submit a thesis. This was solved by the University of Utrecht by awarding
Jelgersma a doctorate h.c. Other physicians went to a university abroad to
defend a thesis.

In 1914 there were 44 foreigners among the doctores honoris causa in
Groningen, one of which was Karl Schwarzschild. The other 43 were mostly
important scientists, notably Léon Duguit, leading French scholar of public
law, Niels Thorkild Rovsing, Danish surgeon, Arthur Louis Day, American
geophysicist and volcanologist, Henri Pirenne, Belgian historian, and British
Alicia Stott (née Boole), female mathematician who never held an academic
position, but nevertheless made a number of valuable contributions to her
field. These were definitely leaders in their field, but probably the most
important scientist was Svante August Arrhenius from Sweden, founder of
physical chemistry, who had received the Nobel Prize for Chemistry in 1903.

The remainder were influential Dutchmen, such as famous architect Hendrik
Petrus Berlage, social democrat but known as communist sympathizer, well-known
poet Albert Verwey, and musician, music teacher, composer and conductor of
classical music Peter Gijsbert van Anrooy, who was a very good friend of
Kapteyn. But also Carel Coenraad Geertsema, prominent politician and public
administrator and at the time also President-Curator of Groningen University.
A special case was the doctorate awarded to steel magnate and philanthropist
Andrew Carnegie. The archives of Groningen University contain a telegram from
him in which he profoundly thanks for the honor but notifies that he would not
be present at the celebrations.

In 1914 there were 43 ordinary professors at the University of Groningen, so
most of these could have proposed two candidates (or more) for these honorary
degrees. A number of them must have been chosen not by individual professors,
but by the governing bodies within the university as a whole, especially the
ones that concerned politicians or public administrators. Since van Anrooy was
a very good friend of Kapteyn, the latter might very well have taken the
initiative and proposed his name. Carnegie might very well have come out of
Kapteyn’s sleeve as well. After all, there was no other connection to my
knowledge between the university of Groningen and Carnegie other than
Kapteyn’s appointment by the Carnegie Institution. It might have been a way of
showing gratitute for this and the excellent and unique opportunities offered
to Kapteyn to conduct his research at the Mount Wilson Observatory. Then it
would make sense that it was Kapteyn himself who had come up with the idea.

Between 1914 and 1922 there were only five more honorary doctorates, four to
Dutch public figures and one to Annie Cannon. Of the 19 awards during
Kapteyn’s professorship and outside the celebrations in 1914, only two were
international scientists (Koch and Cannon), the rest Dutch persons that were
awarded the degree in the ‘corrective’ sense defined above. That Kapteyn was
allowed to propose a degree for a prominent foreign scientist in an ordinary
year was very unusual and testifies to his exceptionally prominent status
within the university. After all, Kapteyn’s fame especially in the US,
evidenced by his collaboration at Mount Wilson and his appointment by the
Carnegie Institution, must not have gone unnoticed by his peers, such as the
other professors and fellow members of the Senate. The numbers suggest that
having proposed three and having that many accepted must have been highly
unusual, but Kapteyn’s status within the university was such that if someone
was allowed that many it would have to be him.

On the other hand we may ask: which Groningen professors were honored with a
doctorate h.c. elsewhere between 1878 and 1922? This information can be found
from the annual summary of events (the ‘Lotgevallen’ or Happenings, literally
‘Fates’) in the University by the Rector Magnificus. However, this may not
always be complete, since the relevant installment fails to mention Kapteyn’s
honorary degree from Harvard! These were:

> $\bullet$ Bernard Hendrik Kornelis Karel van der Wijck, professor of
> philosophy and logic (Edinburgh 1884),
> $\bullet$ Jan Willem van Wijhe, professor of anatomy and embryology, founder
> of the Anatomical Laboratory (Freiburg 1889, Aberdeen 1906),
> $\bullet$ Jacobus Cornelius Kapteyn (Cape Town 1905, Harvard 1909, Edinburgh
> 1921),
> $\bullet$ Hartog Jacob Hamburger, professor of physiology and histology,
> founder of the Physiological Laboratory (Aberdeen 1906, Utrecht 1922, Padua
> 1922),
> $\bullet$ Anton Gerard van Hamel, clergyman and professor of French language
> and literature, editor of the prominent Dutch literary magazine ‘De Gids’
> (Utrecht 1906),
> $\bullet$ Barend Sijmons, professor of German language and literature (St.
> Andrews 1912),
> $\bullet$ Franz Marius Theodor de Liagre Böhl, professor of Hebrew and
> Assyrian (Bonn 1915).

Hamburger’s degree in Utrecht was actually at the centennial of the School for
Veterinary Science. The total count is then twelve degrees bestowed upon seven
individuals in 45 years. Kapteyns degrees from Cape Town and Harvard were the
only ones from outside Europe.

###  2.3 Jacobus Cornelius Kapteyn

Jacobus C. Kapteyn (see Fig. 1), who lived from 1851 to 1922, is one of the
most prominent founders of the field of statistical astronomy. A general
introduction to this field with much backgrouns and Kapteyn’s contributions
can be found in [14], to Kapteyn himself see my biographies of him
[24][JCKERng]. Kapteyn’s professional life’s aim was to determine the
distribution of the stars in space, building on the approach of the Star
Gauges by William Herschel about a century before him. His first contribution
was the provision of a catalogue of stars in the southern hemisphere
comparable to the Bonner Durchmusterung in the north and for this his part was
to measure out the photographic plates taken of the complete southern sky by
David Gill from Cape Town. This took him twelve years of labor. The Cape
Photographic Durchmusterung was published in three installments, the last one
in 1900, the final year of the nineteenth century, when Kapteyn approached the
age of fifty. He developed the method of statistically estimating distances of
groups of stars, so-called secular parallaxes, using their proper motions
which contain a reflection of the motion of the Sun through space in addition
to the peculiar motion of each star itself. His assumption was that these
latter components were homogeneous and isotropic. For his goal of constructing
the structure of the sidereal system he needed two more assumption, namely
that there was no attenuation of light in space by scattering or absorption by
interstellar dust and that everywhere the mix of stars was the same.

By careful study of proper motions across the sky and looking for systematic
patterns he discovered that this first assumption was false, and that in
addition to random motions of the stars there seemed to be a set of two
systematic streams, – when corrected for the motion of the sun – precisely
opposite to each other, accurately directed along the Milky Way, and, using
radial velocities from elsewhere, at a relative speed of about 40 km/sec.
These are Kapteyn’s Star Streams, which he announced at a large international
congress during the St. Louis World Exhibition in 1904. Karl Schwarzschild, as
we will see, soon came up with an alternative explanation that proved in the
end to be the correct one. Of course the presence of the Streams significantly
complicated Kapteyn’s use of secular parallaxes, but did not make it
impossible.

In order to proceed with his program to map the stellar distribution in space,
he devised a very ambitious undertaking, defining 220 areas of sky that
avoided bright stars and unusual crowding of fainter stars to derive
properties of all stars to as faint levels as possible: magnitudes, colors,
proper motions, radial velocities (because of the difficulty of obtaining
sufficiently accurate spectra for all but the brighter stars this was not
restricted to the formal areas), etc.

[t]

Figure 1:  Jacobus C. Kapteyn from a drawing in 1908 by Cornelis Easton.
Kapteyn Astronomical Institute.

One very fundamental new property that he needed to consider as well was the
spectral type, a classification of the stars in terms of the lines in their
spectra that proved later fundamental in understanding differences among stars
and their structure and evolution. This classification scheme was defined
around 1901 by the work of Annie J. Cannon (building also on the work of
others), although at the time the physics and astrophysics underlying it was
not at all understood.

This Plan of Selected Areas was defined in 1906 and Kapteyn succeeded in
assuring commitment of provision of observational material from some twenty
major observatories. In the Introduction I already mentioned Harvard College
Observatory under E. Pickering and Mount Wilson Observatory under George E.
Hale for providing large amounts of observing time to obtain plate material
for the star counts and colors. Some notable further contributions were from
the German Sternwarten of Bergedorf (Hamburg) and Potsdam (Berlin) for
spectral typing using objective prisms, Radcliffe (Oxford), Yerkes, Yale, and
the Cape for proper motions and parallaxes, Lick and Mount Wilson for radial
velocities, and quite a few others. It took decades to complete all of this.

To briefly round up the story on Kapteyn, I note that he did a number of
studies on interstellar absorption (extinction), correctly inferring that it
would be stronger in the blue and therefore give rise to increasing reddening
of stars with increasing distance, but he kept worrying that he automatically
selected in his studies different mixes and therefore colors of stars with
distance. He actually arrived at quite reasonable values for the amount of
extinction, as we know now, although to a significant extent this was
fortuitous. In 1916, he quickly accepted Harlow Shapley’s evidence for absence
of absorption when the latter found that with Kapteyn’s absorption stars in
globular clusters would have to be intrinsically some two magnitudes redder
than observed. The option that dust was restricted to the plane of the Milky
Way did not seem to have occurred to Kapteyn, Shapley or any other influential
astronomer at the time. Although astronomers suspected the presence of
absorbing dust, it took untill the work by Robert J. Trumpler in 1930 on
diameters of star clusters that the case for interstellar absorption was
convincingly settled and it had become clear that it was restricted to a thin
layer in the plane of the Galaxy.

Neglecting absorption Kapteyn near the end of his life used star counts and
proper motions to make a ‘first attempt’ at deriving a rather flat model of
the sidereal system. His final major contribution was to open up the field of
stellar (nowadays usually called galactic) dynamics, where he explained the
equilibrium of the spatial distribution as a balance between the motions of
stars and their collective gravitational force. In the perpendicular direction
the distribution was maintained by random motions, in the plane of the system
in addition by rotation and the associated centrifugal motion which he found
was adequately provided by assuming two systems of stars rotating in opposite
directions, which he identified with his Star Streams. The sun was in the
inner parts, some 650 pc from the center.

Now, this contrasted with Shapley’s more spherical and larger system of
globular clusters. Eventually this was brought together in the picture of a
disk and halo, respectively Kapteyn’s system expanded by a factor of three or
so when extinction was realized to affect the stellar distribution in the
plane, and Shapley’s shrunk by factor two. It took until 1938 before Jan
Hendrik Oort used the accumulating data in the Plan of Selected Areas to
repeat Kapteyn’s analysis allowing for absorption. For detailed discussions
and references to major papers in the development of all of this see
references [24] through [27]. Note that any discussion on Kapteyn suffers from
the fact that his archives, curiously except for the letters David Gill sent
him, have been lost, presumably during the bombing of Rotterdam in 1940 (see
contribution by Petra van der Heijden in the 1999 ‘Kapteyn Legacy’ symposium
[28]).

##  3  Dr.h.c. C. Easton and Kapteyn’s 25th anniversary as professor in 1903

Cornelis Easton (see Fig. 2) lived from 1864 to 1929. He was primarily a
journalist, but in addition an accomplished amateur astronomer (see [2] for
biographical notes; this and the English-language article [29] is based on an
extensive biographical article in Dutch by astronomer Johan W.J.A. Stein SJ of
the Specola Vaticana (the Vatican Observatory) [20]).

After his secondary education (indeed HBS), he spent a few years studying
various subjects at what is now known as the Technical University of Delft,
after which he switched to study French and obtained the qualifications for
secondary school teacher in that subject. But rather than picking up that
profession he took on jobs as a newspaper journalist, eventually as editor
(in-chief) of some daily or monthly newspapers and magazines. He had developed
a strong interest already as a child in astronomy and as a young man started
producing drawings of the Milky Way, some of which he published. He very early
on wondered what it would look like ‘from space’, that is to say from outside
and from other directions, speculating there was spiral structure as in Lord
Rosse’s famous drawing of the Whirlpool Nebula. This gave him the idea that
the Milky Way had in fact the structure of a spiral-shaped star cluster.

So Easton had been intrigued by the observation of spiral nebulae and became
convinced that our Galaxy should not be any different and had to have spiral
structure as well. He therefore derived a model for the Galactic spiral
structure on the basis of the assumption that the sun is well away from the
center and in areas in the Milky Way that were relatively bright we were
looking along spiral arms and in darker areas in between such arms. In 1900 he
published his ideas in an article in the Astrophysical Journal presenting it
in the title as A new theory of the Milky Way. He eventually improved this
using photographs of the Milky Way from many sources giving rise to the
representation in Fig. 3, taken from another Astrophysical Journal publication
by Easton in 1913 [5]. The ring at the outer edge is a sketch of the Milky Way
on the sky between latitudes $+10^{\circ}$ and $-10^{\circ}$, and the inner
part the derived structure. It must be said that Kapteyn seemed to have judged
this to be an unjustified over-interpretation.

[t]

Figure 2:  Cornelis Easton in 1906. This picture comes from the Album
Amicorum, presented to H.G. van de Sande Bakhuyzen on the occasion of his
retirement as professor of astronomy and director of the Sterrewacht Leiden in
1908 [12].

[t]

Figure 3:  Easton’s model for the spiral structure of the Stellar System,
based on a sketch of the Milky Way from many photographs in the outer shell.
Reproduced from Easton’s paper in 1913 [5].

Still, Easton’s efforts were sufficient reason to propose a honorary doctorate
in 1903. It is probably not a coincidence that Kapteyn that year celebrated
his 25th anniversary as a professor. In 1878 he was appointed on a new chair
in astronomy in Groningen, spurred by the fact that a new law on higher
education had come into force. This law stipulated among others that the
curricula at the three state-funded universities should be the same and this
gave rise to a very substantial increase in professors at and in the budgets
of the universities. For Groningen a new professorship in astronomy was
opened, but no accompanying observatory, which resulted in Kapteyn’s
establishment of an astronomical laboratory, an ‘observatory without
telescopes’. So there had been a burst of professorial appointments in 1878
and the Rector Magnificus in Groningen in his annual report on the
‘Lotgevallen’ of the University noted with great satisfaction that of these
many jubilees Kapteyn had been the only one to be honored with a prestigious
Knighthood in the Order of the Netherlands Lion. Maybe the university honored
Kapteyn by allowing him to bestow an honorary doctorate.

From the ‘Lotgevallen’ of the University for the academic year 1902–1903:

> ‘One doctoral award took place honoris causa, in a public and extraordinary
> session of the Senate. On the 13th June Mr. C. Easton from Rotterdam, where
> he is an editor of the Nieuwe Rotterdamse Courant, on the basis of his
> excellent service for the science of astronomy, was promoted honorably to
> doctor in mathematics and astronomy. Promotor was prof. Kapteyn.’

In the NASA/Harvard Astrophysics Data System, Easton has no fewer than 22
publications, of which 11 in leading international journals including in
addition to the Astrophysical Journal, the Monthly Notices of the Royal
Astronomical Society, Nature, Astronomische Nachrichten, and the Bulletin of
the Astronomical Institutes of the Netherlands. Some of these publications
concern correlations between the distribution of bright stars and the
brightness of the Milky Way and distances of features in the Milky Way.

He was also interested in long term patterns in the weather. Kapteyn had been
interested in this also. From widths of tree rings in oak trees in the Trier
area in Germany, Kapteyn had inferred also in the 1880s a periodicity of 12.4
years in the amount of rainfall between 1770 and 1880. Easton analyzed
‘modern’ temperature data for Western Europe since around 1850, less accurate
ones in the hundred years before that and classifications of the harshness of
winters in the centuries before that. Easton then had found that there was an
89-year period in the occurrence of severe winters, about seven times the
period of Kapteyn’s cycle. Attempts to attribute that at least partly to the
solar cycle were not successful.

Easton eventually became chairman of the amateur organization Netherlands
Association for Meteorology and Astronomy and editor-in-chief of its
periodical Hemel & Dampkring (‘Sky & Atmosphere’). For an amateur astronomer
his contributions are remarkable.

##  4  Dr.h.c. K. Schwarzschild and the University’s tricentennial in 1914

Karl Schwarzschild (see Fig. 4) lived from 1873 to 1916 (for two authoritative
obituaries see Hertzsprung [9] and Eddington [6]). Already during his
secondary school he published two papers on determinations of orbits in binary
stars and as a student another paper on variable stars. He studied under Hugo
H. Ritter von Seeliger in München (Munich). Von Seeliger would later produce a
model of the sidereal system much like Kapteyn’s and at about the same time.
However, it was more mathematical, schematic and difficult to follow.
Schwarzschild wrote a PhD thesis on Poincaré’s theory of rotating liquid
bodies, after which he spent some time as assistant at Vienna, where he
developed new methods to derive stellar magnitudes from photographic plates.
This had of course been done before, notably by Kapteyn and Gill in the Cape
Photographic Durchmusterung (and also in the Carte du Ciel), but Schwarzschild
improved the procedure considerably by using extra- or intrafocal exposures,
in the process describing also the characteristic of reciprocity failure –
although not by that name – in photographic plates (that at faint levels it
takes more than twice the exposure time to reach the same level of
photographic density for twice as faint incident light). He derived an
empirical formula describing the relation between exposure (brightness
multiplied by exposure time) and the resulting photographic density. Already
in 1901, after a second period in München, he was appointed professor and
director of the Sternwarte in Göttingen. His most important work at that time
was the Göttingen Actometrie222 I keep the German name Actinometrie similarly
as I did for Durchmusterung in the CPD., a survey of stellar magnitudes
between the celestial equator and $+20^{\circ}$ declination, replacing his
method of out-of-focus exposures by a new method based on a regular shaking of
the plate during exposure to distribute the light over a larger area of
emulsion. The instrument developed for this was called a Schraffierkassette.

[t]

Figure 4:  Karl Schwarz- schild in one of the few pictures available of him.
From the English Wikipedia (copyright expired).

The Göttingen Actinometrie contained 3500 stars. The word Actinometrie was
derived from the instrument that John Herschel had built, with which he had
determined the energy radiated by the sun by measuring the increase in
temperature in a closed volume that was exposed to sunlight. Herschel had
named that an actinometer, from the Greek word ‘aktina’ for ray (of light).
The term has also been used by John A. Parkhurst who published about the same
time as Karl Schwarzschild the Yerkes Actinometry of stars around the north
celestial pole, a kind of precursor of the North Polar Sequence. The Göttingen
Actinometrie produced an important piece of information, namely the very tight
correlation between stellar colors and spectral type, thus establishing Karl
Schwarzschild’s reputation..

In 1909 Schwarzschild was appointed director of the Astrophysikalisches
Observatorium in Potsdam, a very prestigious position. Potsdam took part in
Kapteyn’s Plan of Selected Areas, eventually providing accurate photographic
magnitudes for stars in the northern Areas that were too bright for the deep
plates taken at Harvard and Mount Wilson. The Schwarzschild Archives at
Niedersächsische Staats- und Universitätsbibliothek Göttingen has provided me
with scans of the relevant correspondence. Kapteyn and Schwarzschild
corresponded regularly. The Kapteyns visited Potsdam in May 1911 and there is
also some correspondence between Mrs. Kapteyn and Mrs. Schwarzschild, the
former writing in English apologizing that German is ‘beyond my power’.

Kapteyn and Schwarzschild were on very good terms. The latter had asked
support from Kapteyn to have two younger astronomers invited to spend some
time at Mount Wilson, for which Kapteyn mediated by recommending them to
George Hale. The first person that Kapteyn introduced to Hale on
recommendation by Schwarzschild was Ernst A. Kohlschütter, who had obtained a
PhD from Karl Schwarzschild while the latter was still at Göttingen and had
moved to Hamburg afterwards. This opened what David DeVorkin has dubbed the
‘Pipeline’ (see his contribution to the 1999 ‘Legacy Symposium’ on Kapteyn
[28] and that of Klaas van Berkel in the same volume). Kohlschütter went to
Mount Wilson in 1911 and stayed until the start of WWI. He worked with Walter
S. Adams and they discovered the relation between the width of spectral lines
and absolute magnitude (the dwarf-giant distinction for late types) and
introduced the spectroscopic parallax. Later Kapteyn and Adams had a bitter
argument when Kapteyn felt Kohlschütter did not get the credit he deserved
from Adams.

The second person was Ejnar Hertzsprung, a Danish astronomer working for Karl
Schwarzschild at Potsdam. Kapteyn introduced him to Hale in 1912 and took him
along during his Mount Wilson visit that year. Before that Hertzsprung spent
some time in Groningen and by the time they started out for California he was
engaged to the younger daughter Henriette Kapteyn. They got married in May
1913 and Henriette moved with him to Potsdam that year.

Although the Actinometrie was interesting work to Kapteyn, probably the most
relevant in the context of the honorary degree was Schwarzschild’s theoretical
work related to the structure of the Milky Way as a Stellar System. There are
two things to mention.

Figure 5:  Part of the paper by Karl Schwarzschild in which he developed a
method to solve the integral inversion problem in statistical astronomy. Here
he introduces special analytical forms of the distribution functions. From
[19].

The first concerns Kapteyn’s Star Streams. Not long after Kapteyn’s
presentation of these in 1904, Schwarzschild noted that the interpretation of
two distinct streams of stars moving through one another was not the only
possibility. He had described the distribution of peculiar velocities of stars
in space as much like that of molecules in a gas, which means according to the
Maxwell distribution. These velocities then are distributed as a Gaussian
curve, but isotropic, so Gaussians with the same dispersion (or mean velocity)
in all directions. Karl Schwarzschild proposed a distribution that was also of
this Maxwellian form but with dispersions that were different in three
different perpendicular directions. This then is in the observations
equivalent to two opposite streams in the direction of the largest dispersion.
It became known as an ellipsoidal distribution, which then would mimic two
opposite streams while in fact it was a single distribution that happened to
be anisotropic. Kapteyn has stuck to his interpretation of the distinct
streams on the basis of the observed property that the make-up of stars in
both streams was distinctly different. Prominent British astronomer Eddington
supported that view for a long time as well, until the evidence for different
compositions of the streams disappeared with better data. Schwarzschild’s
proposal was adopted definitely after Jan Oort had discovered Galactic
rotation and from dynamics showed that the long axis of the velocity ellipsoid
had to point towards and away from the center of the Galaxy. That Kapteyn’s
streams deviated from this by $20^{\circ}$, an observational result that
turned out to be correct and referred to as the deviation of the vertex,
remained a problem until it was explained by Oort later as due to the
gravitational influence of spiral structure. To honor his work in this area
the velocity ellipsoid remains to be referred to as the Schwarzschild
distribution.

Figure 6:  Diploma or ‘doctoral bull’ for the honorary degree of Karl
Schwarzschild from the University of Groningen in 1914 [15]

There was however another contribution of Schwarzschild that might have been
the strongest argument in favor of the doctorate h.c. and that has to do with
the derivation of the spatial distribution of stars from counts as a function
of apparent magnitude. Suppose that we know the distribution of stellar types,
and thus absolute magnitudes, at any position in the system. This ‘luminosity
function’ was assumed to be the same everywhere and in principle can be
determined locally using statistical studies and secular parallaxes. Now if we
know this luminosity function and the stellar density, the distribution of
stars, the counts of stars in a particular direction as a function of apparent
magnitude, follows from a summation using these two functions. Take an
apparent magnitude and a position on the sky. For each element of distance
from the sun along the corresponding line of sight each apparent magnitude
corresponds to a certain absolute magnitude. The number of stars seen at that
apparent magnitude from this element then is the total density of stars there
multiplied by the the fraction of stars of the required absolute magnitude,
which is the value of the luminosity curve at that absolute magnitude. All
such contributions from elements at other distances along the line of sight,
corresponding to different absolute magnitudes, then have to be added up to
find the total count of stars of that particular apparent magnitude.

Mathematically this is an integral, which can be evaluated in principle in a
straightforward manner. But the solution required is the inverse. Given the
form of the luminosity curve and the star counts at apparent magnitudes, the
problem is to determine the total density as a function of distance. That
means ‘inverting’ the integral and inversion of integral equations is
notoriously difficult. Now in 1912 Karl Schwarzschild, being interested in
this problem, possibly since he was a student after all of von Seeliger, had
developed a method to solve this problem for the case that the luminosity
curve was a Gaussian function (see Fig. 5). Of course this was of enormous
importance to Kapteyn. Indeed, later in 1920 he and van Rhijn chose the
Schwarzschild method to solve for the distribution of stars in space, which
resulted in the Kapteyn Model.333 When the luminosity function is unknown,
there actually is a second integral equation to be solved simultaneously and
that involves mean parallaxes as a function of apparent magnitude, derived
from proper motions and secular parallaxes. Kapteyn had studied this
mathematical problem around 1900 with his brother Willem, who was a professor
of mathematics in Utrecht. This remained cumbersome and Schwazrschild’s method
was to be preferred.

There are in the archives of the University of Groningen many notes related to
the honorary doctorates in 1914, but although discussions about how to choose
the candidates are recorded in minutes of the meetings of the Senate and
Faculties, there is no record of supporting arguments for the choices. It
seems obvious that Kapteyn had the work described above in mind when he
proposed Schwarzschild. The archives in Göttingen contain the letter to him in
which he was notified of the award. In accordance with the customs of the time
it is in Latin. No explanation of for what work the award was made, was given
in the letter. Also the diploma or bull was phrased in Latin (see Fig. 6).

The question is whether or not Karl Schwarzschild actually came to Groningen
to receive the degree in person and to attend the celebrations of the
tricentennial. The answer is he did. The direct evidence consists of two small
pieces of paper, on which he wrote rather short notes to the Rector
Magnificus. They were of course in German, translated into English they say:

> Potsdam, 20-IV-1914.
> To the Rector and Senate of the University of Groningen
> I express my cordial thanks for the honor you have extended to me. It will
> be a great pleasure to attend the celebration of the three hundredth
> anniversary of the university.
> Your sincerely dedicated
> K. Schwarzschild.

> Potsdam, 5 July 1914
> To the academic Senate of the National University of Groningen,
> I express cordial thanks for the sending of the beautiful medal that will
> help me to keep alive the remembrance of the impressive celebration in
> Groningen.
> Your fully dedicated
> K. Schwarzschild.

The archives of the University of Groningen contain a printed note by the
Rector Magnificus, specifying that the procedure to come to the selection of
doctores h.c. would be that each Faculty (there were five) would nominate six
scientists for de honorary doctorates plus another fifteen representatives of
foreign universities and learned societies, for a total of 45\. The Senate
would take care that the final lists would be uniform in composition of
nationalities, disciplines, etc. There is no good record in the archives of
the Senate, Rector Magnificus or Faculty of Natural Sciences of which
professor proposed which candidate. It would be a safe bet that Kapteyn had
proposed Karl Schwarzschild.

The university had 43 professors in five Faculties: Theology (6), Law (6),
Medicine (11), Arts and Philosophy (10), and Natural Sciences (10). It must
have been too difficult to decide which professor could and which could not
propose a person and have his proposal accepted, so that – in spite of the 45
proposed by the Rector – in the end the number grew to 67.

Figure 7:  The memorial medal that the University of Groningen had had struck
for the celebration of its tricentennial in 1914. From [21].

The file in the Archives with organizational details of the tricentennial
contains long lists of guests and representatives to the proceedings, and
since the hotel accommodations in Groningen were limited, visitors were for
the larger part invited to stay with professors in their private homes. The
archives contain a map of the city of Groningen so that visitors could find
their way around. It was oriented with the west at the top (note that the word
orientation comes from putting the east at the top). The accommodation lists
show that Schwarzschild and another honorary doctor, Peter van Anrooy, had
been staying with Kapteyn. No doubt Karl Schwarzschild came to Groningen to
receive the honorary degree and attend the celebrations. He is also on the
printed list of the 283 attendees of the celebratory dinner, as are Kapteyn
and van Anrooy.

The celebrations started on Monday June 29, with a session where guests were
received. A special commemorative book was presented [21]; it included an over
200 page history of the university written by well-known professor of history
Johan Huizinga (he would move to Leiden in 1915) and various other
contributions on the university in the past or present, including a
description by Kapteyn of his Astronomical Laboratory. Also a special memorial
medal was presented (Fig. 7), of which Schwarzschild later had received a
specimen according to his note above. The formalities of the celebration took
place on June 30 in the Nieuwe Kerk, a major church in Groningen, but not in
the large Martini Church in the center of town where currently such
festivities and ceremonies take place. The program wasfull of speeches by
dignitaries and representatives from other universities and learned societies.
Kapteyn had been asked by the US National Academy of Sciences to be its formal
representative and given a text to read out (see [24] for details); he had
been elected a Fellow of the Academy in 1907. On the following day, July 1, a
special session of the Senate took place during which the honorary degrees
were officially bestowed upon the 67 laureates. The Rector Magnificus took
care of the one for H.M. Queen Wilhelmina, the others were granted by the
Deans of the Faculties, so there was no specific individual promotor. For
Schwarzschild the Dean was biologist Jan Willem Moll. The degrees were
bestowed by pronouncing the official formula in Latin. Karl Schwarzschild was
honored for his ‘exceptional contributions to many aspects of astronomy and
physics’, and was proclaimed – somewhat contradictory – doctor in mathematics
and astronomy.

Nowadays Karl Schwarzschild is principally known for the Schwarzschild radius
of a black hole. This was work he did later. Weeks after the tricentennial the
First World War broke out and Schwarzschild – although already 40 years of age
– volunteered for the German Army, in which he eventually rose to lieutenant
of the artillery. In 1915 he was stationed in Russia where he wrote three
articles on General Relativity and Quantum Mechanics. He solved Einstein’s
field equations for the case of a point mass – or a single piece of very
concentrated matter – and found that there was an event horizon. Gravity is so
strong that light cannot escape through this horizon so that an outside
observer cannot be aware of any events inside. This dimension of a black hole
(a term coined later), defined as that of the event horizon, became known as
the Schwarzschild radius.

In 1915 in Russia Schwarzschild began to suffer from pemphigus, a rare
autoimmune skin disease of which he died in 1916 at the age of only 42.

Interlude: Dr.h.c. P.G. van Anrooy

Peter Gijsbert van Anrooy (1879–1954), who also stayed with the Kapteyns, was
a musician, composer and conductor. He had been director of the symphony
orchestra at Groningen (the Groninger Orkest Vereeniging) between 1905 and
1911; later he worked in the Hague as director of the renowned ‘Residentie-
orchestra’ (Den Haag is the place of residence of the government) and
‘Toonkunst-choir’ (toonkunst is a now somewhat out of date word for music in
general). In 1914 he worked in Arnhem in between these two assignments.

Henriette Hertzsprung-Kapteyn wrote a biography of her father [10], from which
I quote (in my English translation, also available on [10]):

> ‘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
> had 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 to 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 Anrooy, 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’, 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.’

At the funeral of Kapteyn, van Anrooy played on the organ the final choir from
Bach’s Matthäus Passion. Considering this friendship and teacher-pupil
relation it is not unlikely that van Anrooy was actually also proposed by
Kapteyn for a doctorate h.c. and was subsequently asked to put him up during
the festivities. The Dean of the Faculty of Arts and Philosophy, professor in
English literature and Sanskrit, Johan Hendrik Kern, praised van Anrooy in his
laudatory as ‘expert musician, excellent and skillful orchestra conductor, who
in our country successfully raised music to a higher standard.’ He was
promoted to doctor of arts.

I note before ending this section that Andrew Carnegie, who I suspect had been
proposed by Kapteyn, received an honorary doctorate in the Faculty of Law for
‘his dedication to the laws of war and peace, not only in words but
particularly in deeds’.

##  5  Dr.h.c. A.J. Cannon and Kapteyn’s retirement in 1921

Annie Jump Cannon (1863–1941) (see Fig. 8) studied physics from 1880 to 1884
at Wellesley College in Massachusetts, which was (and is) one of the top
academic schools for women in the USA. Her middle name ‘Jump’ was her mother’s
maiden name. She returned home, developing an interest in photography and
studied all aspects of it, perfecting her skills and as a photographer she
traveled through Europe. In 1893 she published her photographs after returning
home which drew some attention. At about the same time she suffered from
scarlet fever which left her almost deaf. After her mother’s death, she
returned to Wellesley where she was hired to teach physics. It was also there
that she developed an interest in astronomy and spectroscopy. She studied
astronomy at Wellesley and for this took courses at Radcliffe College not far
from Harvard College, where Edward Pickering, who taught there, noted her and
hired her in 1896 as one of his assistants. She finished her Masters at
Wellesley in astronomy in 1907. At Harvard, Cannon became a member of the
Harvard group of female computers that Pickering had hired.

[t]

Figure 8:  Annie Jump Cannon in 1922. From Wikimedia [30], copyright expired.

When the first spectra of stars had been obtained it became quickly clear that
there was a large variety among them. The usual attack then is to design a
classification scheme to order the characteristics. The resulting sequence
then may be, but does of course not have to be, interpreted as a evolutionary
one. This happened with stellar spectra, giving rise to terms ‘early’ and
‘late’, while in fact the sequence is to a large extent one of mass.

Figure 9:  Reproduction of the original introduction of the spectral types as
we know them today in 1901. At the top the proposed order (‘N’ later dropped
from being part of the sequence and ‘O’ was moved to the front). Below some
typical stars and their classifications. From [3].

Stellar classification started by the work of Pietro Angelo Secchi SJ of the
Specola Vaticana, who in the 1860s examined spectra of a few thousand stars.
He divided them into five classes, of which classes I to III are more or less
what we now would call ‘early’, ‘middle’, and ‘late’. The current scheme is
defined in the Henry Draper Catalogue. Henry Draper was a physician, but also
an accomplished amateur astronomer. Draper was one of the first to
photographically record the spectrum of a star at his private observatory and
recognized the similarities in the many stellar spectra he obtained to that of
the sun. After his death his widow funded the spectroscopic work of Pickering
at Harvard through a fund called the ‘Henry Draper Memorial’. Pickering
obtained photographic spectra of hundreds of stars and these were classified,
in first instance by Williamina Fleming into a new system, splitting up
Secchi’s type I in types A through D, type II in E through L, designating type
III as M and introducing a new type O (from ‘Orion’ since some stars in that
constellation belonged to this type). The result was published as the Draper
Catalogue of Stellar Spectra in 1890. This was refined by Antonia Caetana de
Paiva Pereira Maury, but particularly by Annie Cannon (using also spectra from
the southern station of Harvard College Observatory in Peru), who defined the
now universally adopted sequence O, B, A, F, G, K, M, each subdivided into ten
sub-classes. She is told to have been able eventually to classify 200 stars
per hour or 18 seconds per star. This resulted in the Henry Draper Catalogue,
published by Cannon and Pickering in nine volumes of the Harvard Annals
between 1918 and 1924, containing about 250,000 stars (and later extended with
the Henri Draper Extension).

Fig. 9 shows two tables from the original publication in 1901, in which Cannon
proposed more or less the sequence as we know it now and its subdivisions
(although the second capital letter later disappeared). The type ‘N‘ was later
considered not part of the sequence and the ‘O’ was moved to the top of it.

The proposal by Kapteyn to award Cannon an honorary doctorate is conserved in
the Groningen Archives. To preserve the authenticity I will reproduce it in
translation in full.

> ‘It is known that an entirely new field of research in classical astronomy
> has been added by the ascent of spectroscopy. A field in which insights have
> been given that in the past seemed impossible. – What we know about the
> evolution of cosmic bodies is based entirely, or at least for a major part,
> on spectroscopic observations and with the researches in the last few years
> it appears that the role spectroscopy will play in the study of the
> structure and motions of the sidereal world will become an almost equally
> fundamental one.
>
> In the meantime the faintness of stars are raising obstacles for the study,
> obstacles that for the moment can be overcome only for the brightest stars
> and then only after inexhaustible patience and tireless energy. Science
> however has to no lesser extent a need for spectra of fainter stars. It is
> especially for the faint stars, stars of magnitude 12, that statistical
> investigations have led to results concerning the structure of the universe.
> It is reasonable to expect that when such investigations can be done
> separately for each of the spectral types that the harvest of important
> results will be much richer.
>
> Unfortunately we are not there yet, but the most important step in this area
> appears to have been taken. After the Observatory in Potsdam in 1883 had
> obtained for a part of the sky spectra for 4000 stars, nothing was done in
> this field that in importance is a match to the work at the observatory at
> Harvard. In 1890 the so-called Draper Catalogue was published for all stars
> that can just be seen by a sharp, naked eye. In total 10,000 stars.
>
> Until almost the present time this is all that astronomers in general terms
> had at their disposal. The prospect of much extended determinations was not
> good. Telescopic stars need much more work and their number grows
> disturbingly quickly with every next unit of magnitude. Ten years ago no
> astronomer would have expected that now in 1921 we would have available
> 200,000 carefully determined spectral types of nearly all stars up to the
> $9^{th}$ magnitude plus a large number even beyond that of $10^{th}$ or even
> the $11^{th}$ magnitude. A quarter of those, comprising four tomes is
> already available to astronomers, the rest is in press and will presumably
> be accessible to everybody within a year.
>
> This gigantic effort, a true monument of organization, skill and
> perseverance is due to a single woman
>
> Annie J. Cannon.
>
> For all those who wish to study the evolution and the structure of the great
> stellar system the appearance of this publication is the most important
> event in the last few years. It opens the possibility described above to
> study separately stars of different spectral classes, classes just as
> diverse as the classes in the animal kingdom. What is missing to reach the
> same limit for all stars will also not take too long because completeness
> for the faintest stars will not be required.
>
> In what urgent need the work of Miss Cannon will provide, is most clearly
> illustrated by the fact that the Harvard Observatory as reaction to urgent
> insistence of a large number of astronomers has made, by mail, available the
> manuscript with an enormous mass of data. The Groningen Laboratory would not
> have been able to complete its latest publications without the thousands of
> spectra that Miss Cannon with extraordinary kindness has made available.
>
> I believe that it is a real obligation for astronomers to give a proof of
> their gratitude and bring homage for a piece of work for the satisfactory
> completion of which except for Miss Cannon possibly no one else in the world
> would have had the skill, perseverance and self-sacrifice; a piece of work
> so urgent and of such far-reaching significance. And since Groningen,
> possibly more than any other university, had the fortune to profit from this
> work, I believe that the Senate and University of Groningen should not forgo
> the privilege to associate itself with the author
>
> Miss Annie J. Cannon
>
> by offering her an honorary doctorate in Mathematics and Astronomy.’

The Groningen work Kapteyn refers to is that of van Rhijn, who was making
preparations, by determining mean parallaxes as a function of apparent
magnitude, to repeat the analysis for the distribution of stars in space
separately for various spectral types. The proposal was approved and Annie
Cannon was duly informed of the decision. No record of this is present in the
Groningen Archives, nor among the scans that were made for me of the Cannon
Archives files on the Groningen and Oxford honorary doctorates at the Harvard
University Archives – Pusey Library. The letter in reply is available at
Harvard in handwritten and typed form, the latter being identical to the
handwritten version that was actually sent to the Groningen Rector Magnificus.
In part it reads:

> ‘… To be ranked among the scholars of that ancient and renowned seat of
> learning founded just before the Pilgrim Fathers left Holland for the
> wilderness of my own Country, to be thus linked with the Groningen
> Astronomical Laboratory, made famous by Professor Kapteyn, the world’s
> greatest astronomer of to-day, is indeed the highest honor of my life, far
> exceeding my fondest dreams.
>
> It will be impracticable for me to go to Groningen to receive the diploma in
> person, and therefore I shall be most grateful to you if you will be kind
> enough to forward it by mail. …’

So Cannon did not come to Groningen, which may have been a real disappointment
for Kapteyn. There was then no special celebratory session of the Senate at
which the doctorate would have been bestowed. The files on her Oxford honorary
doctorate in the Cannon Archives actually contains a letter from Kapteyn
concerning her Groningen degree, dated 10 June 1921:

> ‘Dear Miss Cannon,
> Let me be the first to congratulate you on the well earned honour conferred
> on you by the University of Groningen. As far as I know you and
> Schwarzschild are the only persons upon whom the doctorate in Mathematics
> and Astronomy ‘honoris causa’ has been bestowed. I hope you will find in
> this tribute at least some small return for a work which even in Astronomy
> has hardly a parallel, a work which is so urgently demanded for the further
> progress of science and which will earn for you the gratitude of all who try
> to penetrate somewhat further in the mysteries of the stellar Universe.
>  Very truly yours,
>  J. C. Kapteyn’

Kapteyn fails to mention Easton, so maybe he did not consider him to be a real
astronomer. However the phrasing is such that Easton should have been
mentioned. The diploma was sent from Groningen on July 6, accompanied by a
letter from the Rector expressing regret she would not come but ‘understanding
her motives’. It is noteworthy that the files in the Cannon Archives do
contain a large number of congratulatory letters, telegrams and cards, mostly
from friends and organizations (such as the President of Wellesley College),
but only one from an astronomer (Otto J. Klotz, director of the Dominion
Observatory in Canada).

Among the further honors Cannon received were honorary doctorates from
Wellesley College, her Alma Mater, where she had been both a student and a
teacher, and from Oxford University in the UK. She had received only a
Master’s degree form Wellesley but had not continued to submit a PhD thesis,
so this was a special honor. This was in 1925 and the two proposed dates for
the ceremonies almost excluded Annie Cannon from attending both. Not much
correspondence is available on the two degrees in the Cannon Archives. The
only relevant piece is a draft of a letter to the President of Wellesley
College, which begins as follows:

> ‘It is with the deepest appreciation of the high honor proposed for me by
> the trustees of Wellesley College that I write you my acceptance and
> expectation to be present on May twenty-ninth. It is the very day I was
> booked to sail, but I find that I can change to a boat going a few days
> later which will put me in England in due season. …’

There are essentially no congratulatory letters, telegrams, etc. in the Cannon
files associated with these two honorary degrees, so these must have filed
somewhere else.

There is a well-known photograph of Annie Cannon where she wears the gown and
cap associated with her doctorate h.c. from Oxford. She seems to have hoped to
receive such a gown also from the University of Groningen. Both the Cannon and
Groningen files contain a letter from the Rector Magnificus (original in
handwriting, copy in Groningen typed) in response to a (missing) letter from
Cannon. The reply letter is dated October 26:

> ‘Dear Miss Cannon,
> I have your letter of 10 October, for which my thanks. There is no gown or
> hood connected with the doctorate in this country; it was abolished already
> more than a century ago. Only the professors of the universities wear on
> official occasions a black velvet gown with white ‘chabot’ and black velvet
> cap. The universities have no colours nor utterly decorations, – all in the
> severe puritan style, as you see. I hope you will console yourself about
> this!’

This all is not to say Annie Cannon would not have appreciated the high honor
from Groningen. She obviously wore the Oxford gown and cap with pride. It is
fully understandable she would have been more taken by and would not fail to
attend the ceremony of the honorary doctorates from her own Wellesley College
and the renowned University of Oxford.

##  6  Discussion

In the first place I briefly discuss Kapteyn and his honors. He has been
awarded three doctorates h.c. It is of interest to note that there are none
from universities in England, France, Germany, Russia, etc., that were leading
nations in astronomical research. This may not be significant since Kapteyn’s
honors (listed in [24], Appendix A.4) show that he definitely received high
honors from a number of these nations, such as knighthoods, medals and
memberships of academies and learned societies. By the way, the honorary
degree from Edinburgh is unrelated to Kapteyn’s collaboration with Gill, who
after all was Scottish; he was raised in Aberdeen and studied there under
Maxwell, and had nothing to do with Edinburgh.

It is interesting to briefly compare Kapteyn to his famous pupils de Sitter
and Oort. Willem de Sitter has been awarded four honorary doctorates:
Cambridge 1925, Cape Town 1929, Wesleyan 1931 and Oxford 1932. Did he take the
initiative for Leiden University to award honorary degrees to astronomers?
Yes, in three cases he did, namely for Robert T.A. Innes in 1923 and Friedrich
Küstner and Henri-Alexandre Deslandres in 1928.444The site of Leiden
University, that lists honorary doctorates [13], erroneously lists Deslandres
as having been awarded the doctorate in 1900. Innes was director of the Unie
Sterrewag in Johannesburg, with which de Sitter at about the same time had
negotiated a collaboration and the establishment of Leiden’s southern station
at its premises in Johannesburg. Küstner was emeritus director of the
Sternwarte Bonn, and Deslandres was a French astronomer, director of the
l’Observatoire de Meudon et l’Observatoire de Paris. The award was on the
occasion of the Leiden General Assembly of the International Astronomical
Union, of which de Sitter was president at the time. There is more to note
about this; de Sitter wanted to end the isolation of Germany that after the
First World War had been excluded from international organizations (see below
for more on this) and he used his prerogative as President to invite
individual German astronomers. In this spirit he took the initiative to honor
a French and a German astronomer with a doctorate h.c. He had some difficulty
convincing the Leiden Senate to award two honorary titles in one discipline at
the same time. While Küstner worked in astronomy relevant to the Leiden
program – he had continued the tradition of the Bonner Durchmusterung by
setting up photographic programs for positional astronomy, in particular for
proper motions –, Deslandres did not, his field was the sun and its
atmosphere. He was the ranking French astronomer and had been one of the vice-
presidents of the IAU since 1922 and was ending his term in Leiden.

More remarkable is Jan Oort, who had ten honorary doctorates, including
Oxford, Cambridge and Harvard (see [25], Appendix A.5), yet has not taken even
a single initiative to award one by Leiden University. In fact the next
astronomer after Küstner in 1928 to receive a Leiden doctorate h.c. is later
Nobel Prize laureate Reinhard Genzel in 2010!555 In a formal sense this is
incorrect, since Jacob Evert Baron de Vos van Steenwijk received one in 1959
in the Faculty of Law. He held a PhD in astronomy (from 1918 under Ernst van
de Sande Bakhuyzen), but became a politician and administrator, with
appointments such as Mayor, Queen’s Commissioner and President-Curator of
Leiden University. But throughout his career he has been actively involved in
organizational bodies concerning Dutch astronomy, and attended IAU Genral
Assemblies. Pieter van Rhijn did not receive any honorary doctorates and did
not take the intiative to have the university of Groningen award an other one.
In fact no astronomer received a doctorate h.c. from Groningren after Cannon.

What about Kapteyn’s choices for Schwarzschild and Cannon? Who else might he
have considered? Some of the most critically important persons for him in
establishing his career and international fame had been David Gill (for the
Cape Photographic Durchmusterung), Simon Newcomb (for making his work known in
America and his native Canada, and inviting him in 1904 to the St. Louis
Congress), George Hale (for inviting him to Mount Wilson as Carnegie research
associate and adopting the Plan of Selected Areas as prime observing project
for the 60-inch) and Edward Pickering (for a major contribution from Harvard
to the Plan).

Gill and Newcomb had died by 1914, but Pickering lived until 1919 and would
have been an option in 1914. Kapteyn had received his honorary doctorate from
Harvard in 1909 and could have reciprocated the honor. Kapteyn was very
sensitive about giving credit where it belonged and realized that the work for
the Henry Draper Catalogue must have been done really for the most part by
Annie Cannon. There had been disagreements with Pickering, especially about
the latter’s insistence to include a Special Plan of areas in the Milky Way,
which meant much extra work; it had annoyed Kapteyn that he had to accept this
to ensure Pickering’s cooperation. In the end he would have considered
Schwarzschild’s work more suitable and had better relations with him. In any
case, from a scientific as well as personal point of view Schwarzschild
probably seemed a more agreeable choice.

Hale certainly would have been a possibility in 1914. This might have resulted
in an uncomfortable situation with Andrew Carnegie receiving the same honor at
the same time, who after all in a sense was Hale’s superior or ‘boss’. In
fact, it is far from ruled out that Kapteyn had been involved in selecting
Carnegie (who else on the Groningen university faculty would have a connection
to him?) and then also including Hale in addition might have been too much.

Then there is Anders S. Donner, who Kapteyn had first met at the great Carte
du Ciel congress in Paris in 1887. Donner was director of the Helsingfors
Observatory (Swedish for Helsinki), where he acquired plates for his
participation in the Carte du Ciel. Kapteyn and Donner had quickly become very
good friends (they were among the younger attendants in Paris and not part of
the establishment), and over the years Donner had collected an enormous amount
of photographic material for Kapteyn, which resulted in quite a number of
joint papers in the Publications of the Astronomical Laboratory at Groningen.
Until 1925, 39 volumes appeared in these Publications, of which no fewer than
nine were based on plates taken by Donner. However, Donner had not published
himself any significant papers; his vocation seemed to have been operating the
telescope (at least in winter; at his latitude the observatory was closed from
April through August when at night there is no end to astronomical twilight)
and providing photographic plates for others.

[t]

Figure 10:  Arthur Eddington and Jacobus Kapteyn. It is not known when, where
and by whom this photograph has been taken. Oort was presented with a copy of
this by S. Chandrasekhar. For the full story see [25]. It would seem that this
photograph has been taken around 1920. From the Oort Archives.

There is one other person who has meant much to Kapteyn, and who might
certainly have qualified for an honorary degree and that is Arthur S.
Eddington (see Fig. 10). Without doubt Eddington was one of the most prominent
astronomers of the twentieth century. When Kapteyn had presented his Star
Streams in 1904, within two years Eddington, who had just become chief
assistant at the Royal Greenwich Observatory after having been one of the
brightest students in Cambridge, devised a quantitative test and applied this
to a set of stars in the Groombridge Catalog for which new proper motions were
available at Greenwich. Where Kapteyn used brighter stars all over the sky,
Eddington’s material constituted more stars over a limited part of the sky,
which also were fainter. His results did, as he wrote, ‘strongly support
Kapteyn’s hypothesis of two star-drifts’.

Now in 1914, Eddington was only 32 years of age and this would seem a bit
young for a doctorate h.c. But in 1921 he certainly could have been a good
candidate. In addition to confirming the Star Streams, in his book Stellar
Movements and the Structure of the Universe from 1914, which had drawn much
attention, Eddington had prominently discussed the concept. In his
introductory address on the occasion of the centenary celebration of the RAS
in 1922 he would list the in his opinion six ‘outstanding landmarks in these
hundred years’ and Kapteyn’s Star Streams was one of these. He remarked: ‘But
I think the great impetus on sidereal astronomy came from Kapteyn’s
discovery’. And he wrote later an obituary of Kapteyn of unusually high
praise. This was after 1921 of course, but he must have had these opinions
before that and been a promoter of Kapteyn’s work throughout the years.

The selection in 1921 at the time of Kapteyn’s seventieth birthday and
retirement has to be seen as well in the light of another development. Kapteyn
had alienated a significant number of leading astronomers in particular in
England (and maybe other parts of the UK as well), France, Belgium and the
USA, because of his stance after Wold War I on international organizations.
Especially in these countries there was a strong sentiment and movement to ban
the defeated nations from such organizations that were being formed just after
the end of WWI. Kapteyn had vehemently opposed setting up these without
participation from Germany, Austria, and the other Central Powers, in
particular the International Research Council and the International
Astronomical Union. More background on this can be found in an authoritative
book by Kevles, The Physicists [11]. Together with his friend, physiology and
psychology professor Gerardus Heymans, Kapteyn took the initiative to
circulate an open letter strongly protesting and condemning this attitude,
that in the end was signed by almost 300 persons (however Kevles fails to
emntion the role of Heymans; see [24] for an excerpt from this letter).

When Kapteyn’s 70th birthday was approaching in 1921, de Sitter took the
initiative to prepare the publication of Kapteyn’s selected works. A committee
that was formed quickly ran into opposition in the UK, apparently because
German Küstner was a member. So, when Frank W. Dyson, who then was at
Greenwich, solicited help among British astronomers, he did get support from
Arthur Eddington, but strong opposition from many others as well. French
astronomer Jules Baillaud, Belgian George Lecointe, and English astronomer
Herbert H.Turner from Oxford were among the most outspoken among European
astronomers in arguing for exclusion of German scientists after the war. They
and others incorrectly accused Kapteyn of having accepted the Orden pour le
Mérite from the German Kaiser at the same time as the captain of the submarine
that had sunk the Lusitania. The Orden pour le Mérite was a German distinction
bestowed by the Kaiser. Note that Orden is German. It was a very high honor
for a foreign scientist to receive that distinction (a few foreign astronomers
had preceded Kapteyn, such as Simon Newcomb, David Gill and Edward Pickering,
as well as German astronomers Freidrich Argelander and Arthur von Auwers, but
also famous scientists had received it, such as Charles Darwin, Lord Rayleigh,
Hendrik Lorentz,and others). Kapteyn had received the Orden in 1914 just
before the War, at the same time as von Seeliger and Max Planck. This was in
the Scientific Class, but there also was a Military Class, which the captain
of the submarine that sunk the passenger ocean liner Lusitania – an act that
enraged the world and played an important role in the US joining the War –,
had received, but that was in 1917! Yet his stance did not prevent Kapteyn
from being appointed not long after the War a corresponding member of the
French Académie and foreign member of the UK Royal Society. De Sitter’s plan
came to naught and he and Kapteyn’s successor in Groningen Pieter van Rhijn
might have resorted to a plan to have a doctor h.c. selected by Kapteyn.

Now Eddington was the first Englishman, who was prepared to work towards
reconciliation. George Hale seemed to have sided with the ones opposing
Germany, but he in any case defended Kapteyn as being far from pro-German.
After all, in their correspondence during the War, Kapteyn had expressed
repeatedly his view that Germany was to blame for the War and had also very
strongly condemned the sinking of the Lusitania. In 1920 Eddington was the
only Englishman that attended the congress of the Astronomische Gesellschaft.
Whether avoiding further escalation played a role in not selecting Eddington
in 1921 remains a matter of speculation, but considerations like these must
have played a role, regardless of how appropriate it had been to honor him
this way on scientific grounds.

In 1922 an argument against selecting Hale for an honorary doctorate might
have been Kapteyn’s very poor relations with Walter Adams, who was Hale’s
deputy. Adams might have seen this as a provocation against him, which Kapteyn
would have wanted to avoid.

The choice of Annie Cannon seems to have had, in addition to this, two other
aspects. In the first place a genuine appreciation and admiration for the work
of stellar spectral classification at Harvard as a very fundamental and vital
contribution to astronomy. Cannon in that case would easily qualify as the
obvious first exponent of this work, as Kapteyn expressed in his arguments to
the Senate when he proposed her. Kapteyn was rather strict about giving the
credit to where it belonged as is clear from the Adams-Kohlschütter case. Of
course the Henry Draper Catalogue was compiled under Pickering’s directorship,
but with the latter dead the honorary degree could easily go to Cannon, where
Kapteyn must have felt the credit belonged, without passing over Pickering.
The second thing is that what might have played also a role is that Kapteyn
would have welcomed the fact that it then would be a woman, who would be
honored. Kapteyn has a record of supporting women’s rights.

One of his best friends in Groningen was the well-known professor of
philosophy and psychology Gerardus Heymans (see [24] or [26]). Heymans was
among other matters interested in differences between men and women. He
researched this with questionnaires, asking professors and lecturers at Dutch
universities about character traits such as individuality, ability for
abstraction, memory, etc., and whether they felt predominantly male or female.
Kapteyn and his calculators helped reduce these data and maybe contributed to
this as well. Although Kapteyn and Heymans were good friends, they often had
different opinions, but when it came to women’s suffrage, they were in perfect
agreement. It is true that Heymans lobbied more actively for women’s suffrage,
but according to a thorough study concerning New participants in science:
Female students and teachers at the University of Groningen, 1871–1919 [4],
Kapteyn was certainly involved in Heymans’ circle of acquaintances who worked
for women’s emancipation.

It is also remarkable that Kapteyn was the first astronomer in the Netherlands
with a female PhD student, Etine Imke Smid. She obtained her PhD degree in
1914 on a thesis, which concerned the study of proper motions of more than one
hundred stars. She was the first woman in the Netherlands to obtain a
doctorate on an astronomical dissertation. After her defense, Etine Smid
worked for Kamerlingh Onnes in Leiden for some time, but moved to Deventer
after she got married and left science.

And I quote from Henriette Hertzsprung-Kapteyn’s biography:

> ‘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 medicine and I took law as subjects, which gave rise to much
> criticism in these turbulent days of the fight for women’s 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.’

The eldest daughter indeed studied medicine in Groningen, the younger law, for
a while in Groningen, but she later switched to English in Amsterdam. The son
went to Freiburg to study mining engineering; the fact that he did not go to
Delft for this had to do with the lower costs of studying in Freiburg. His
choice to send his daughters to university meant Kapteyn had to support three
children during academic studies and that was a major financial burden.
Professors did earn a decent salary, but many of them, including Kapteyn, did
extra teaching in order to be able to finance their children’s studies.
Kapteyn chose to carry the financial burden of his daughters entering
university, even if this meant that his son would have to go and study in
Germany. Female students at universities were still quite rare; the first
female student at a Dutch university (that of Groningen) had been Aletta
Henriëtte Jacobs, who entered the university to study medicine in 1871,
followed six years later by her sister Charlotte who studied pharmacy, hardly
a generation before the Kapteyn daughters entered university.

So, Kapteyn was a supporter of women’s emancipation. The fact that he was an
unconventional man in a number of ways may have been a reaction to his
strictly religious upbringing. But he did not allow reform at all fronts. His
marriage was along conventional lines; his wife took care of the household,
while Kapteyn saw the finances as his exclusive responsibility.

Still, he had caught two birds with one stone: honoring the great progress in
stellar spectroscopy and selected a female doctor h.c. Excluding the avalanche
of honorary degrees in 1914, Cannon’s was the first honorary doctorate from
Groningen to go to a foreign scientist since Robert Koch in 1884. And the
second female doctor h.c. from abroad in the history of Groningen University
after Alice Boole Stott in the 1914 tricentennial. Thos drought would last
until 1958, when Elizabeth Caroline Crosby, American professor of neuroanatomy
would become the third.

After his formal retirement Kapteyn left Groningen. He and his wife bought a
house in Hilversum and after some travel in Europe moved in with their
daughter and her husband in Amsterdam before settling in their new house.
Leiden Observatory offered him a parttime appointment to take charge of the
astrometric department. But them the first signs of what would prove to be his
fatal illness appeared and he died still in Amsterdam within a year of his
retirement. Henriette Hertzsprung-Kapteyn tells us that Kapteyn and his wife
left Groningen quietly. They had said farewell to their friends during dinners
in small numbers and left unnoticed one day at seven in the morning. It is a
pity that Annie Cannon could not come to Groningen to accept the honor in
person during a special ceremonial meeting of the Senate to mark the departure
of possibly Groningen’s most successful and celebrated professor.

##  7  Summary and conclusion

Kapteyn at three occasions proposed doctores honoris causa for the University
of Groningen. The first one, in 1903, coincided with his 25-th anniversary as
a professor in Groningen and went to Cornelis Easton. Easton was a journalist
and amateur astronomer. The degree was awarded to honor him for his work of
mapping of the Milky Way by making drawings. This he followed up by efforts to
derive from this the spiral structure of the Galaxy.

In 1914 he proposed Karl Schwarzschild particularly for his theoretical
contributions to the study of the structure and kinematics of the stellar
distribution in space.

Kapteyn most likely also proposed musician, composer and teacher Peter van
Anrooy, who not only developed his taste and appreciation for music but taught
him lessons as well to better understand music.

It is very well possible that he also had a determining role in the selection
of industrialists and philanthropist Andrew Carnegie, the benefactor of the
Institution that bore his name and provided Kapteyn’s position as research
associate, which facilitated much of his research including his annual visits
to Mount Wilson Observatory.

Maybe for this reason the option to select George E. Hale was not favored,
while the otherwise real option of Arthur S. Eddington was put aside because
of his relatively young age.

There were a few controversies that may have limited the choice in 1921 and
which have played an adverse role in many aspects of Kapteyn’s legacy. One was
the rather poor relation with Walter S. Adams from Mount Wilson Observatory,
which prevented his return to California after the War and which might have
escalated were Hale selected.

Another was Kapteyn’s stand on the in his view extremely objectionable policy
of excluding defeated nations as Germany, Austria, etc. from membership of
international political and scientific organizations. Eddington also very
strongly defended the same position as Kapteyn, but selecting him for an
honorary degree might have escalated tensions with others.

When Kapteyn retired in 1921 he proposed Annie Cannon for her fundamental and
essential contributions to the Harvard spectral classification and the Henry
Draper Catalogue. He might have felt that in this way she would receive the
credit she deserved, which for women would naturally go to the male director
under whom she worked. The fact that Edward Pickering had died helped, since
it meant that he did not have to be passed over. An important argument for the
selection of her very likely was that Kapteyn supported women’s equality and
rights and welcomed the idea that she would be the first foreign female thus
honored prominently (i.e. not as part of a large contingent) by the University
of Groningen.

The imminence of the outbreak of the War, his enlisting in the German Army and
death during the War is probably the reason Schwarzschild’s honorary title is
seldom or not at all quoted, certainly not in obituaries. Cannon’s degree is
usually mentioned but is overshadowed by the one from Oxford. Her decision not
to come to Groningen meant no formal ceremony to mark Kapteyn’s retirement and
departure from Groningen.

Acknowledgements I am grateful to the staff of the Archives at
Niedersächsische Staats- und Universitätsbibliothek Göttingen and of the
Harvard University Archives – Pusey Library for providing scans of the
relevant correspondence in the Schwarzschild and Cannon archives. I thank
historians Klaas van Berkel and David Baneke for a critical reading of a draft
of this paper and making very useful comments and suggestions. I thank the
staff of the Kapteyn Astronomical Institute for support and help and for
hospitality extended to an emeritus professor as guest scientist.

## References

  * [1]
  * [2] Blaauw, A., 2014: Easton, Cornelis, in The Biographical Encyclopedia of Astronomers, eds. T. Hockney, V. Trimble, T.R. Eilliams, et al., Springer Publishers, ISBN 978-0-387-30400-7.
  * [3] Cannon, A.J. & Pickering, E.C., 1901: Spectra of bright southern stars photographed with the 13-inch Boyden telescope as part of the Henry Draper Memorial, Annals of Harvard College Observatory, 28, 129.
  * [4] de Wilde, I., 1998: Nieuwe deelgenoten in de Wetenschap: Vrouwelijke studenten en docenten aan de Rijks-universiteit Groningen 1871–1919, van Gorcum ISBN-10 : 2-9063-5761-8.
  * [5] Easton, C, 1913: A Photographic chart of the Milky Way and the spiral theory of the Galactic System, Astrophysical Journal, 37, 105.
  * [6] Eddington, A.S., 1917: Obituary notice, Associate: Schwarzschild, Karl, Monthly Notices of the Royal Astronomical Society, 77, 314\.
  * [7] Harvard University, 2020: Honorary Degrees, www.harvard.edu/on-campus/commencement/honorary-degrees.
  * [8] Heffernan, M. & Jöns, H., 2007: Degrees of influence: The politics of honorary degrees in the Universities of Oxford and Cambridge, 1900-2000, Minerva: a Review of Science, Learning and Policy, 45 (4).
  * [9] Hertzsprung, E., 1917: Karl Schwarzschild, Astrophysical Journal, 45, 285.
  * [10] Hertzsprung-Kapteyn, H., 1928: J.C. Kapteyn; Zijn leven en werken, Wolters, Groningen (1928), electronic version available at www.dbnl.org/tekst/hert042jcka01_01/, for more information and my English translation see www.astro.rug.nl/JCKapteyn/HHKbiog.html.
  * [11] Kevles, D.J., 1995, The Physicists: The history of a scientific community in modern America, Harvard Univ. Press, ISBN 978-0-674-66656-6.
  * [12] Leiden Observatory, 1908: Album Amicorum, presented to H.G. van de Sande Bakhuyzen (1908), see local.strw.leidenuniv.nl/album1908/book_info.html.
  * [13] Leiden University, 2020: Eredoctoraten en prijzen, www.universiteitleiden.nl/over-ons/feiten-en-cijfers/eredoctoraten-en-prijzen.
  * [14] Paul, E.R., 1993, The Milky Way Galaxy and Statistical Cosmology, 1890-1924Cambridge Univ. Press, ISBN 978-0-521-35363-2.
  * [15] Schwarzschild Archives, 2020, Niedersächsische Staats- und Universitätsbibliothek Göttingen.
  * [16] Merriam-Webster Dictionary, 2020, www.merriam-webster.com/dictionary/honorary+degree.
  * [17] Nieuwsblad van het Noorden, 2018: www.dvhn.nl/groningen/Dit-is-waarom-Ban-Ki-moon-een-eredoctoraat-van-de-Rijksuniversiteit-Groningen-verdient-23674056.html.
  * [18] Oxford University, 2020: Honorary degrees, Early history, www.ox.ac.uk/news-and-events/The-University-Year/Encaenia/Honorary-Degrees-Early-History.
  * [19] Schwarzschild, K., 1912: Zur Stellarstatistik, Astronomische Nachrichten, 190, 361.
  * [20] Stein SJ, J.W.J.A., 1929: Dr. C. Easton, in memoriam, Hemel & Dampkring 27, 225 & 258\.
  * [21] University of Groningen, 1914: Academia Groningana, MDCXIV-MCMXIV; Gedenkboek ter gelegenheid van het derde eeuwfeest der Universiteit te Groningen, uitgegeven in opdracht van den Academischen Senaat.
  * [22] University of Groningen, 1916: Academia Groningana 29 Juni – 1 Juli 1914: Verslag van de herdenking van het 3de eeuwfeest der Universiteit te Groningen.
  * [23] University of Groningen: Overzicht eredoctoraten 1717–2018, www.rug.nl/about-ug/profile/prizes-and-awards/honorary-doctorates/eredoctoraten-1717-2018.pdf.
  * [24] van der Kruit, P.C., 2015: Jacobus Cornelius Kapteyn: Born investigator of the Heavens, Astrophysics and Space Science Library, Vol. 416, Springer Publishers, ISBN 978-3-319-10875-9, accompanying Webpage www.astro.rug.nl/JCKapteyn.
  * [25] van der Kruit, P.C., 2019: Jan Hendrik Oort: Master of the Galactic System, Astrophysics and Space Science Library, Vol. 459, Springer Publishers, ISBN 978-3-319-10875-9, accompanying Webpage www.astro.rug.nl/JHOort.
  * [26] van der Kruit, P.C., 2021a: Pioneer of Galactic Astronomy: A biography of Jacobus Cornelius Kapteyn, Springer Biographies, Springer Publishers. ISBN 978-3-030-55422-4.
  * [27] van der Kruit, P.C., 2021b: Master of Galactic Astronomy: A biography of Jan Hendrik Oort, Springer Biographies, Springer Publishers, ISBN 978-3-030-55547-4.
  * [28] van der Kruit, P.C. & van Berkel, K., 2000: The legacy of J.C. Kapteyn: Studies on Kapteyn and the development of modern astronomy, Astrophysics and Space Science Library, Vol. 246, Springer Publishers, ISBN 0-7923-6393-0.
  * [29] van E., E. (E. van Everdingen), 1929: Dr. C. Easton, Nature 124 (3130): 659.
  * [30] Wikimedia (2020): commons.wikimedia.org/wiki/File:Annie_Jump_Cannon_1922_Portrait.jpg.