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Leiden University archives, Observatory, directorate W. de Sitter

Identifier: ubl135

Scope and Contents

The archive consists of scientific and other professional correspondence, manuscripts and notes, course notes, and papers regarding the management of Leiden Observatory.

The archive contains papers related to general relativity and cosmology, including correspondence with Albert Einstein, A.S. Eddington, Georges Lemaître and H.A. Lorentz, as well as detailed (and dated) notes by De Sitter.

It also contains a large amount of letters from Ejnar Hertzsprung, both concerning his scientific work and the daily management of the Observatory.


  • Creation: 1894-1935
  • Creation: Bulk 1908-1934


Language of Materials

Dutch or English unless stated otherwise (there are some papers in German and French)

Conditions Governing Use

Since the archive is digitized, the original papers are not free for public access, unless with permission of the curator after a written request with motivation. Other regulations that apply during the use of these materials can be found on the website of Leiden University Library.

Biographical / Historical

Willem de Sitter (1872-1934) studied astronomy at Groningen University. From 1897-1899, he worked at the Royal Observatory at Cape of Good Hope. After he obtained his doctorate in 1901, he remained in Groningen as the assistant of J.C. Kapteyn.

In 1908, De Sitter was appointed professor of astronomy at Leiden University. In 1918, he also became director of the Observatory in that city. He conducted a radical reorganisation of the observatory, introducing new research and education programmes and expanding the observatory. The most notable members of the new scientific staff were Ejnar Hertzsprung and Jan Oort.

De Sitter is mostly remembered for his work on the astronomical and cosmological consequences of the general relativity theory. He worked on this subject in 1916-1917, when he wrote three classic articles in the Monthly Notices, and again in 1930-1934. Apart from this, he mainly worked on celestial mechanics, esp. the theory of Jupiter’s satellites.

From 1925-1928, De Sitter was president of the International Astronomical Union.

Willem de Sitter was awarded honorary doctorates from the several universities, including Cambridge, Oxford and Cape Town. He received the Gold Medal of the Royal Astronomical Society in 1931 and the Bruce Medal in the same year.

De Sitter was succeeded as director of the observatory by Ejnar Hertzsprung.


253 items (c. 2,5 metre)

Abstract in Dutch

Archief afkomstig van Willem de Sitter, hoogleraar sterrenkunde aan de Universiteit Leiden (1908-1934) en directeur van de Leidse Sterrewacht (1918-1934). Dit archief bestaat uit wetenschappelijke en zakelijke correspondentie, manuscripten en aantekeningen (waaronder algemene relativiteitstheorie en hemelmechanica), collegedictaten en papieren die betrekking hebben op de reorganisatie van de Sterrewacht Leiden in 1918-1924.

Abstract in English

Archive created by Willem de Sitter, professor of astronomy at Leiden University (1908-1934) and director of Leiden Observatory (1918-1934). The archive consists of scientific and other professional correspondence, manuscripts and notes (including about the general relativity theory and celestial mechanics), course notes, and papers regarding the reorganisation of Leiden Observatory in 1918-1924.


The inventory consists of the following categories:

Correspondence by correspondent (no. 1-70)
Correspondence by subject: scientific (no. 71-80)
Correspondence by subject: students and visitors (no. 81-89)
Correspondence by subject: non-scientific (no. 90-106)
Manuscripts and notes: courses followed by De Sitter (no. 107-111)
Manuscripts and notes: courses taught by De Sitter (no. 112-119)
Manuscripts and notes: Studies (no. 120-146)
Manuscripts and notes: General Relativity and Cosmology (no. 147-156)
Manuscripts and notes: Jupiter Satellites (no. 157-192)
Manuscripts and notes: other (no. 193-214)
Committees (no. 215-222)
Observatory management (no. 223-244)
Miscellanea (no. 245-253)

To facilitate researchers, each file has been given a short title. In some cases these titles were original designations by the authors. The other titles are given by the author of this inventory. The main subject of each file is indicated in the inventory. This is not an exhaustive description, but a first indication, intended for researchers.

Correspondence has been ordered alphabetically by correspondent, following the Dutch system (disregarding prefixes such as van, van der etc).

Within the files, correspondence has been ordered chronologically. Other papers are preserved in the order in which they were found.

"Correspondence with ..." indicates that the file contains letters from both correspondents; "letters from …" indicates that only one side of the correspondence has been preserved in this archive.

BAN Bulletin of Astronomical Institutes of the Netherlands
IAU International Astronomical Union
IEB International Education Board (Rockefeller Foundation)
UOJ Union Observatory Johannesburg

Physical Location

Leiden University Library, Special Collections

Custodial History

This archive was created by Willem de Sitter in the course of his work. The papers were preserved in Leiden Observatory until they were transferred to Leiden University Library in 2010.

In 1974-1975, Carla Kahn, wife of visiting professor F.D. Kahn, searched in the archive for the Einstein correspondence, which she found (Kahn 1975). In the course of her search, she made a card index of the correspondence. In 1998, Reinold de Sitter (grandson of Willem) made an inspired but incomplete inventory of the De Sitter papers (R. de Sitter 1998).

Over the years, many people have consulted the archive, either for research or out of pure curiosity. Also, several papers have been temporarily removed for research purposes or for exhibitions. These papers all returned to the archive.

Some files come from the collections of Leiden University Library. They have now been reunited with the Observatory Archives. These files are indicated in the inventory.

Immediate Source of Acquisition

The archive was transferred to Leiden University Library in 2010, by agreement between Leiden Observatory, the Leiden University Library and the Department of Documentary Information and Archive Administration of Leiden University.


In several cases, only a selection of a file has been preserved. This concerns large files of routine calculations, mostly related to the theory of Jupiter’s moons. In these cases, a section of the file has been preserved by way of example. These files are indicated in the inventory.

Some official papers send in by committees or organisations have been removed. This concerns annual reports, budgets, and minutes of meetings. These papers can be found in the archives of the relevant organisations.

The selection was made by David Baneke, advised by Frans van Lunteren (Leiden Observatory) and Huib Zuidervaart (Huygens Institute).


No future additions are to be expected.

Existence and Location of Copies

The archive is digitized. The images are available in the inventory.

Related Materials

This archive is part of Leiden Observatory Archives, which contain the papers of the Observatory and its directors from c. 1830-1945.

The collection of Leiden University Library also includes the scientific papers of Jan Oort (director of the Observatory from 1945-1970).


  • Baneke, D., Teach and travel. Leiden Observatory and the renaissance of Dutch astronomy in the Interwar years, Journal for the History of Astronomy , 2010.
  • Guichelaar, J., Willem de Sitter. Een alternatief voor Einsteins heelalmodel (Amsterdam 2009).
  • Herk, G. van, H. Kleibrink en W. Bijleveld, De Leidse sterrewacht. Vier eeuwen wacht bij dag en nacht (Zwolle 1983).
  • Kahn, C. en F. Kahn, Letters from Einstein to de Sitter on the nature of the Universe, in: Nature, 257 (1975) 451-454.
  • Kerszberg, P., The invented universe. The Einstein -De Sitter controversy (1916-17) and the rise of relativistic cosmology (Oxford 1989).
  • Röhle, S., Willem de Sitter in Leiden – Ein Kapitel in der Rezeptionsgeschichte der Relativitätstheorien, Dissertation Johannes Gutenberg-Universität (Mainz 2007).
  • Sitter, R. de, Grandfather, a charcoal sketch (s.l., 1998).
  • Sitter, W. de, Short history of the observatory of the university at Leiden 1633-1933 (Haarlem 1933).
  • Sitter-Suermondt , E. de, Willem de Sitter, een menschenleven (Haarlem 1948).

Processing Information

The papers have been reordered and repacked. The original files were maintained as much as possible. Obviously misfiled papers have been refiled, and obviously related papers have been put together, while obviously unrelated papers have been separated.


Collection guide of the Leiden University archives, Observatory, directorate W. de Sitter (1894-1935)
Archieven van de Universiteit Leiden - Sterrenwacht, directoraat W. de Sitter
David Baneke, 2010
Language of description
Script of description
Code for undetermined script
Language of description note
Beschrijving is in het Engels.

Revision Statements

  • 4 September 2015: latest update

Repository Details

Part of the Leiden University Libraries Archives & Collections Repository

Witte Singel 27
Leiden 2311 BG Netherlands
+31 71 527 2857