Leiden University is the oldest university in The Netherlands. In 1574, Prince William of Orange took the first step towards establishing the university, as a reward for the city's brave resistance to the Spanish besiegers. The university was founded on February 8, 1575. Throughout the centuries many great scholars and scientists have brought fame and respect to Leiden University.
Astronomy in Leiden has a long tradition. In 1633, Leiden University established an observatory to accommodate the so-called quadrant of Snellius (the Dutch scientist best known for his laws of refraction). This founding date makes the Sterrewacht Leiden the oldest still operating university observatory in the world. In the first two centuries of its existence it served mainly an educational purpose. The construction of a spacious new observatory building in 1861, under the supervision of F. Kaiser, marks the beginning of the modern era of astronomical research in Leiden. This building was the home of Leiden astronomy for over a century, and still has powerful symbolic value.
Already in the late 19th and early 20th century Leiden was an exciting place to work, in the vicinity of many famous physicists, such as H.A. Lorentz, P. Zeeman, P. Ehrenfest, and H. Kamerlingh Onnes. In 1919 W. de Sitter became director of the Observatory. He added an astrophysical (spectroscopy and photometry of stars) and a theoretical division to the existing astrometric one. De Sitter himself was head of the Theoretical Division and among other things worked with Einstein on the cosmological implications of general relativity. The head of the Astrophysical Division and the successor of De Sitter as Observatory Director was E. Hertzsprung, famous as co-inventor of the Hertzsprung-Russell diagram of stars, a tool still widely used today.
In 1924 de Sitter brought to Leiden the man who would become the most famous of all Leiden astronomers, J.H. Oort. Oort worked on the motions of stars relatively close to the Sun and discovered the rotation of our Galaxy. In 1945 Oort succeeded Hertzsprung as Observatory Director and he remained in office until his retirement in 1970. Few astronomers have made so many important contributions to so many different fields in astronomy as Oort has. His interests range from comets (the Oort cloud) and our Solar System, to stars, galaxies (the Oort constants), clusters and the large-scale structure of the Universe. In 1944 he encouraged a student from Utrecht, H.C. van de Hulst, to calculate whether neutral hydrogen could produce observable radiation. Van de Hulst predicted that the 21-cm hyperfine ``spin flip'' transition might be observable because of the large number of hydrogen atoms expected along a line-of-sight through the Galaxy. The detection in 1951 at Harvard by Ewen and Purcell, confirmed within a few weeks in the Netherlands by Muller and Oort, and in Sydney by Pawsey, marked the birth of spectral-line radio astronomy. Van de Hulst later became a professor in Leiden and has worked on a range of problems concerning interstellar dust and radiative transfer. Oort was also one of the founding fathers of the European Southern Observatory, established in 1962. The most recent three directors of ESO, A. Blaauw, L. Woltjer, and H. van der Laan, were all Leiden professors, as is the current director, Tim de Zeeuw.
In 1974 Leiden Observatory moved from Kaiser's old Observatory building to the Huygens Laboratory, and recently to the new J.H. Oort building. The name Sterrewacht Leiden was kept. Now, astronomy is done with modern means and the Institute has become larger than ever before, still standing at the forefront of astronomical research.