Foundation of the Astrophysical Observatory Potsdam (AOP)

In the middle of the 19th century, Gustav Kirchhoff and Robert Bunsen developed spectral analysis. It provided the possibility of obtaining information about the physical parameters and chemical abundances of stars through spectral analysis of their light. Foerster recognized these possibilities and, in 1871, initiated the construction of a solar observatory as a memorial to the crown prince, emphasizing the importance and benefit of solar research. This mandate was soon extended to the entirety of astrophysics.

The chosen site of the observatory was a hill south of Potsdam, known as the Telegrafenberg, where a relay station of the optical telegraph system for sending military information from Berlin to Koblenz had stood from 1832 to 1848. The Astrophysical Observatory Potsdam was founded on July 1, 1874. Before construction of the observatory had even begun, Gustav Spörer made solar observations from the tower of the former military orphanage in Potsdam’s Linden Street. Construction started in 1876, and the main building of the observatory and its equipment were completed in the autumn of 1879.

To begin with, a board of directors comprised of Wilhelm Julius Foerster, Gustav Kirchhoff and Arthur Auwers managed the AOP, but in 1882 Carl Hermann Vogel was appointed as sole director of the observatory. The main focus of his work was on stellar astrophysics. He was the first person to successfully determine radial velocities of stars photographically and, as a result, he discovered spectroscopic binaries.

In 1899, what was then the largest refractor in the world, with 80cm and 50cm lenses, manufactured by the firms of Steinheil and Repsold, was mounted in a 24m dome. It was inaugurated by the German emperor, Wilhelm II, with considerable celebration. Although the Great Refractor of Potsdam did not realize all the aspirations astronomers had for it, it is nonetheless worth mentioning the important discoveries of interstellar calcium lines in the spectrum of the spectroscopic binary delta Orionis by Johannes Hartmann in 1904, and the presence of stellar calcium emission lines, a hint towards understanding stellar surface activity, discovered by Gustav Eberhard and Hans Ludendorff in about 1900.

Ten years later, one of the most famous astrophysicists of the century, Karl Schwarzschild, became director of the observatory. In only a few years of work (he died of an illness in 1916) Schwarzchild made fundamental contributions to astrophysics and the general theory of relativity (GTR). Only a few weeks after Einstein’s publication, Schwarzschild found the first solution to the highly complicated system of Einstein’s equations. Now known as the “Schwarzschild solution,” it is of fundamental importance in the theory behind our understanding of black holes.

There are further close links between the AOP and Einstein's GTR. In 1881 Albert A. Michelson performed his experiments that attempted to demonstrate the movement of the Earth through the hypothetical ether in the cellar of the main building of the AOP. His negative results were only fundamentally reconciled in 1905 by Einstein's special relativity theory.

The aim of a solar tower telescope, built between 1921 and 1924, which was initiated by Erwin Finlay Freundlich and is now called the Einstein Tower, was to prove the redshift of spectral lines in the gravitational field of the sun – an effect proposed by Einstein's GTR. Although it was not yet possible to measure gravitational redshift at that time, important developments in solar and plasma physics were set in motion here and the architect, Erich Mendelsohn, created a unique scientific building with his peculiarly expressionistic tower design.

Besides the work done by Schwarzschild, in the following decades, important observational programmes such as the ''Potsdamer Photometrische Durchmusterung'' and the outstanding investigations by Walter Grotrian on the solar corona were recognized throughout the world.