In 1824, the observatory acquired a Fraunhofer refractor, which at the time was the best and the largest dioptric telescope in the world. In order to harness the potential of the new telescope, the building was reconstructed according to the design of George Frederic Parrot as a rotating dome observatory. The new design of the observatory’s dome turned out to be so successful that it was emulated in a number of new observatories (such as those of Helsinki and Pulkovo, for instance) built during the next half century.
Research conducted at the Old Observatory has led to many discoveries that have changed our understanding of the Earth and the universe. The scientists working here have focused on geodesy, astronomy, seismology, time measurement, theoretical and experimental physics. The observatory’s place in the world’s history of science was cemented by its long-time head Friedrich Georg Wilhelm Struve, who in 1835 was the first to measure the distance of a star from the Earth and who determined the position of thousands of double stars.
As the first point in the Struve Geodetic Arc, the Old Observatory was included in the UNESCO World Heritage list in 2005. In 1816-1852, F. G. W. Struve and Carl Friedrich Tenner, a Russian general born in Estonia, led a series of surveys to plot a geodetic arc stretching from North Norway to the coast of the Black Sea. The arc, which became known as Struve Geodetic Arc, played a pivotal role in the development of astronomy, geodesy and cartography.
After the construction of a new observatory in Tõravere (15 miles south of Tartu) was completed in 1964, astronomers moved out from the venerable structure on Toome Hill. The observatory later accommodated a branch of Tartu City Museum. When the branch was closed, the building was taken over by the Science Centre AHHAA. Since 1963, the observatory has also been home to an astronomy club.
Restoration of the Old Observatory started in 2009 with support from the Regional Competitiveness Improvement Programme of the Enterprise Estonia Foundation. The observatory was opened as a museum in the spring of 2011.
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