Built between 1808 and 1824 on the top of Toomemägi, the Tartu Old Observatory is one of the most historically significant scientific sites in the Baltic states. The observatory was where Friedrich Georg Wilhelm von Struve carried out the measurements for the Struve Geodetic Arc — the first precise determination of the shape of the Earth, accomplished between 1816 and 1855 using a chain of triangulation points crossing nine modern countries.
The observatory is now a museum open to visitors, with the original Fraunhofer refractor telescope still in the dome. The meridian room, where Struve's measurements were made, is preserved as it was in the 19th century. On clear evenings, the dome is sometimes opened for public stargazing — check the schedule on arrival.