Neither Academia Gustaviana nor Academia Gustavo-Carolina had a separate observatory during the 17th century; however, astronomy was part of the University’s programme. The University’s first telescope was bought by its forward-looking Professor of Mathematics Sven Dimberg. We only have fragmentary descriptions of the instrument. We know it was ordered from England and was 12 cubits long. Presumably it was a 6 metre refractor telescope with tubes that fit into each other. The telescope reached Tartu in 1692 and Dimberg presented it to his colleagues on May 4. When the University moved to Pärnu in 1699, they planned to erect an observatory but unfortunately the time in Pärnu was cut short.
An observatory was part of the Imperial Tartu University’s plan. At first, the architect of the time imagined it in the towers of the ruins of Tartu Cathedral but the University’s mathematicians pointed out that a separate building was needed. They began constructing it on the ruins of the former bishop’s castle. Before the construction was finished, the Observatory was housed in two temporary buildings: first in an attic apartment and then in the house of Andreas von Lambert, the land surveyor of the nobility. This was the office of the Observatory’s first directors: Ernst Christoph Knorre, Johann Wilhelm Andreas Pfaff and Johan Sigismund Gottfried Huth. They ordered the Observatory’s first instruments from England (such as the Dollond and Herschel telescopes on display to this day), helped engineer the Observatory building, and Pfaff published a collection of articles called Astronomische Beyträge.
The first heyday of the Observatory began when a young energetic and talented Friedrich Georg Wilhelm Struve began working here. The construction of the Observatory was finished in 1810 and Struve set things in motion, ordered several new instruments, including the Reichenback-Ertel Meridian Circle and the world’s largest telescope of the time – the 9-inch Fraunhofer Refractor. He organised the measurement of the world's longest meridian arc (Struve Geodetic Arc) in order to fix the shape of the Earth and create a basis for making more accurate world maps. During his observations with the Fraunhofer Refractor, Struve found over 3000 binary stars and measured a star’s parallax, which made him the first person to calculate the exact distance of a star. Tartu became the most important observatory in the Russian Empire. The Emperor of Russia invited Struve to St Petersburg where he built an even bigger and grander observatory in Pulkovo and appointed Struve as Director.
Struve was succeeded by astronomer Johann Heinrich Mädler who had become famous for his Moon maps. His reason for coming to Tartu was probably the Fraunhofer Refractor, which he used in various observations: partly continuing Struve’s work with binary stars, partly dedicating himself to his own subjects of interest such as the Moon, the planets and comets. While in Tartu, he wrote articles for the local newspapers and a bulky book called Popular Astronomy. Mädler was succeeded as Director by Thomas Clausen whose achievements were mostly in the field of theoretical science and mathematics. He calculated the orbits of comets and worked out precise methods for using instruments and analysing data. During the service of Peter Carl Ludwig Schwarz, the Observatory participated in the Bonn star catalogue observation programme. In 1874, P. C. L. Schwarz travelled to Southeast Siberia to observe a rare astronomical event, the transit of Venus across the Sun. On the expedition he had a Repsold Heliometer, which survives to this day. Schwarz was married to renowned Baltic German artist Julie Hagen.
The turn of the century and the next Director, Grigori Vasilyevich Levitsky, brought a whole new research direction to Tartu – seismology, the study of earthquakes. Levitsky began studying earthquakes in the Observatory basement and set up his equipment in the gunpowder cellar. At the beginning of the 20th century, people started dreaming of an Observatory far from the noise of the city. The new energetic Director Konstantin Pokrovsky managed to replace the ageing Fraunhofer Refractor with a new main telescope from the Carl Zeiss manufacturing company in Jena. The telescope was accompanied by a Petzval astrograph or photo camera. However, the new building remained only a dream. Pokrovsky was known for his astronomy lectures, which were so packed with people that they had to be held in the assembly hall of the main building. Pokrovsky is also part of the history of Estonian literature as the cosmography teacher in Maurus’ School in Anton Hansen Tammsaare’s work Truth and Justice.
The first Director of the Observatory after the University’s teaching language became Estonian was Taavet Rootsmäe whose main research fields were stellar astronomy and theoretical astronomy. He was a remarkable populariser of science and promoter of Estonian scientific vocabulary. However, the star Director of the Observatory was Ernst Julius Öpik. His scientific interests were very varied: small Solar System bodies, the age of the Solar System, the development of stars, and the distance of galaxies. He was the first person to determine the distance of the Andromeda Galaxy, proving it was a separate galaxy and not a star cluster within our galaxy. This discovery ignited a debate among astronomers of the world for a long time. Öpik also worked out a method for counting meteorites, which he was invited to introduce at Harvard University. The connections he made in Harvard saved him from a German concentration camp and got him a position in Armagh Observatory in Ireland where he worked until his death.
During the Soviet Era, the Observatory became part of the ESSR Academy of Sciences. With the beginning of the Space Age, astronomy became a preferential field of study. A new larger observatory was constructed away from the city lights and vibrations in Tõravere. There they built a reflector telescope with a 1.5 metre diameter and Estonian astronomers Vladimir Riives, Harald Keres, Grigori Kusmin, Aksel Kipper, Arved Sapar, Jaan Einasto and many others had the chance to conduct astronomy research at a world level. The Old Observatory became the office space of astronomers and physicists. The Western Hall received an inserted ceiling due to lack of space and the Eastern Hall was given to a branch of Tartu City Library where they displayed old scientific equipment.
The Old Observatory was given back to the University of Tartu in 1996. At first it became the home of Estonian Education and Research Network (EENet) and AHHAA Science Centre. In 2004, it was decided that the Observatory would become a museum. The construction work began in 2009 and was finished in December 2010. The Old Observatory opened its doors as a museum in April 2011.