1A grand and shiny research instrument made of brass stands on granite pillars at the Western hall of the Tartu (Dorpat/Yurjew) Old Observatory. In 2022, exactly 200 years passed since its arrival in Tartu making it the right time to discuss some aspects from the history of the instrument.

2At the turn of the 18th and 19th century, the fields of mechanics and optics were both making progress. On the British Isles, the industrial revolution had occurred somewhat earlier and as a large maritime nation, they needed navigational tools. Thus, the best astronomical instruments were made there. Between 1806 and 1814, the continental blockade established by Napoleon and the resulting interruption in exporting astronomical instruments to the mainland unwittingly favoured the technical development of continental Europe: on the one hand, craftsmen learnt to make achromatic lenses on the continent as well, on the other hand, improved machines for dividing circles were invented, allowing to larger circular scales, more precisely divided than before, significantly improving measurement accuracy. The efforts towards precision in observatories influenced scientific culture in general. The Earth and the Sky had to be measured. Most astronomers were at the same time geodesists and governments were ready to pay for better maps, based on triangulation.

3In earlier historical research on the Observatory of Tartu, its instruments are usually mentioned (Levitski, 1899; Zhelnin, 1969), but there is a primary emphasis on the famous Fraunhofer large refractor (Struve, 1825), while the other instruments have fallen into the background. In the following, I focus on the questions of why exactly the Reichenbach & Ertel Meridian circle was selected for the Tartu observatory, how was it compared to similar instruments at the time of purchasing, how the instrument reached Tartu and which are the most important research projects it helped to perform.

4The author makes use of well-known publications as well as correspondence available in the National Archives of Estonia (RA, EAA) and the University of Tartu library manuscript department (UTL MD) that has not been widely used in research as of yet. All 19th-century dates are in the Julian calendar, which was then in use in Russia and was 12 days back from the Gregorian calendar.

1.  The University of Dorpat/Tartu and Its Observatory

5For the reader who has not come into close contact with Baltic history, it is perhaps necessary to make a small historical excursion in order to explain its geopolitical situation at that time. Tartu, with the German name Dorpat (and Russian Derpt or Yurjew), is located in old Livonia, an area that has been part of the sphere of influence of German culture since the 13th century, when the Crusaders arrived here. The local peasantry retained the Estonian (or Latvian) language, but the educated elite — the nobility and townspeople — were mostly German-speaking and managed to maintain their autonomy through Danish, Swedish, Polish and Russian periods of power until the World War I. The university was opened in Tartu for the first time in 1632 under Swedish rule. It operated intermittently until 1710, when the Estonian area became part of the Russian Empire during the Northern War.

8The construction of buildings necessary for teaching and research (library, anatomical theatre, observatory, botanical garden, etc.) was also planned in the budget for the new universities, but they were not completed everywhere at once. In the early 19th century, stationary non-military observatories existed only at the Academy of Sciences in St. Petersburg and Vilnius University, both obsolete. The Tartu observatory was completed already in 1810 and was the most modern in the Russian empire at that time, belonging to a new generation of observatories (Markkanen, 2013). It is logical that the modern building should also be given up-to-date content.

2.  The Director of the Tartu/Dorpat Observatory, Friedrich Wilhelm Struve, and His Wishes

12Since the necessary amount — 400 Rhenish guilders or 2,000 silver roubles — was not available at the time in the Dorpat observatory’s budget, Struve hoped that he could pay for the instrument in instalments, and find ways to save operational costs to do so. However, the curator demanded a guarantee, and so it was decided to sell some instruments that the observatory no longer considered necessary (Baumann’s repeating circle, Troughton’s reflecting circle, Baumann’s sextant mirror, Williams’s transit instrument). Three of the former were sold to Moscow University, the latter to Kazan University (Moscow University observatory lost its equipment in a fire during the war in 1812, the Kazan observatory was created only in 1810 and was in stage of completing), earning 3,800 Assignation roubles in total. The entire amount received covered only half of the required sum (the silver rouble was worth about three times more than the Assignation rouble). It was deposited at the Tartu pawnshop to earn interest income for the observatory.

3.  Struve’s Trip Abroad in 1820

• 12  Struve’s proposal to the UT council 10.03.1820. - Fr. G. W. Struve’s correspondence about organizi (...)

It seems to me that if such a large sum was spent on it [building the observatory], there is hope that the observatory will achieve something important soon in the realm of science and that the local astronomer will employ his best effort and mind to work towards the goal so as not to lag behind the best observatories. To facilitate the achievements, it is necessary to: firstly, furnish the observatory with instruments that it has lacked thus far; and secondly, make sure that the astronomer would mount the instruments in the correct manner, use them properly for observation and plan the observations as is expedient.12

15Struve continues to say that the third main instrument, the meridian circle, has been already ordered from Reichenbach in Munich, but the tool is new and lacking available written sources, thus it would be necessary to travel to Munich, see how the instrument maker has set it up and learn from him. It would be hopefully easier to solve in person the matter of the order being delayed. Struve mentions also the opportunity of turning to Repsold in Hamburg (who had made a wonderful meridian circle for Göttingen two years ago) if it turns out that Munich is a hopeless cause.

16As it often happens, Struve failed to stay on schedule with the trip and had to apply for an extension to his summer holidays intended for the trip (three months became five and a half). In his request to the council of the University of Tartu, Struve wrote from Munich:

• 13  Struve’s letter to the Rector of the UT from Göttingen 6.8/25.07.1820. - Fr. G. W. Struve’s corres (...)

The Reichenbach’s meridian circle is the most perfect instrument among its like and awoke in me a great yearning to see the instrument ordered for Tartu in its completed form. Unfortunately, it seems that this will not come to fruition soon, and there may be no hope at all. Reichenbach has become the director of constructing waterways and roads in Bavaria, and will supervise the building of a channel connecting the Danube and the Rhein, for which he will travel to England this autumn. He has abandoned mechanics-related works for now, handed over his workshop to the Viennese polytechnic institute and all his most skilful workers transferred with it. If the meridian circle is nearly complete, we may receive it soon, if it is not, then in many years or not at all, so now may be the only way and opportunity to get such an instrument from Reichenbach.13

17When Struve finally came back, he had to submit a thorough report of the trip. Among other things, it includes his thoughts on visits to the Göttingen observatory directed by Carl Friedrich Gauss (1777-1855) and to the Königsberg observatory directed by Friedrich Wilhelm Bessel (1784-1846). Gauss used both Repsold’s and Reichenbach’s meridian circles in parallel. Having inspected both, Struve clearly preferred Reichenbach’s instrument:

• 14  Bericht des Professors der Astronomie W. Struve an Rector (Kopie). Nov. 1820. RA, EAA, 5374.1.384, (...)

Having closely inspected the instruments, one of which is just like the meridian circle ordered for Tartu, and cast a critical glance at the observation journals, I am convinced that this is the best among the instruments in use up to now, both in terms of mechanics as well as the light intensity of the optics originating from Fraunhofer’s workshop in Munich.14

18In Königsberg, Struve had made exciting future plans with Bessel. Let us read the travel report:

• 15 Probably the Histoire céleste française, appeared in 1801.
• 16 Bericht des Professors der Astronomie W. Struve an Rector (Kopie). Nov. 1820. RA, EAA, 5374.1.384, (...)

Therefore I spent 12 days [...] in Königsberg, and it was at least as useful as the time spent in Munich in scientific terms. As the first result of our consultations, Professor Bessel and I agreed that he will endeavour to extend his trigonometric measurement up to the border of the Russian state so that we would have the opportunity to combine it with the Baltic degree measurement. Furthermore, we agreed upon a large astronomical project that would involve both of the observatories for at least the following five years, with the purpose of mapping the exact locations of all, even the smallest, stars in the sky with the help of the new meridian circles. The work would be similar to what the Lalandes15 did 20 years ago, only with substantially more imperfect instruments. We hope that the new Abo observatory will also contribute to this great work, also perhaps the observatory to be built to South Russia in the future. Thus, we could achieve greater perfection and most of the work would be done by Russian astronomers. To conclude, we discussed the current state of astronomy and the most practical placement of instruments, especially Reichenbach’s, and found that while in earlier times, it was essential to place the transit instrument and meridian circle side by side to organise the astronomer’s work most expediently, thanks to the characteristic construction of Reichenbach’s meridian circle there is no need for it now, since observations one had to make with two instruments simultaneously can now be performed only with Reichenbach’s meridian circle and in a better manner. The transit instrument will continue to have a function independent of other instruments.16

19Struve continues to say that this also eliminates the necessity of reconstructing the observatory, which he had wanted to do some time ago. The ulterior motive of this report did not concern the purposeful use of the meridian circle only, since Struve continues with a cautious request: as economies were made (in reality, no reconstruction plans had been approved and no funds had been assigned), perhaps the large refractor that Struve saw in Fraunhofer’s workshop could be ordered for Tartu (Riekher, 1990, p. 172-175). However, this is a different story and we will not discuss it further at this point.

4.  Works Performed with the Meridian Circle in Struve’s Times

22By systematically combing through the visible sky, Struve found more and more binary stars. His work attracted attention also outside the borders of the Empire of Russia. In 1826, the Astronomical Society of London awarded Struve their gold medal. This was the first time a scientist of the Russian Empire received this honour. By 1827, the 25th anniversary of the foundation of the imperial university of Dorpat, he was able to publish his new catalogue of binary stars that included 3,112 binary stars, most of them unknown thus far (Struve, 1827). This brought Struve the Royal Society’s gold medal.

23His main assistants were Ernst Wilhelm Preuss (1793-1839), who had exchanged the trade of a weaver for the astronomer, and Karl Knorre (1801-1883), son of Tartu’s first Professor of Astronomy, who later became the director of the Nikolaev (Mykolaiv) observatory, created in 1821. Vassili Feodorow (1802-1855), who was dispatched to study in Tartu by St. Petersburg Imperial orphanage and later became the founder of the University of Kyiv’s observatory, also contributed. A few Russian military surveyors who had been sent to Tartu to further their education also helped.

5.  The Meridian Circle and the Tartu Geodetic Arc

25In many observatories that have measured the geodetic arc, the meridians coincide with the corresponding instrument optical axis. This was so for example in Göttingen at Gauss’s where Struve learnt many things that were necessary for his degree measurement. However, in Tartu, the meridian passes through the centre of the Tartu Old Observatory’s dome, while the meridian circle is several metres to the west of it. Why so? The simple answer is that Struve started the Baltic degree measurements already in 1821 (or 1816 when we consider the Livonian triangulation as part of the Struve Geodetic Arc measurement), while the meridian circle reached Tartu in the autumn of 1822. Still, it had an impact — to pinpoint precise moments, the meridian circle was used to measure the accurate coordinates and culminations of select stars. Struve mostly did this himself so as not to create inaccuracies that may occur due to different observers. For this purpose, he usually observed Alphecca in the Northern Crown, but sometimes also Procyon or the North Star. In 1826, Struve tied the Tartu point, the centre of the Baltic degree measurement, properly to the meridian circle (Struve, 1831).

26While Struve started his degree measurements in the Baltic provinces, the University of Tartu became a vital teaching and training location for Russian military surveyors. Among other things, they practiced observation with the meridian circle in Tartu. The collaboration between military and civil surveyors led to the joining of the triangulation chains of Struve and military surveyor Karl Tenner (1783-1856) in 1828, and the final result of that was the Struve Geodetic Arc, stretching from Hammerfest to the Black see (2800 km, UNESCO Heritage since 2005) (Struve, 1857-60). The meridian of Tartu was the best-measured meridian in the Russian empire, and could serve as prime meridian, until it was connected to the observatory of Pulkovo in 1844 via chronometric expedition, after that the prime meridian of Russia started to be in Pulkovo.

6.  Ludwig Schwarz and Zone Observations

27The next big project after Struve’s catalogue of double stars and long geodetic arc measurement was participation in zone observations in the second half of the 19th century. The idea that Struve discussed with Bessel already in 1820, measuring the coordinates of all visible stars with the help of the meridian circles, distributing the work between observatories located at various latitudes, was brought up anew in the 1860s. At this time, societies were in fashion, and various kinds of societies with different purposes were constantly being established. During the second meeting of the German astronomical society that was founded in 1863, it was proposed to compile a catalogue of the Northern celestial hemisphere, re-measuring the coordinates precisely with the meridian circles. The catalogue would have had a practical value in specifying the exact time as well as in navigation. Tartu was prepared to join the project right away. At the time, Johann Heinrich Mädler (1794-1874) was still the Tartu observatory’s director, and communicated closely with the German Society of Naturalists, which had been the starting point of the astronomical society (Eelsalu, Hermann & Hamel, 1985). The main part of this large project was performed when Ludwig Schwarz (1822-1894, head of the Tartu Old Observatory from 1872 to 1894) was the director of the observatory.

28This international project is known under the name of zone observations. The entire Northern celestial hemisphere was divided into zones, each of the participating observatories — 10 at the beginning — was to measure the coordinates of all stars up to the apparent magnitude of 9 in the zone that had been entrusted to them (in Tartu’s case, this included the declination* range from 70 to 75, a little more than 3,000 stars) as at the year 1875.

29Unfortunately, the meridian circle had somewhat deteriorated by that time, they did not succeed in observing stars with the apparent magnitude of 9 and repolishing the lens yielded no results, so it was replaced with Merz’ lens in 1877. The old lens was mounted on the Dollond transit instrument, the tube of which had to be shortened. There was also a need to supplement the reading system, as the divisions of the vernier had faded to such an extent that they were very hard to read (Zhelnin, 2020, p. 90). Schwarz investigated the bending of the meridian circle — it was his doctoral dissertation (Schwarz, 1871) — and he also complained that the observatory’s old and rather thick walls caused the air to ripple in the observation openings and the traffic on nearby streets created vibrations.

30The name of the city was changed from Dorpat to Jurjew in 1893, the language of teaching in the university was changed from German to Russian, director Schwarz died, and new director Grigori Levitski (1852-1917) was appointed in 1894. Still, the observatory selflessly endeavoured to do their bit in zone observations. Measurements performed in Tartu were published in volumes 17-20 of Tartu observatory’s publications (Schwarz, 1887, 1891, 1893; Lewitzky 1899). Most of the work done by the observatory of Dorpat, was repeated in 1905-1906 by Leopold Courvoisier (1873-1955) in Berlin-Babelsberg (Zhelnin, 1969, p. 80). The compilation of the entire catalogue took a long time, and it was published in instalments from 1890 to 1910, including the exact position of 140,000 stars. The astronomical society’s catalogue was the last large catalogue from direct observations; the following ones of this type were prepared with the help of photography.

31While Tartu’s participation in zone observation has been deemed somewhat unsuccessful, it was still a great training opportunity for the young generation of observers and mathematicians. Observer Heinrich Bruns (1848-1919) later became the director of the Leipzig observatory, Anders Lindstedt (1854-1939) a Professor at the Royal Institute of Technology in Stockholm, Oskar Backlund (1846-1916) an academician of the Russian Academy of Sciences and director of the Pulkovo observatory, F. W. Struve’s grandson Ludwig Struve (1858-1920) became head of the Kharkiv observatory etc. Among the mathematicians who were responsible for the recalculations for the figures to reflect the beginning of 1875, Gustav v. Grofe (1848-1895) later became an Associate Professor of the University of Tartu, Theodor Molien (1861-1941) a Professor of Tomsk University, Hans Seyboth (1864-1938) garnered fame as a compiler of chess exercises.

32The meridian circle was considered valuable enough to evacuate it during World War I and remount it in its original location after the war. In 1920 electric lighting and a cover were installed for it but no known large projects were undertaken in the interwar period. After World War II, Estonia became part of the Soviet Union, and the former university observatory was included in the Academy of Sciences. To make more space the building was reconstructed and the openings for observation were closed. Already in 1964, the new observatory was opened outside of the city in Tõravere and the old observatory was not used anymore for scientific observations. At the beginning of the 2000s the old observatory was returned to Tartu University. The building was reconstructed again and opened as a museum in 2011.

7.  Conclusions

33At the time of purchasing the Reichenbach & Ertel meridian circle, it was arguably the best instrument available. Together with the transit instrument made by Dollond and the large refractor made by Fraunhofer, it turned the observatory of Dorpat/Tartu into a land of dreams for entrepreneurial astronomers such as Struve. Differently from the Fraunhofer refractor with its equatorial mounting that was tailor-made to match the location, and can be therefore called a solo instrument, the meridian circle was rather part of an orchestra. Observers tried to use similar instruments together. Struve’s catalogue of binary stars with its supplements remains the most important scientific achievement of the Tartu Old Observatory’s meridian circle. As the meridian circle arrived in Tartu at a time when Struve had already begun measuring the Baltic Geodetic Arc, it is not on the Tartu meridian (that passes through the centre of the Old Observatory) but is still connected to it. This means that it is also linked to the Struve Geodetic Arc, a UNESCO World Heritage object. In the first half of the 19th century, the Tartu meridian circle supported astrometric works of Pulkovo and it has been used in instruction for at least 140 years (1822-1964). During all this time the meridian circle was used also for everyday astronomical timekeeping. In the second half of the 19th century, the observatory under the leadership of Ludwig Schwarz participated in compiling the astronomical catalogue of the German astronomical society using a meridian circle that by then was slightly deformed and outdated. This project was never successfully completed, but the people of Tartu contributed to it for a quarter of a century, contributing to the promotion of a new generation of astronomers.

34The meridian circle still stands at its original location in the Western hall of the Tartu Old Observatory. Alas, one cannot perform observations with it anymore, as the observation opening was closed during reconstruction following World War II.