The Reichenbach-Ertel Meridian Circle Bought: 1822 Company: Reichenbach-Ertel, Munich

The main aim of the transit instrument was to determine exactly the astronomical time and this work is done until today by the instrument. It was the astronomer’s special interest to study the movement of the celestial bodies. To measure it, it was necessary to establish extremely exactly the coordinates of the studied objects. This is how the Danish astronomer Ole Römer came to an idea to add the extremely accurate circular scale on the level of the meridian to the transit instrument with the help of which it was possible to measure the culmination height together with the culmination time. Knowing the latitude of the place of observation, it is easy to calculate the declination of the culminating celestial body. In England they began to call the new instrument the transit circle, in Germany the meridian circle (Meridiankreis). The last name is the source for the Estonian name.

Georg Friedrich von Reichenbach and his meridian circles. Reichenbach (1771-1826) who was born to the family of engineers started independent business in Munich in 1804. In 1809 he established a joint company with Joseph Fraunhofer (1787-1826) who had just found a technical solution for making big achromatic objectives. In 1814 he left Fraunhofer and together with Traugott Ertel (1778-1858) established a separate workshop. In 1819 their first meridian circle was completed which F. Bessel had ordered for the Observatory of Köningsberg. In this instrument the circular scale with the verniers was used thus making the accuracy of determining the declination comparable with the accuracy of the hourly angle (up to one second of the arc). After Reichenbach’s death Ertel continued building meridian circles with his son.

The meridian circle of Tartu was one of the Reichenbach’s first and also largest instruments. The telescope’s aperture is 10 cm, the focal length 1.5 m, the diameter of the circular scale 95 cm. In the centre of the first program, started by Struve, the determination of the location of binary stars and the control of the accuracy of the meridian circle was important. In 1822-1826 Struve made 13,128 documented observations at 748 nights.

Another bigger work was the participation in a wide scale observation program started by the German Astronomical Society – the repeated measurement of stars in the Bonn star catalogue (BD – Bonner Durchmusterung) the so-called zone observations. The catalogue was completed by Friedrich Argelander, who was from Prussia and later worked in Finland. He was not satisfied with his measurements and as a founder member and the first chairman of the society wanted with common efforts to “perfect” the catalogue. The program was started in 1869. The declination from +70 to +75 degrees was given to the Tartu Observatory. It was necessary to measure all the stars, up to the 9th magnitude, all in all 13,102 stars, 35 observation registers with 3,492 sheets have been preserved from the most intensive observation period of 1845–1882.

The meridian circle is now in its original place in the Western hall of the observatory and ready to function, but, however, nothing can be observed because the openings in the wall and the roof have been closed in the course of the post-war reconstruction.