The Meridian Circle

When the observatory moved to Bergedorf in 1909, it obtained a meridian circle with a focal length of 2,3m and an aperture of 190mm, constructed by the company A. Repsold & Söhne.

The Meridian Circle is an astronomical telescope, which pivots only North-South.  With its help, one can define the transit time of a star through the meridian. The meridian is the supposed line that celestial bodies pass at their highest point over the horizon during the day.

The exact observation of meridian passages combined with the exact measurement of the altitudes above the horizon allows one to accurately determine a star’ s celestial coordinates. Conversely, pre-determined stellar coordinates allow one to calculate the exact time (sidereal time). Such methods help determine the local time, the time for the time zones (CET) and the universal time (GMT or UT). Until World War II, the calculation and the announcement of time was one of the main functions of Hamburg Observatory.

Until 1964, the Meridian Circle in Bergedorf was used in two major projects to survey current star positions (AGK2 and AGK3). In 1967, the instrument was modernised and shipped to Perth (Australia) for mapping the southern hemisphere. In 1989, the Meridian Circle was transferred to the Deutsches Museum in Munich.

Fig. 1: The Meridian Circle

Fig. 2: Observer and time reader at the instrument

 

Mapping of the Northern Hemisphere

During the 1920s and 1930s, the observatories in Hamburg-Bergedorf, Bonn and Pulkovo (near Leningrad/St. Petersburg) collaborated to create a new catalogue containing the current position of stars (AGK2). In Hamburg, the project was very successful, so that the duties of Pulkovo passed over to Hamburg.  

The function of the Meridian Circle was to determine the exact position of a network of reference stars. The positions of all other stars were measured relative to those stars on photographic plates. A star’ s coordinates are constantly changing by a small amount because of the precession of the Earth’s axis and the star' s proper motion. So in 1953, prior to the completion of AGK2, astronomers started work on a new catalogue. Besides Hamburg Observatory, several other international observatories participated in the AGK3, which was completed in 1964.

 

Fig.: Astronomers working on the AGK3