Meridian circle

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Meridian circle of the Kuffner observatory , Vienna.
Meridian circle instrument from the precision engineering workshop of Georg von Reichenbach and Traugott Ertel (Munich, 1825)

The meridian circle is the classic instrument for measuring star locations in the meridian . It corresponds to a very large theodolite without a standing axis , which can only be pivoted in the meridian plane ( south point - zenith - north point ), and is therefore one of the straight through or passage instruments .


A meridian circle is mounted on two pillars that are deeply rooted in the ground , on which the two axle journals (precise, circularly ground axle ends) of the horizontal west-east axis rest. This precisely leveled tilt axis of the instrument is relieved with 2 or 4  counterweights via lever constructions so that the axle bearings do not wear off. The precise measuring telescope with a focal length of 1–3 m sits vertically rotatable on this axis. With the telescope, 1 to 2 large vertical circles rotate with reading microscopes and dragonflies .

History and meaning

The meridian circle was created by Olaf Romans from the freiäugig used mural quadrant developed (wall circle) and the most important was from about 1810 to 1950 meter of many observatories . With around 20 globally distributed meridian circles, all exact star catalogs and proper movements were measured, for example AGK1 (1868–1908) and AGK2 (1924–1933, 200,000  stars with an accuracy of ± 0.1 ).

Even today, special star catalogs are measured with meridian circles, e.g. B. for space travel or for comets - astrometry . Precise optoelectronic instruments are those in Bordeaux and the Carlsberg meridian circle on La Palma . The latter has been working with an accuracy of ± 0.003 ″ since 1984 .

Usage and corrections

The time and angle of elevation of the stars are measured when they reach their highest point ( culminate ) in the meridian . When the telescope and axle journal are calibrated , the maximum level occurs exactly on the vertical thread. The time difference between two stars corresponds to their right ascension difference, the declination follows from the elevation angle and geographical latitude .

In the past, full measurement accuracy was obtained by measuring the time on around 20 parallel threads in the field of vision using the ticking of a pendulum clock . To measure the height , place the star on the horizontal thread , behind which it is bisected for a few seconds . This largely eliminates the influence of the unrest in the air .
The inclination of the axis is controlled by a hanging level (large dragonfly), its direction by two collimators (auxiliary telescopes some distance away). The height index error is determined by measuring perpendicularly to a bowl with reflective mercury .

Early on, instead of visual measurements on the thread net,impersonal micrometers ” were used to track the stars. Around 1920, many instruments were converted to photographic circular reading , from the 1970s to optoelectronic measuring methods and more recently to  CCD .


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