Meridian circle

The meridian circle is the classic instrument for measuring Sternörtern in Meridian. It corresponds to a very large theodolite without a vertical axis, the only in the meridian plane ( South Point - Zenit - North Point) is pivotal, and thus belongs to the so-called passage or passages instruments.

A meridian circle is mounted on two pillars on which the two spindles ( precision, circular polished shaft ends ) rest the horizontal east-west axis. The precise measurement of 1-3 m telescope focal length is rotatably mounted on this axis. The telescope is 1-2 large vertical circles twist with Ablesemikroskopen and dragonflies.

The meridian circle was further developed by Olaf Romans from the freiäugig used mural quadrant ( Wall circle) and was from about 1810 to 1950 the most important instrument of many observatories. With about 20 globally distributed meridian circles all accurate star catalogs and proper motions were measured, such as the AGK1 ( 1868-1908 ) and the AGK2 ( 1924-33, 200,000 stars to ± 0.1 " exactly ).

It measures time and elevation angle of the stars when they reach ( culmination ) in the Meridian high. This occurs precisely at the vertical thread if the telescope and journals are calibrated. The time difference of two stars corresponds to their Rektaszensionsdifferenz, from elevation and latitude follows the declination.

Were early on instead of visual measurements on reticle " impersonal micrometer " used that one tracks the stars. By 1920 they built many instruments on photographic circle reading to, from the 1970s on optoelectronic measuring methods and lately on CCD.

The full measurement accuracy was obtained earlier by timing at about 20 parallel threads in the visual field by the ticking of a pendulum clock. For the height measurement is to the star on the thread line, behind which one biseziert him for a few seconds. Thus, the influence of atmospheric turbulence falls out mostly. The inclination of the axis is controlled by a suspended level (high level), their direction by two collimators ( auxiliary telescopes at some distance ). The index error is determined by vertical measurement to a shell with reflective mercury.

Even today special star catalogs are measured with meridian circles, eg for space or for Comet astrometry. Precision opto-electronic instruments that are in Bordeaux and the Carlsberg Meridian Circle on La Palma. The latter works since 1984 to ± 0.003 " accuracy.