Wall quadrant

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A wall quadrant is a historical astronomical instrument used to measure the exact elevation angles and positions of stars on the meridian . It was in use until around 1800 and is the predecessor of the meridian circle .


The basic construction is the same as for the classic quadrant . It consists of

In contrast to the smaller quadrants , which were held in the hand or set up on a tripod , the wall quadrant is firmly mounted on a precisely vertical wall running north-south. Due to this exact alignment to the meridian, stars (especially fixed stars ) can be reliably recorded at their culmination and their transit time can also be measured precisely. Due to the stable installation of the highly magnifying telescopes , the measurement accuracy was noticeably increased.

Tychonic wall quadrant

Tycho's wall quadrant, around 1600
The approximately 2.5 meter high wall quadrant by John Bird , London 1773

Until the invention of the telescope , Tycho Brahe's quadrant of the wall was the most accurate of its kind. It stood on Tycho's Danish observatory Uranienborg and had a quarter circle of two meters radius, which was supposedly readable to an accuracy of 10 ″ (0.1 mm). Copernicus' instrument (around 1520) might not have been much smaller, but it was mounted on a wooden plate, and the circle was divided to 0.5 °, which allowed angle estimates of around 0.05 °.

As an experienced observer, Tycho was able to target the stars with free eyes to about 1 ′ (one arc minute ), which corresponds to the optical selectivity of the eye . He achieved measurements that were 2 to 5 times more accurate than before, which was a crucial prerequisite for Johannes Kepler's planetary theory. Kepler himself, his assistant and successor, hardly took any measurements because he had an eye defect.

Main instrument in observatories of the 17th / 18th centuries Century

With the increasing importance of celestial mechanics and improved star catalogs , the demands on accuracy increased. When they therefore end of the 17th century on measuring Telescopes with stronger magnification passed, big, due south were aligned wall quadrant to the main instrument of many observatories . The reference to the mathematical horizon was established by vials for seconds , and eyepiece micrometers were even attached to some devices (invented by Ole Römer around 1680), which enabled fine measurements in the field of view of the telescope.

With these developments, measuring accuracies of 1–2 ″ were achieved as early as 1700. Fixed quadrants now allow not only time determination (to about ± 0.1s) but also the measurement of precise star locations and geographical latitude . At the Specola observatory in Bologna, around 1710 even 0.8 ″ was reached. With such an instrument, James Bradley discovered the annual aberration of the "fixed stars" in 1725 , which was the first time that the heliocentric view of the world could be demonstrated.

Later meridian instruments

The wall quadrant was replaced in the course of the 18th century by the meridian circle , in which the telescope can sweep the entire meridian from the south point through the zenith to the north point. Astronomical and geodetic instruments were also developed, which achieved higher accuracies with smaller dimensions, because a more compact design is less sensitive to instrumental errors and temperature influences. In addition to the transit instruments and the passenger instrument , more flexible universal instruments were increasingly used, with which 0.1 ″ could be reached by 1920. Today the astrometry of planetary and star positions is carried out with automatic meridian circles and with so-called astrometric satellites .

See also

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