Zenith telescope

Zenith telescopes are high-precision measuring instruments of astronomy and geodesy geodetic. They are a special development of the meridian circle and are used for accurate determination of Sternörtern and the perpendicular direction, which is most favorable near the zenith because of low atmospheric interference.

Important measurement results are in addition to the stellar coordinates the astronomical latitude and longitude or the local sidereal time. In cooperation with other observatories can be established from the polar motion of the earth and a unified, international length and time system.

Instrument technology and research objectives

Zenith telescopes are a few meters long pipes with the focal length and 20 cm to 30 cm opening width which are mounted in the vertical plane of the local meridian. Special methods and micrometers ( see Horrebow - Talcott method) differences of zenith distance of star pairs and / or the timing of their meridian crossings are measured. Which can be converted

  • Calculate accurate positions of stars near the zenith, what in the global network of some of these observatories, the establishment of the "fundamental system " allows the celestial coordinates ( similar methods are used also by scanning astrometry ), or

The restriction to the zenith nearby stars is indeed an obstacle for large-scale surveys and analyzes, but increases the accuracy quite substantially - namely about 0.01 " (30 cm to a radius of the earth 6357-6378 km).

Specifically, the zenith is chosen for such precision measurements because

Zenith telescope in the 17th and 18th centuries

First ideas for a Zenitteleksop go back on the 17th century. Robert Hooke had a prototype, called by him " Archimedean Engine" built into the Gresham College, in order to prove the heliocentric system. On the basis of the star etamin the dragon who goes almost through the zenith of London, he wanted to (in vain ) to measure the suspected annual change in position ( parallax).

After the fire of London in 1666 used Hooke and Christopher Wren for this experiment, the Doric column erected as a memorial to the fire and brought to its tip to a lens. But she was not stable enough against the wind. 50 years later, James Bradley and Samuel Molyneux repeated the experiment with a much smaller, but more stable assembled zenith telescope. Instead of the (very small ) parallax they discovered while the aberration of starlight (20 " ), which in 1728 was to prove the motion of the Earth for the first time.

Large Zenith Telescope

Reflecting telescopes with a liquid primary mirror can be constructed only as zenith telescopes of principle. The Canadian Large Zenith Telescope has an aperture of at least six meters. His mercury level is held by rotation in its parabolic shape.

As the largest Zentitteleskop However, the radio telescope of Arecibo qualified with a diameter of about 300 m.

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