Vertical circle
The term vertical circle (outdated vertical circle ) is used in astronomy for vertical great circles and associated measuring instruments , in geodesy, however, for the glass circles built into theodolites for height measurement .
Vertical circle in trigonometry
In terms of spherical trigonometry , a vertical circle is any great circle that is perpendicular to the mathematical horizon . All these circles, also called “vertical”, go through the zenith and nadir of the observation site. The planes spanned by these circles are called vertical planes , they are orthogonal to the horizon plane .
The vertical circle going through the west and east point is called “ first vertical ”, the one running through the north and south point is called the “(celestial) meridian ”.
Vertical circle as a measuring instrument
As a term used in instrument science, it is an astronomical angle measuring device similar in structure to the meridian circle , rotatable in height and azimuth . It consists of a telescope (in the Astro Geodesy isolated also a mirror objective , for example, when ". Core DKM3 ") mounted on a horizontal tilt axis is rotatably adjustable and a stable Altazimuth - mount is worn. The angular movements around the tilting and standing axes are measured with large, finely divided circles and a reading microscope .
These instruments represent the predecessors of the theodolite and allow the elevation and horizontal angles of stars or star pairs to be determined in the horizontal coordinate system.
- If a vertical circle can be pivoted or folded over, but is designed without a special horizontal circle, it is called a straight-through or passage instrument. Such massive instruments, around 50 cm high, are used to determine the exact time ( sidereal time ) and - in international measurement campaigns - to determine the differences in geographical longitudes .
- Equipped with a Horrebow level (a particularly precise level ) and an optical micrometer , vertical circles were used for the Horrebow-Talcott method for determining latitude until around 1980 (see International Polar Motion Service ).
As an independent type of instrument, the vertical circle slowly fell out of use in astronomy from the 19th century, as it was inferior to the passage instrument and especially the meridian circle in terms of measuring accuracy . For example, in 1796 a vertical circle from Carry (London) with a 2- foot (61 cm) focal length was used at the Gotha observatory, but was resold after a few years. In geodesy, however, it was further developed - in a more compact design - into a universal instrument.
Geodetic measuring device
Geodesists understand by the "vertical circle " the vertical or height circle of an opto-mechanical measuring instrument ( theodolite , tachymeter ). This pitch circle , made of glass or alloyed metal, is used to measure zenith angles . They can be read with the help of micrometers or electronically to an accuracy of about 1 " , with larger universal instruments even to 0.1".
The reference to the plumb line is established by a very sensitive level or a height compensator that aligns itself in the plumb direction due to gravity ; the remaining deviation is called the altitude index error .
To increase the accuracy of the measurement , the theodolite telescope is "punched through" after the first measurement. H. brought into the second vertical circular position. The two readings should complement each other to 360 ° (or 400 gon ). Any deviation from this is related to the collimation or target axis error . By measuring the angle of elevation (zenith angle) in both telescope positions and forming the difference between the measured values, it is not necessary.
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