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vertical circleA great circle on the celestial sphere that passes through an observer's zenith and cuts the horizon at right angles.
an astronomical instrument for determining by observation the zenith angles of heavenly bodies, which serve as material for calculating their declination, as well as the geographic coordinates of the place of observation. The optical tube of the instrument for sighting the heavenly body can rotate around a horizontal axis. An accurately divided circle placed on this same axis serves to measure the vertical angles. The entire instrument can be set in any vertical by rotating it around the vertical axis; how-ever, the vertical circle is usually used for observation in the meridian. Observations with the vertical circle are conducted mainly when compiling absolute catalogs of declinations of stars. The idea of the vertical circle was proposed in the 19th century by V. Ia. Struve, the founder of the Pulkovo Observatory. The first vertical circle was set up at Pulkovo in 1839. A photographic vertical circle with a largely automated observation process was built at the Pulkovo Observatory in 1962.
a large circle of the celestial sphere passing through the zenith and nadir of the place of observation and a given point on the celestial sphere. The vertical circle that passes through the north and south points coincides with the celestial meridian. The vertical circle that passes through the east and west points is called the prime vertical.