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meridian circle:see transit instrumenttransit instrument
telescope devised to observe stars as they cross the meridian and used for determining time. Its viewing tube swings on a rigid horizontal axis restricting its movements to the arc of the meridian.
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meridian circleSee transit circle.
(also transit circle), an astronomical instrument for the exact determination of the right ascensions and declinations of celestial bodies by measuring the time of meridian transit and the zenith distances of the celestial bodies. The meridian circle was invented at the end of the 17th century by O. Roemer, and its theory was developed by T. Mayer (18th century) and F. Bessel (19th century). The advantages of the meridian circle in comparison with other astronomical instruments resulted in its widespread use in the 19th century. In the 20th century it is the principal instrument used for the exact determination of the equatorial coordinates of celestial bodies.
The modern meridian circle has a telescope with an objective having a diameter of 15-20 cm and a focal length of 150-250 cm. The horizontal axis of the instrument is mounted on massive posts such that the telescope, which is fixed perpendicularly to the axis, rotates as exactly as possible in the plane of the celestial meridian. Small deviations in the line of sight of the meridian circle from the meridian, which depend on the correctness with which the meridian circle has been mounted and on the instrument’s errors, are taken into account in correlating observations with results of special investigations.
To register the moments of transit across the meridian that are necessary for the determination of the right ascensions, the eyepiece part of the meridian circle tube is equipped with a recording micrometer. The observer shifts the vertical wire of the eyepiece micrometer, directing it at a star moving across the field of vision; at this time, the contacts are periodically closed and the moments of closures are recorded on a chronograph. Accurately graduated circles are mounted on the horizontal axis to measure the angles and meridian plane. The horizontal wire of the eyepiece micrometer is directed at a star in measuring declinations and the divisions of the circles are counted off. The divided circle is read by four measuring microscopes placed in cylinders that are mounted on the posts of the meridian circle.
In some meridian circles most processes of actual observation are automated. The meridian circle is placed in a special room whose ceiling slides apart to form a wide slit along the meridian for observations. Maximal stability and minimal reaction to temperature variations are among the requirements imposed on the meridian circle. The precision of the determination of equatorial coordinates on a meridian circle is characterized by a mean square error for the right ascension (α) of ± 0.020 sec 8 and for the declination (δ) of ±0.35”.
REFERENCEPodobed, V. V. Fundamental’naia astrometriia, 2nd ed. Moscow, 1968.
V. V. PODOBED