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The meridian circle is an instrument for timing of the passage of stars across the local meridian, an event known as a transit, while at the same time measuring their angular distance from the nadir. These are special purpose telescopes mounted so as to allow pointing only in the meridian, the great circle through the north point of the horizon, the zenith, the south point of the horizon, and the nadir. Meridian telescopes rely on the rotation of the Earth to bring objects into their field of view and are mounted on a fixed, horizontal, east-west axis. The similar transit instrument, transit circle or transit telescope is likewise mounted on a horizontal axis, but the axis need not be fixed in the east-west direction. For instance, a surveyor's theodolite can function as a transit instrument if its telescope is capable of a full revolution about the horizontal axis. Meridian circles are often called by these names, although they are less specific. For many years, transit timings were the most accurate method of measuring the positions of heavenly bodies, and meridian instruments were relied upon to perform this painstaking work. Before spectroscopy, photography, and the perfection of reflecting telescopes, the measuring of positions (and the deriving of orbits and astronomical constants) was the major work of observatories.〔 , at (Google books ) 〕〔 , at (Google books ) 〕〔 , at (Google books ) 〕 ==Importance== Fixing a telescope to move only in the meridian has advantages in the high-precision work for which these instruments are employed: *The very simple mounting is easier to manufacture and maintain to a high precision. *At most locations on the Earth, the meridian is the only plane in which celestial coordinates can be indexed directly with such a simple mounting; the equatorial coordinate system aligns naturally with the meridian at all times. Revolving the telescope about its axis moves it directly in declination, and objects move through its field of view in right ascension. *All objects in the sky are subject to the distortion of atmospheric refraction, which tends to make objects appear slightly higher in the sky than they actually are. At the meridian, this distortion is in declination only, and is easily accounted for; elsewhere in the sky, refraction causes a complex distortion in coordinates which is more difficult to reduce. Such complex analysis is not conducive to high precision. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Meridian circle」の詳細全文を読む スポンサード リンク
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