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transit instrument

transit instrument or transit, 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. In the field of view of the eyepiece are threads of spider web or fine lines ruled on thin glass. The threads or lines are parallel in a north-south direction and odd in number. Precise adjustment places the middle line exactly on the meridian. After the observer has noted the times at which each line is passed by the star, he averages them to learn the instant at which the star was on the meridian. In modern transits, known as meridian circles or meridian telescopes, the observer merely presses a button as the star crosses each line. Electrical impulses are recorded on a revolving drum at one or two second intervals as they pass through a chronograph. The meridian circle is equipped with precisely graduated circles mounted on the horizontal axis. Stationary verniers, or reading microscopes, mounted on the fixed supports of the telescope enable the observer to read the circles. The meridian telescope gives the altitude of a star as well as the transit time. This information yields the right ascension and declination, i.e., the location of the star in the celestial sphere. The meridian circle has largely replaced the transit as the equipment of observatories, although the older transit instrument is still used to some extent for determining sidereal time. For a discussion of the transit used by engineers, see surveying.

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