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transit circle(meridian circle) An optical telescope mounted in such a way that it can swing only in a vertical north-south plane. It is used to measure the positions of celestial bodies with great accuracy as they cross the meridian of the telescope site (see culmination). The altitude of a body as it crosses the meridian is now usually measured very precisely by electronic devices; earlier instruments used an accurately graded circular scale set in the vertical plane. The declination of the body can then be determined using the latitude of the observatory. Measurement of the precise local sidereal time at the meridian passage determines the right ascension of the body.
transit circle[′trans·ət ‚sər·kəl]
A type of astronomical transit instrument having a micrometer eyepiece that has an extra pair of moving wires perpendicular to the vertical set to measure the zenith distance or declination of the celestial object in conjunction with readings taken from a large, accurately calibrated circle attached to the horizontal axis. Also known as meridian circle; meridian transit.