an astronomical instrument, mounted on an E-W axis, in which the reticle of a telescope is always in the plane of the meridian. It is used to time the transit of a star, etc, across the meridian
transit instrument in American English
1.
a telescope mounted at right angles to a horizontal east-west axis so that it can be rotated only in the vertical plane of the meridian at its site: used to observe and time the transit of celestial bodies across the meridian