A telescope adapted to the observation of the passage, or transit, of an astronomical object across the meridian of the observer. The astronomical transit instrument is the classic instrument of positional astronomy, which is the study of the positions and motions of astronomical objects. (The specific categories of astronomy concerned with these investigations are astrometry and celestial mechanics.) The chief variants of the classic design include the vertical circle, the horizontal transit circle, the broken or prism transit, the photographic zenith tube and, most commonly, the meridian or transit circle.