How did transit instruments help astronomers map the celestial sphere?

Astronomical transit instruments are specialized telescopes designed to map the celestial sphere by restricting their motion to the plane of an observer's meridian—a great circle passing through the north and south points of the horizon and the zenith.1

By aligning the telescope's optical axis on a north-south line, astronomers observe celestial objects as they rotate across this meridian, a process known as transiting. This restricted movement provides high stability, allowing for precise measurements of an object's position. Specifically:1

• Right ascension: Determined by transit circle telescopes by timing the object's crossing of the meridian.1
• Declination: Measured by vertical circle telescopes or transit circles by observing the object's zenith distance.12

The data collected from these observations are compiled into star or planetary catalogs, which serve as the foundation for accurate celestial mapping. Historically, taking thousands of such observations from a standardized meridian allowed astronomers to create consistent maps essential for navigation and timekeeping.12