Meridian Circle

A meridian circle, also called a transit circle, is a precision astronomical instrument, essentially a telescope mounted so that it can rotate only along the north-south meridian line, used to measure the exact moment a celestial object crosses the meridian together with its angular altitude, from which its precise position in the sky can be calculated. Developed and progressively refined from the seventeenth through nineteenth centuries, building on the earlier transit instrument pioneered by astronomers including Ole Romer, the meridian circle became the standard instrument of major observatories for compiling accurate star catalogues and for maintaining precise civil and astronomical timekeeping, since a star crossing the meridian provided an exact reference against which clocks could be checked. Meridian circles at observatories such as Greenwich played a central role in nineteenth century positional astronomy and in establishing the geographic and time standards, including the prime meridian itself, that underpinned global navigation, before the instrument was gradually superseded in the twentieth century by photographic and, later, electronic and satellite-based astrometric techniques.