The Transit Circle at the Royal Observatory, Greenwich

This circle and telescope were originally proposed by the seventh Astronomer Royal, George Biddell Airy, to supersede both the transit instrument (AST0982) and the mural circle (AST0973) introduced his predecessor, John Pond. Constructing the huge circle and telescope was a major undertaking but, as Airy told the Observatory's inspectors, 'Whatever we do, we ought to do well'. It was designed by Airy and the engineering was carried out by agricultural machinery experts, Ransomes & May of Ipswich, while the prestigious London instrument makers Troughton & Simms constructed both the optical parts and the main body.

A transit circle combines the operations of a mural quadrant or circle and a transit telescope by allowing the observer to measure both the right ascension and declination of a celestial object as it moves across the meridian (north-south line) of the telescope's cross-hairs. Between 1851 and 1954 this instrument was to make some 600,000 observations. Built to the east of the previous transit telescope, the transit circle defined a new meridian at Greenwich from 1851 until the present day. After an international conference was held in 1884, it was agreed that this Greenwich Meridian would be the Prime Meridian, marking 0├òÔòØ longitude. It was also agreed that the international day started at midnight in Greenwich. Thus this instrument defined time and space for the world.