Airy transit circle

This telescope was originally proposed as a more accurate, more modern version of the transit instrument introduced by the Astronomer Royal, John Pond [AST0982]. Constructing Airy's huge transit circle was a major undertaking. The engineering was carried out by agricultural machinery experts, Ransomes & May of Ipswich while prestigious London instrument makers, Troughton & Simms constructed both the optical parts (including the 206 mm object lens) and the main body. As a whole however the instrument was designed by then Astronomer Royal, Sir George Biddell Airy.

The telescope was first used on 4th January 1851. Airy had intended for it to come into use on the 1st day of the new half century but bad weather prevented it. The transit circle was in continuous use until 1938& in 1954 the last official observation was made (by Gilbert Satterthwaite). The telescope was used to observe clock stars which provide the basis for GMT predominantly. It was also used to observe the Sun, Moon, planets & minor planets & other stars to improve knowledge of their positions and proper motion. In 1880 GMT based on the meridian of this instrument was made legal time in the UK & in 1884 it became the prime meridian of the world. The telescope was generally operated by two observers, one on the telescope, the other on the circle microscopes.

As the transit circle was not wall mounted, repeated use often meant that the telescope fell out of true vertical alignment and needed to be realigned (or collimated) weekly. From 1854, a recording apparatus, or chronograph, was introduced to record its observations on a rolling drum of graph paper. The last observation was taken in 1954 just before the astronomers finally left the bright lights and polluted skies of Greenwich for their new site on the grounds of Herstmonceux Castle in Sussex.
Troughton & Simms