LB Greenwich
Greenwich (SE10)
Royal Observatory, Meridian Building
Distance (S) from Greenwich
OS map detailsOS Explorer: 161 or 162
OS grid refTQ 38883.77322
WGS84 lat/long51.477815, -0.001502
Marking date1851
AccessRestricted (charged entry)
Greenwich Meridian Marker; England; LB Greenwich; Greenwich (SE10)


Currently viewing images from 29 Sep 2011
Click to choose image set
29 Sep 2011

The Airy Transit Circle (ATC) has defined the Greenwich Meridian since 1851. It was this meridian, that it was suggest should be adopted as the Prime Meridian of the World at the International Meridian Conference in 1884. The resolution passed, stated: ‘That the Conference proposes to the Governments here represented the adoption of the meridian passing through the centre of the transit instrument at the Observatory of Greenwich as the initial meridian for longitude’. This therefore is the key location for the discerning visitor to the Meridian to seek out. For more information on the ATC, and the International Meridian Conference, see links on the left.

The first image in the 1850s image set, is the first of 16 plates published by Airy in the 1852 volume of Greenwich Observations as part of his description of the Transit Circle. It ‘gives a perspective view from the north-west, supposing the ladders removed, and the pillars which support the roof broken off’. The official drawings were used as the basis of illustrations by others, as can be seen from the other 1850s images. In the first of the 1880s images an observation is being made of the Sun and a shield has been fitted to reduce the heating and subsequent distortion of the telescope tube by the Sun’s rays. The second image in the 1890s set, shows the mercury trough, which was used to provide a reflected image of a star whose position was being measured. The appearance of the eye end of the telescope was altered, following the fitting of the travelling wire micrometer in January 1915.

The first recorded observation with the Instrument was taken on 4 January 1851 and the last on 30 March 1954. It ceased to be used for the determination of time in 1927.

Airy Transit Circle
Royal Observatory visitor attraction (part of the National Maritime Museum)