The Greenwich Meridian

Currently viewing images from 29 Sep 2011

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The Astronomical Mark

From the earliest of times, it was a priority for astronomers to get an accurate determination of the difference in longitude between the Observatory at Greenwich and ones elsewhere. Of interest here, are the telegraphic determinations that took place between 1888 and 1902. In Brief:

1888: Erection of wooden hut in courtyard, containing a small reversible transit instrument mounted on the Bradley Meridian.

1891: Erection of new transit pavilion to replace the wooden hut.

1902: Third telegraphic determination of Greenwich Paris longitude difference. For this determination, a south meridian mark was set up: ‘ … the 7½ -inch object-glass of about 51 feet focal length, which had been made in 1887 for use with the personal machine, was utilised for the purpose. A strong shelf was attached to the girder on which the dome of the transit pavilion rests, and the object-glass was set in a stand, so that it could be placed, when necessary, in a vertical position on this shelf, and a mark on the north wall of the main observatory building [ie the present Meridian Building] could be viewed through it by the telescope. The mark was formed by a small hole in a metal plate, behind which an electric light was placed, and when seen in the telescope it was very similar to the image of a star of about the 4th magnitude. The mark was 55 feet south of the centre of the observing pier, and the object-glass, when in position, 6 feet south of the same point.’ No trace of the mark remains, nor has it be identified in any images.

1927: Small transit instrument(s) used for time determination (until 1957), replacing the Airy Transit Circle which had been used for this purpose since the start of 1851 (see 1940s image).

1958: Transit Building demolished.

Wall Plaque(s)

The present plaque denoting the Bradley Meridian dates from the Observatory’s 1992–93 refurbishment. Prior to this, and roughly in the same location, there was a short vertical metal wall strip, with a sign above and an explanatory sign alongside. This earlier marking probably dates from about 1967 when the Meridian Building was opened to the public for the first time following extensive renovation work. It was certainly in place no later than 1970. Prior to then, the position of the Bradley Meridian appears to have been unmarked at this location.