The Greenwich Meridian

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The Troughton Mural Circle was commissioned in 1807 with the intention that it would replace the two mural quadrants (Halley’s and Bradley’s) and possibly the Bradley 8-foot Transit Instrument as well. A new Circle Room (now the site of the Airy Transit Circle (ATC)) was built to house it. The instrument was erected in 1812. The first published observation of NPD (a measure of the height of a star) was made on 11 June and the first transit observation on 15 June. The circle was only used for transit observations until 3 March 1814, as the instrument did not prove steady enough. It was briefly used for this purpose however, for the eleven days that Pond was without a transit instrument in 1816, while the new Troughton instrument was being prepared following the dismounting of the Bradley one.

Pond was keen to use two circles simultaneously and was able to persuade the admiralty to let him have the virtually identical circle being made by Jones for the new Cape Observatory. It was erected in 1824 with the first recorded observation, taking place on 18 January 1825. The two instruments were mounted facing each other, on separate piers with the Troughton to the west and the Jones to the east.

The published observations show that the Troughton Circle was used with both a north and a south mark. No description of either is given. The first reference to a mark appears on 21 July 1812 in connection with the transit observations. Philip Laurie, in his 1955 history of the Observatory wrote: ‘On September 7, 1812, a South Meridian mark was put up but had to be replaced three years later when it was discovered that the original had been removed by mistake by some workman engaged in the construction of a new building.’ Regrettably, Laurie does not quote a source – whatever it was, it was not the published observations.

There are 19 dates on which there are references to the marks in the published observations in the period up to 6 July 1813, (with the majority being associated with the transit observations), with only a tiny handful after that – the last being in 1833 when Pond recorded ‘on the 1st March, the meridian mark appeared bisected by the central wire’. There is an identical entry for the Jones instrument on the same date. This is the only reference to a meridian mark for the Jones instrument in the whole of Pond’s published observations.

According to Airy, the pier on which the Troughton instrument was mounted was on virtually the same meridian as the western pier of the Airy Transit Circle, whilst the Jones one was a few feet to the east of its eastern one. This would put the meridian of the Troughton Circle roughly in line with the western edge of the roof opening above the Transit Circle and the meridian of the Jones, a few feet to the east of its eastern edge. According to Google Earth (© 2005), the ‘Troughton’ meridian would have clipped the SE corner of the red brick section of the Ranger’s House. When the data set was updated in late 2006, it was aligned slightly differently to the grid and indicated that the Jones rather than the Troughton Meridian strikes the building at about that point. Microsoft’s Virtual Earth (© 2006) indicated likewise.

This is where the iron plate is mounted. Its height corresponds with the top of the park wall. The plate is four inches square, with a central vertical slit a little over ¾ inch long and about ¼ inch wide (equivalent to about 2.7” of arc at the distance of the mural circles). Since it appears to have no other purpose, it is assumed that it is a meridian mark for one of the two circles – or possibly both of them if its main purpose was to check their stability. The presence of the railings does however present a problem, for unless they are of a later date; they are likely to have interfered with the view.

When the Jones instrument was first erected, a series of daily observations were made of a ‘window at Blackwall’ with both circles – their purpose presumably to check the stability and relative calibration of the circles rather than their alignment. The identity of the building is unknown, but it is plausible that it was this window rather than a bespoke mark that Pond used as his north mark when the Troughton Circle was first set up.

So is the iron plate a meridian mark? The jury is out. Let us know what you think.