The Greenwich Meridian

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The use of meridian marks by astronomers

Ground based marks accurately placed on the meridian were regularly used by the Greenwich astronomers as a quick means of determining the alignment errors of their transit instruments. Ideally, both a northern and a southern mark were used.

The need for a mark at Chingford

With the existing northern mark erected at Blackwall in 1816 often rendered invisible by the rigging of ships or mists from the Thames, Pond, the Astronomer Royal sought to erect an alternative. In July 1819, it was ‘Ordered that Colonel Mudge be requested to assist the Astronomer Royal in carrying into effect the establishment of a permanent north meridian mark.’ Three years later, on 4 July 1822, the Observatory’s Board of Visitors resolved ‘That the Astronomer Royal be requested to apply to the proper persons to obtain an estimate of the expense of erecting a granite column upon proper foundations for a meridian mark, and to ascertain the conditions upon which the proprietor of the land will consent to the erection and that he do report the same to the council’. The Minutes of their meeting on 14 November 1822 show that the terms agreed were 'fifty pounds for a long term of years at a peppercorn rent and the lessen to be paid for any damage that may be done to the land in carting materials over the same.’ The minutes of the meeting on the 13 February 1823 give the estimated cost of erecting the obelisk as fifty pounds. However, when the final bill was presented, it came to one hundred and seventy six pounds, fifteen shillings and two pence (ref: Royal Museums Greenwich ADM/BP/45). The first recorded observation of it was made on 28 March 1824. In 1923, it was decided not to renew the lease and it was allowed to expire.

The temporary mark

A temporary mark was however erected first, and it seems likely that communications between Greenwich and Chingford would have been carried out using a system of flags in a manner perhaps not dissimilar to that described by Revd Weld in his account of placing meridian marks for the Stonyhurst transit circle in 1851. A temporary mark was clearly in place by the summer of 1822 as it was selected as one of the stations in a re-determination by Captain Kater of the difference in the longitudes of the Greenwich and Paris observatories by Trigonometrical means. In the autumn of that year, the mark was removed by Kater, and ‘a staff was put up in its place, having a triangular board fastened to it, the base of which was parallel to the horizon, and the vertex coinciding with the staff … The angular distance from the meridian to the staff was then measured by means of the micrometer of the transit instrument [at Greenwich] … By means of the roughly computed distance from the Royal Observatory to Chingford, and its angle with the meridian, the distance of the station from the meridian of Greenwich was found to be 20 inches’. Kater later positioned an Argand's lamp on the Meridian and asked Pond to observe it from back in Greenwich to confirm the accuracy of its placement. This observation, which is recorded in Greenwich Observations, was made on 15 November 1822.

Colonel Colby had intended to use Chingford as one of the stations of the Trigonometrical Survey of Great Britain. Accordingly, the theodolite belonging to the Ordnance was placed at Chingford between 21 July and 28 August 1823, and with it, the angles were obtained that had not been measured the preceding autumn, before work stopped for the winter.

Kater later marked the site with a piece of Portland stone about one foot square and four or five feet long that he had procured for the purpose. It was sunk endways until level with the ground and had the word ‘station’ along with the year cut into it. Kater gave the latitude of the Chingford station as 51°38’ 9.59” and its distance from the centre of the transit instrument as 57,847.66 feet. The station was used again between 19 April and 29 May 1848, at which time, its locations was recorded as: ‘13.54 feet from the centre of the Greenwich north meridian mark, 9.67 feet from the south-west corner, and 10.12 feet from the south-east corner of the base of the obelisk;’ and as being marked by ‘a round hole bored in a square stone’. Whether or not this stone still survives has not been established.

Apart from the reference to the Kater’s lamp, there are just two other entries regarding the temporary mark in the published observations. The first is dated 12 October 1823 when Pond recorded ‘The temporary Signal at Chingford was this day remarkably well defined, and appeared about its own diameter, or the thickness of the transit wire apparently to the eastward.’ The second was about four weeks later on 9 November.

Airy, who succeeded Pond in 1835 was disinclined to make use of meridian marks. In his introduction to the 1836 volume of Greenwich Observations, he tells us that from 23 January of that year: ‘The use of the meridian marks at Blackwall and Chingford was given up: the former being too broad for the new fine wires, and being frequently hidden by the rigging of ships in the river; and the later being visible only in the finest weather’. Airy went on to explain how the azimuth error was to be determined instead.

The accuracy of the mark

Although the use of the Chingford mark had been discontinued, it was still observed until 1843 when atmospheric conditions were particularly favourable – not to determine the azimuth errors of the telescope but the errors of the mark itself. Five observations were made in total. Following the first on 24 August 1836, Airy concluded ‘…it appears that the mark, probably, is within a second [of arc] of its proper position: which is as near, I apprehend, as the thickness of the old wires and the uncertainty of horizontal refraction would allow of placing it’ – an error equivalent to less than 3½ inches on the ground. An observation made in particularly favourable circumstances on 6 April 1842, lead him to conclude that it was just 0.02 seconds (of arc) west of the meridian.

In 1839, in his report to the Board of Visitors, Airy wrote: ‘The board are aware that a meridian mark (a pillar of granite) was erected under the direction of Mr. Pond in the year 1824, at Chingford in Essex, upon one of the hills projecting from Epping Forrest, on a piece of ground leased to the Board of Admiralty for ninety-nine years from that time. Although this mark is not used by me in the same manner as by Mr. Pond, in the occasional adjustments of the Transit, yet I am fully aware of the advantage of keeping it in repair and sometimes observing it, for the purpose (if there were no other) of noting the direction of a line of several miles in length, which in 1824 was as nearly in the meridian as it could be placed by the best instruments in the world. Some small steps have therefore been taken by me for the preservation of the fence, &c.; and I propose to regard it as an object whose careful conservation is well worthy of attention.’

In 1869, Airy reported that the Obelisk was ‘now backed by [the] trees of Epping Forest, and, when it is to be used, it will be necessary to attach to it a lofty pole.’ The following year, he reported that he had since discovered that the trees belonged to Richard Hodgson, late secretary of the Royal Astronomical Society, who had given a spontaneous ‘undertaking that the trees shall be cleared away whenever it shall be necessary, to the extent that may be required to make the Obelisk visible’. A Francis Frith photo from 1911, which was also issued as a postcard (see images above) shows the pillar not only enclosed by metal railings but also surmounted by an arrow or vane like structure, which was presumably added by the Ordnance Survey. The railings were subsequently removed and were certainly missing by 1934.

The arrival of the first plaque

In 1935 Spencer Jones reported to the Board of Visitors that: ‘It [the pillar] is now an object of curiosity to visitors to the site. The pillar is on land vested in the Epping Forest Commissioners (City of London Corporation). The Corporation has agreed to fix an inscription to the pillar, explaining the purpose for which it was erected and used. The Greenwich meridian, as changed in 1850 and adopted by international agreement in 1884 as line of zero longitude, passes 19 feet to the east of the pillar.’ The erection of the plaque coincided with the start of the retriangulation of Great Britain for which the nearby trig point was erected. It was probably a no lesser authority than the Astronomer Royal himself who started the myth that Greenwich Meridian was actually adopted (rather than adopted in principle) as the Prime Meridian in 1884 – a result no doubt of the need for brevity not only on this plaque, but a similar one back at Greenwich that was erected at about the same time.

The final observations

The mark was brought briefly back into use in 1953 when triangulations were being carried out to fix the position of the new instruments at Herstmonceux. Observations were made with the Airy Transit Circle during the periods 8 June to 7 August, and 11 September to 9 October. To facilitate this, the Admiralty maintained a beacon lamp placed 0.122 m east of the vane in the centre of the obelisk. Both the lamp and the remains of the vane are visible in the 1953 image set, where they have been artificially coloured to make them more visible.

Other plaques and inscriptions

The York stone paving surrounding the base of the obelisk carries a number of inscriptions, dating back to 1824. They are easiest to see when lit by the Sun when it is low in the sky (as in the October 2007 image set). The plaque commemorating the writer TE Lawrence (Lawrence of Arabia) was added in 2008.

Listing

The obelisk is a listed structure (grade 2). Listing date: 24 February 1987.

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