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. When both a northern and a southern mark were available, they allowed errors in both the direction in which the telescope was pointing and the alignment of the optics within the tube (errors in collimation) to be detected.

When Halley’s Transit Instrument was retired by Bradley in 1750, it was replaced with an 8-foot Transit Instrument by John Bird in a new building about 120 feet to the east. In continuous use until 1816, it was used with a variety of Meridian marks to both the north and the south. None survive.

Maskelyne’s collimation problems

When Maskelyne became Astronomer Royal in 1765, the north and south marks were both attached to distant buildings. Having to negotiate access and never having the marks under his full control would not only have been inconvenient, but could also lead to unwitting errors. The truth of this was brought home to Maskelyne in November 1768, when he discovered a collimation error of around 30” of arc on reversing the telescope during an observation of Polaris. By comparing the recorded transits of the high and the low stars prior to this time, he was able to establish that the collimation problem originally arose in late August. Reporting the whole episode in the published observations, Maskelyne observed: ‘This error might have been discovered sooner if the North Mark had not been lately removed, on repairing the House on which it was fixed.’ Maskelyne then added: ‘This Mark was later replaced’. References by Maskelyne to collimation checks are few and far between occurring when they do, mainly in the summer months when the days are longer. Maskelyne records two later occasions when large collimation errors (in excess of 7” of arc) occurred. The first, he concluded had arisen on 21 July 1795 and was picked up a week later on 28 July. The second occurred later that year on 25 November, but remained undetected until 29 May 1797. For the error to have remained undetected for so long, suggests both that, Maskelyne no longer had a north mark at this time, and that he had not learned the lesson from 1768 either, by developing a methodology for reducing and analysing his observations more quickly.

The enclosure of the Observatory Courtyard

In 1785, the American astronomer David Rittenhouse described how he had set up a near meridian mark after his view to a more distant one was blocked by a new building. Although he referred to it as ‘a new invention’, the only difference between it and Maskelyne’s existing mark(s) which like that of Rittenhouse, were observed with a supplementary object glass (OG) was its proximity and the method of mounting the OGs. Maskelyne would almost certainly have been aware of Rittenhouse’s mark, but at that time (even if he had been so minded), would have been unable to set one up of his own, as there was no suitable location – the present courtyard having not yet been enclosed. Plans to enclose the courtyard were put into action by Maskelyne while his extension to Flamsteed House was under construction. Known originally as the Front Court, he was given the go-ahead to enclose it in early 1791 (WORK16/126). The north-east corner where the tree now stands consists of made-up ground. The new boundary railings appear to have been continued directly in front of the telescope. Since they would have either blocked or interfered with the view to the north mark on Crowley house, one can only conclude that by then either the mark no longer existed, or that it was no longer in use, or that it was deemed expendable.

The erection of a near mark

The collimation problem detected by Maskelyne in May 1797 appears to have been the trigger for a setting up a near mark in the courtyard later that year. Although there is no reference to the mark in the published observations, its arrival on 13 October was recorded by Maskelyne in one of his Memorandum Books: ‘Set up a hewn stone in the court-yard with a moveable mark for examining line of collimation of transit instrument by inversion It is at the distance of [unrecorded] feet from the end of the tube of transit instrument, to which an object glass of that focal length is applied when this mark is used to render it distinct’. The reason why Maskelyne did not insert the distance figure can only be speculated upon. The inventory of 13 July 1798 – the first to be taken after the Mark’s erection, gives the distance as 78 feet.

On firm foundations

Prior to the courtyard’s enclosure, the ground in front of what is now called the Meridian building fell away to the north-east, implying that much of the courtyard, including the area where the new mark stood consists of made up ground. A shifting of the mark by less than five-thousandths of an inch to the east or west would have been the equivalent of one second of arc when viewed from the telescope. Ensuring absolute stability would therefore have been paramount. The foundations would need to have been bedded firmly on the compacted gravels that make up this part of the hillside – something that may well have involved excavating to a depth of 12 feet or more. Given its proximity to the Observatory, the mark had the potential to be illuminated and observed during the night as well as during the day. That said, there is no evidence in the published observations that it was used in this way. Indeed, if anything, the evidence suggests the contrary, for on 15 October 1803, Maskelyne recorded that the telescope having been put back together after some major work to its axis, was not then ‘adjusted, it being too dark to see the meridian mark’.

How successful or stable the near mark was, is impossible to judge, as there are no references to it in the published observations of either Maskelyne or Pond. Although the base of the stone pier on which the mark stood has survived, the mark and supplementary object glass have not.

Who made the mark?

There are no known descriptions of the mark. Nor are there any signs on the surviving stonework as to how or at what height the mark might have been attached. Nor is it known who made it. After the death of Sisson in 1783 however, Edward Troughton was employed by Maskelyne to make repairs and alterations to the telescope. Troughton went on to make not only the mural circle and new transit instrument at Greenwich but also the transit instrument for Admiral Smyth’s new observatory at Bedford. The Bedford instrument, like Bradley’s, was used with a near meridian mark. Whether or not Troughton made the two marks can only be speculated upon, as can any similarities or differences between them. Both though were adjustable. The Bedford mark is described in Volume 2 of George Chambers’ A Handbook of Descriptive and Practical Astronomy, Volume 2, Instruments and Practical Astronomy (link below)

The method of mounting the Bedford OG was similar to that used by Rittenhouse, but different to that used at Greenwich, where ‘Maskelyne employed a cap with a lens of long focus, to slip over the object-glass of his transit …’. Although the OG was mounted differently, it is likely that the Greenwich mark was adjusted in a similar way to the Bedford one, even if the form of the mark was different. How potential issues arising from any variation in the alignment of the optical centres each time the Greenwich OG was applied remains at this point, a mater of speculation.

The alteration to the railings

When the new Troughton 10-foot Transit Instrument was brought into use on the same meridian by Pond in 1816, the courtyard railings immediately to the north of the stone block, if not actually blocking the view of his new north mark at Blackwall, would have appeared within the field of view and it is presumed that the small gate (now sealed) was inserted to prevent this from happening.

Epilogue

In January 1920, long after it had ceased to have any use supporting a meridian mark, a night-sky camera was installed on the stone pier. Its purpose was to record the amount of cloudiness at night as a supplement to that given by the sunshine recorder during the day. It remained in use until the end of the year when it was replaced ‘by another, giving a larger scale.’ The new instrument remained in use until 31 July 1952, when it was dismounted and taken to Herstmonceux, where ‘it was temporarily set up upon the roof of the solar building alongside the “Herstmonceux” recorder. Simultaneous exposures were obtained with both the instruments over the period September 25–26, 1952, to mid-January 1953.’ An analysis of the results showed that whilst there were certain differences between the two cameras, the overall agreement in the Polaris trails was good to about 1%. Further comparisons with the exposures taken when the instruments were separately at Greenwich and Herstmonceux lead Spencer-Jones the then astronomer Royal to the conclusion that the number of hours of ‘clear’ night sky was about 10% higher at Herstmonceux (the new home of the Observatory) than Greenwich.

The current sign

A sign was erected on top of the pier in March 2011 – partly to inform people of its purpose and partly to prevent them from climbing on it. Unfortunately, the manner of its securing has caused permanent damage to the stonework. Also unfortunate was the typo on one of the dates (1970 instead of 1750) and the unsubstantiated statement about the nature of the mark. The typo is visible in the March 2011 image set, but has been corrected by the time the September 2011 images were taken.