The Greenwich Meridian

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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.

This spot, marks the site of the south mark used with Halley’s 5-foot Transit Instrument – the first of the transit instruments to be used at Greenwich and the instrument that defined the Greenwich Meridian from 1721 until 1750. The first recorded observation with the telescope was made on 1 October 1721. In Halley’s time, the telescope appears only to have been used with a southern mark. In about October 1743, Bradley set up a northern mark (location unknown) on a chimney in Greenwich.

The information we have about the south mark comes from Halley’s successor Bradley who refers to it first on 1 September 1726:

I went to Greenwich; and Mr. Graham and I adjusted the meridian telescope at right angles to the axis. […] After the line of collimation was truly at right angles to the axis, we set [the] thread exactly to the mark made for a meridian on the park wall, near Admiral Hosier’s house, …’. (SP Rigaud)

When Bradley arrived at Greenwich as Astronomer Royal in June 1742, the telescope had been unused for over ten years. His first act was to add to extra cross-wires and have it balanced for the first time. He then set about checking the alignment.

… the line of collimation of the transit instrument was adjusted about July 24th, but it could not be properly directed till I had a clear view of the mark which Dr Halley had made on the park wall; the sight of which was intercepted by the boughs of trees which had grown up since Dr Halley had used this instrument: these being cut away about the end of July. 1742, I then set the instrument by his mark, which I at first supposed to be exactly in the meridian, but afterwards found that it lay 12” or 15” in Azimuth to the west of the true meridian.

On 29 January 1743, Bradley altered the position of the mark:

This afternoon I made a new mark on the park wall (for adjusting the Transit Instrument) one inch 13/16 more easterly than made by Dr Halley which I hitherto have made use of. The middle of his mark was 2 15/16 Inch from the east end of the brick upon which it was made and the middle of the present mark is 1 1/8 inch from the same end.

Bradley does not spell out what the new or old mark consisted of. If it was simply a groove cut into the brickwork, it is likely to have painted white in order to increase the contrast and make it more visible. In June 1744, about eight months after he had erected a north mark, Bradley concluded that the wall on which the south mark was mounted must have moved. The north marks he refers to having observed in 1726 where distinctive features on buildings rather than bespoke marks.

The instrument now when I look northward points very near the same marks as it did when Mr Graham and I adjusted the line of collimation in the year 1726; but as the southern mark has been made nearly two inches more easterly than it was before, it is certain one or other of the marks must have changed its position; and I judge, from the present circumstances, that ‘tis the mark on the park wall that has altered, for I find the marks that I made on a chimney in Greenwich do exactly correspond with the same distant objects with which I formerly compared them. The same consequence may be likewise gathered from the transits of Capella above and below the pole; for, when the instrument was adjusted to the northern mark on the chimney (as it was till after I new-rectified the line of collimation on June 6), the star was the same time in moving through each semicircle; but after this last rectification, the star was longer by 3” or 4”, in passing the western semicircle, than it should have been had the southern mark by which it had been rectified been in the true meridian. (Rigaud)

If Bradley was correct, and the wall was moving, then the south mark would become increasingly unreliable. In anticipation of this, he erected ‘a moveable mark on the park wall’. He referred to it for the first time on 1 July 1745. The wall has been rebuilt since Bradley’s time and none of the Marks survive.