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.

When Halley’s 5-foot 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. In the preface to his first volume of Greenwich Observations (published in 1776), Maskelyne describes the marks as he found them when he arrived at the Observatory in 1765, along with his subsequent alterations.

There are two marks to the south, for adjusting the transit instrument to the meridian. They are formed of circular holes made in iron plates, through which the sky appears. These plates are fixed by short iron rods, to the side of a chimney of a well-built brick house, at the distance of about a quarter of a mile from the Observatory; and the light of the sky coming through the hole, to which the telescope is adjusted, is almost hid by the middle vertical wire, but so that an equal quantity of light appears on either side of it. The mark furthest from the chimney, which appears the eastermost, but is really the westermost, is the true meridian mark. From May 7 to June 14th, 1765, the transit instrument was adjusted to this; but, on June 14th, being uncertain which was the right one, and being inclined, from some observations of the polar star, to think the other so, I adjusted the instrument to that, and kept it there till July 13th; when, having found from comparing all the observations of the transits of the polar star, and other circumpolar stars, both above and below the pole, as well those made with the instrument adjusted to the eastern as western mark, making a proper allowance for the horizontal interval of the marks, which I found by a micrometer to be 7” 6/10 , that the westermost mark or that farthest from the chimney was the right one, and truly in the meridian, I re-adjusted the instrument to it. The other mark seems to have been put up before the meridian was accurately found.

The chimney on which the south marks were mounted is on the south-east side of the Ranger’s House. In the first image of the 2007 image set, based on Maskelyne’s description, the mark would seem to have been fixed to the central chimney and located in the gap between it and the left hand (more westerly) one. Built in 1699, at the time when Maskelyne was writing it was occupied by a Mr Hulse, having been previously been lived in from 1749 until his death in 1773 by the fourth Earl of Chesterfield. Although Maskelyne did not record the dimensions of the marks in the preface, he did in one of his notebooks: ‘They are formed by two circular holes of 62/100 inch in diameter made on 2 square iron plates of 3.3 inches diameter fastened to the wall by a short iron rod’.

It is apparent from Bradley’s published observations that a south mark though not necessarily a north mark was used from the earliest of days. The first published observation with the telescope is dated 2 September 1750. The first reference to a mark occurs a few days later on 8 September: ‘The observations of the Pole Star shew that the mark on Lord Chesterfield’s House, by which the Transit Instrument is set, lies about 5” in Azimuth to the East of the Meridian’. On 17 September, Bradley adjusted the mark and at the same time gave a brief description:

I moved the Tin-Plate at Lord Chesterfield’s ½ an Inch more Westerly, which corresponds to 6”; so that I now judge that it lies very nearly in the Meridian, both the Sun and the Circumpolar Stars shewing that it was before about 6” to much to the East. NB. The Diameter of the Hole in the Tin-Plate is 5/8 of an Inch, and the Breadth of the Plate 3¼ Inch, which I found subtended an Angle of 39”; therefore the Diameter of the Hole corresponds to 7”½; and the Distance of the Mark from the Observatory is 2,16 furlongs, or 1425 Feet.

At no point in the published observations of either Bradley or his immediate successor Bliss is there a reference to the existence of the second plate that Maskelyne mentions. Although the estimates by Bradley and Maskelyne of the hole’s diameter are in close agreement, (to within five-thousandths of an inch), their estimates of its distance from the transit instrument differ by more than 10% – Maskelyne’s estimate for a replacement mark on the same chimney being 1606 feet (and comparable to a modern estimate).

In March 1769, Jean Bernoulli III, Astronomer Royal to Fredrick the Great of Prussia visited Maskelyne. He subsequently wrote: ‘A quarter of a mile away is an iron bar fixed to a chimney; this carries a plate with a hole in it and this is a mark made with great care to verify the direction of the telescope; it remains to be seen if one can count on the solidity of the chimney’. It seems the chimney was solid enough, but not so the fixings. On 15 December 1755, ‘The South-Mark was displaced by the high wind, and afterwards refixed’. On 20 October 1781, it became loose again. Maskelyne recorded: ‘The old meridian mark having got loose from the Chimney to which it was fastened, a new one and much firmer was put up, and fastened to the two Chimneys on each side, and put as near as could be in the same Position as the former’. Exactly how the mark was supported is not stated, nor whether it was perpendicular to the meridian. Nor is it clear if the new mark was still in the form of a circular hole pierced through a metal plate, for on 29 May 1797, Maskelyne recorded: ‘ I caused a slip of white paper to be pasted on the plate of the meridian mark, parallel to the upright aperture of the mark and in the direction of the vertical wire when the axis of the instrument is reversed’. The mark certainly appears to have been altered at some point, for when observing with the successor instrument (The Troughton 10-foot Transit) on 25 September 1816, Pond recorded: ‘The central Wire appears now a Tangent to the luminous Parallelogram which forms the South Meridian Mark’. Prior to this, on 20 March 1812, Pond described ‘the Meridian Mark’, (not specifying if it was the north or the south one) as consisting of ‘an oblong perforation in an iron plate’, the diameter of its luminous image being ‘estimated at 4”.0.’ This would correspond with a slit width of about 0.37 inches at the distance of the south mark. The mark was apparently adjustable and with some precision, for on 14 May 1805, following an improvement to the transit instrument’s pivots (in 1803), Maskelyne ‘moved the meridian mark 0.64 inch to the westward, which answers to the angle 6”,81, at the distance of 1606 feet, which is that of the mark’. On 28 May, it was adjusted again; this time by moving it 0.41 inches eastwards.