This astronomical quadrant was used by Edmund Halley to set his Prime Meridian Line in 1725. It is preserved today and is on public display at the Royal Observatory in Greenwich.
A plaque outside near the current Prime Meridian explains this instrument, and how Halley used it while he was Astronomer Royal at Greenwich as follows:
"HALLEY'S MERIDIAN LINE
Edmund Halley, the second Astronomer Royal, measured the height of stars above the horizon in time to their passage (or ‘transit’) over the meridian marked by the crosshairs in his telescope with an accurate pendulum clock. Astronomers opened hatches and the roof to observe the stars, with one of the most important observations being the Sun at solar noon.
Edmund Halley's meridian line of 1725 is shown on the wall to your right.
[drawing]
Halley used a large-scale mural quadrant like this (inside the building ahead of you) to provide accurate transit measurements."
From the Royal Observatory website: (
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Telescope: Halley’s 8-foot Iron Mural Quadrant (1725)
Mounted on a wall in the plane of the meridian, this instrument consisted of a graduated quadrant fitted with a telescopic sight. Used in conjunction with an accurate pendulum clock, it allowed its user to find right ascension and declination of a heavenly body by measuring its zenith distance as it crossed the meridian together with the sidereal time at which this happened. In later years it was used to measure zenith distance only, a transit instrument being used to make simultaneous measurements of the right ascension.
Commissioning of the instrument
When Halley arrived as Flamsteed’s successor as Astronomer Royal, he found the Observatory devoid of any instruments. The Quadrant was commissioned as a replacement for Flamsteed’s Mural Arc which had been removed together with all the other instruments, by Margaret Flamsteed when she left the Observatory in early 1721 following the death of her husband.
After an unsuccessful attempt by the Government’s law officers to recover the instruments, Halley petitioned the King to bestow upon him a sum of money sufficient to re-equip it. He was awarded £500 of which £61.10s (plus 2.5% duty and other fees) was spent on a transit instrument. Halley also spent a small sum re-equipping the Great Room (Octagon Room). The rest was destined for two mural quadrants which Halley commissioned from the celebrated London instrument maker George Graham. His plan was to have them mounted facing opposite directions on the two sides of a newly built freestanding wall, thereby enabling a view of the whole sky to be obtained. In the event, the money ran out and only one instrument was completed. It was mounted on the east side of the wall looking south.
Rather than re-use Flamsteed’s meridian wall which was known to be subsiding, Halley had a new freestanding wall erected immediately to its north and fractionally to its east (about 1.85m). Made from eleven large pieces of stone (a foundation slab with five rows of two stones on top), it was positioned in the midle of a newly erected building. Although there is no known plan or detailed description of the building, it is believed to have been either square or rectangular in shape with its north-eastern corner breaking though the Observatory’s eastern boundary.
The instrument’s construction
The Mural Quadrant consisted of an 8-foot radius calibrated brass arc supported by an iron framework made by Jonathan Sisson, the heavenly bodies being observed through a braced and counterbalanced radially mounted telescope. As with Flamsteed’s Mural Arc, the scale divisions were marked out at the Observatory itself. However, whilst those of the Mural Arc were marked with the instrument in situ, those of the Quadrant were marked out with the instrument lying on the floor of the Great Room. This part of the operation was carried out in an innovative way by Graham using a beam compass. Unlike any previous quadrant, this one was marked with two independent concentric sets of scale divisions. The inner scale was divided into 90 divisions, each of one degree, which were then subdivided into 12 equal parts of five minutes of arc. The outer scale was divided into 96 equal parts, each of which was also subdivided. With the help of a vernier and later a micrometer, both could be read to the nearest second of arc. The reason for having the second scale was that it was likely to be more accurate as unlike the degree scale, it could be marked off by simple a process of multiple bisection. Further subdivisions of the two scales were read off with a vernier. In use, both scales were read and recorded with the outer scale reading then being converted into degrees by simple arithmetic. Click here to see some examples as recorded by Maskelyne in 1774 (note: some of the observations were made with the later Bradley Quadrant; those made with the Halley Quadrant are marked NQ. or N.Q. in the widest column.
The accompanying clock was month going and also made by Graham. Known as Graham 2 since Airy’s time, it remained in the Quadrant Room, most probabaly on the south wall, until the first part of the nineteenth century (1834?) when it was moved to the Circle Room. . . ."