Nevil Maskelyne & the Zenith Sector


A review of Constructing an Instrument: Nevil Maskelyne and the Zenith Sector, 1760-1774, by Nicky Reeves.

This study offers a biography of the zenith sector, a device that by the 1760s would become the most precise portable measuring instrument in the world. To earn such kudos required decades of negotiation, labor and performance. Indeed, another subtitle of Nicky Reeves’ dissertation might well be “performing precision.” To attain reliable — as in, repeatable in different places and times in the hands of different users — measurements required, as Reeves convincingly shows, several performances: instrument makers performed with disciplined hands both in their workshops and with the apparatus in the field; instrument users performed as they literally wrapped their bodies around the device in a sequence of finely choreographed moves; learned audiences witnessed these performances in the field and by performing praise legitimated the precision of the measurement; and the patrons of this expensive exercise, the Royal Society of London and Britain’s Board of Longitude, performed precision by setting goals for the enterprise. We might also say that Nicky Reeves’s study illustrates how scientific instruments become “black-boxed,” namely, how they move from being unruly objects on which one experiments to being trustworthy tools with which one experiments on other phenomena. As a study of performing precision and black boxing, Reeves’s dissertation contributes to a growing body of literature on the material culture of science.

The function of the zenith sector sounds deceptively simple. By means of a plumb suspended on a 6 to 12-foot wire, the device was supposed to point precisely to the center of the Earth and thereby to define “straight up” for its user. By supporting a telescope tube of similar length parallel to the wire, pivoted at its upper end so that it could swing several degrees from the vertical, the device allowed users to measure how far stars deviated from the vertical at a given instant (recall that all stars move incessantly across the sky at a rate of one degree every four minutes of time). The goal of the zenith sector was to measure these deviations to seconds of arc. Do the math: for a zenith sector 12 feet in length, a deviation of 1 arc second measures about 7 ten-thousandths of an inch at the bob. To perform precision on arc seconds, the zenith sector’s wire had to hang perfectly still and the telescope’s position had to be judged, by the user, against that wire to distances less than the thickness of the finest human hair!

And why worry about “straight up”? As Reeves shows, some of the most fundamental theories of eighteenth-century science made claims that could be judged only by knowing “straight up” very precisely. Newton’s theory of universal gravity predicted an oblate shape for the spinning Earth. Copernicus’s theory of heliocentrism predicted an annual parallax or slight wobble in the position of stars. The theory of the finite speed of light predicted that a telescope, on a moving Earth, must be tilted slightly to observe distant stars. And a spinning, oblate Earth, as was discovered with the zenith sector (this was not predicted), wobbles slightly in a movement known as nutation. Such big cosmological questions, all of which required exact measurements of “straight up,” prompted British astronomers and their patrons to develop the zenith sector.

Although his actors were mostly British, Reeves’s story plays out on a global stage. It begins in the 1670s with Robert Hooke (1635-1703), the famous experimentalist of the Royal Society, mounting a zenith telescope into the roof of his rooms in London. In the 1720s, Samuel Molyneux (1689-1728) and James Bradley (1693-1762), both Fellows of the Royal Society, mounted a similar device in the former’s home. In the 1730s, the French academician Pierre-Louis Moreau de Maupertuis (1698-1759) carried a portable sector to the Arctic Circle, seeking to measure shape of the Earth. But the central actor in Reeves’s story is Nevil Maskelyne (1732-1811), who in 1765 became Astronomer Royal at Greenwich. In 1761, Maskelyne took a 10-foot sector to St Helena, hoping to observe annual parallax. Instead, he found problems with the design and function of his instrument, and returned to London with ideas for fixing and improving the sector. Most of this dissertation deals Maskelyne’s efforts to perform precision after his return from St Helena. His sectors travel to British America as Mason and Dixon survey a boundary between Pennsylvania and Delaware; they move around Greenwich as Maskelyne seeks to legitimate the precision of other instruments in the Royal Observatory; and after achieving “black box” status they move to Scotland where Maskelyne confirms Newton’s claim that mountains or other irregularities in the Earth’s density can gravitationally pull a plumb away from the vertical.

Precision, of course, is another way of saying “good enough”. If the 1761 Venus transits yielded measurements of solar parallax ranging from 8.56 to 9.73 arc seconds, why did contemporaries judge these campaigns as not good enough? Or why might Maskelyne have worried that Mason’s and Dixon’s degree of latitude erred by as much as 640 feet (out of a total length of some 360,000 feet)? Was the quest for precision, we might ask, primarily a moral quest for its own sake, or were practical matters of theory testing, navigation or land surveying also at stake? Reeves’s dissertation opens up questions on how eighteenth-century actors felt about precision qua precision.

While some of these stories have been told by previous scholars (Derek Howse’s biography of Maskelyne, Edwin Danson on the Mason-Dixon line and on the mountain experiments, Eric Forbes on “mathematical cosmography”, Harry Woolf on the transits of Venus), Nicky Reeves’s dissertation tells a fascinating story of astronomical practice in eighteenth-century Britain. Reeves’s study is not just a tale of building and using instruments. Rather he shows that getting instruments to work reliably, getting them to travel, getting users’ bodies trained to dance gracefully around the instruments, and legitimating the enterprise by performing precision for selected audiences required a complex interplay of skilled hands, moral fortitude, rich patrons and dedicated negotiators (like Maskelyne). By giving agency to what philosopher Hans-Jörg Reinberger has called “experimental systems,” Reeves’s dissertation wonderfully illustrates the power and potential of a biographical approach to scientific instruments.

Richard L. Kremer
Department of History
Dartmouth College

Primary Sources

Manuscript minutes of the Board of Longitude, the Royal Astronomical Society and the Royal Society
Autograph journal of the astronomer Nevil Maskelyne
Philosophical transactions of the Royal Society of London
Transactions of the American Philosophical Society

Pierre-Louis Maupertuis, The figure of the earth, determined by observations … at the Polar Circle (London 1738)
John Bird, The method of dividing astronomical instruments (London 1767)

Dissertation Information

University of Cambridge. 2008. 239 pp. Primary Advisor: Simon Schaffer.

Image: Painting of Nevil Maskelyne, the British astronomer, dated 1836. Between pp. 20-21 of The Gallery of Portraits with Memoirs, Volume VI. Edward Scriven, 1775-1841, printer. Wikimedia Commons.

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