The production of this issue received great support from the Cahiers editorial team. We are thankful for their guidance and patience shown to us throughout the long process. At the beginning of the project, we also received enthusiastic responses from many potential contributors. One of these individuals was Paolo Brenni. When we told him about our project, Paolo showed his usual enthusiasm, suggested some leads, given some advice, proposed the names of some colleagues and especially had promised us an article... We dedicate this work to him.

1.  What Was a Meridian Circle?

2.  A Brief History of Meridian Circles

4Like the designs of many other scientific instruments, the designs of meridian circles underwent changes over time. Improving the various parts of the instrument made it more and more useful for astronomers, gradually leading to being considered the most accurate scientific instrument of its time. Such developments included improvements made to the graduation of circles (Chapman, 1983; Bennett, 1987), micrometers (Brooks, 1991), optical lenses (Jackson, 2000; Jahn, 2008), the illumination system (Gressot & Jeanneret, 2022), better materials for the construction of telescope tubes, more stable pillars, and new developments within the theory of errors applied to calculations (Canales, 2001; Adler, 2003). The elite makers of meridian circles were mainly based in Europe during the nineteenth century. Instruments made by Repsold and Reichenbach were among the first meridian circles set up for regular use at observatories at Gottingen and Konigsberg. They developed a distinct style for the instrument and championed keeping the sizes of the instruments relatively small. Ertel & Sohn were trained by Reichenbach and built upon their knowledge to continue the development of the instruments. Pistor & Martins furnished the Berlin Observatory with their own design of meridian circles. Wilhelm Eichens (1818-1884) and Marc Secretan (1804-1867) designed instruments for the Paris Observatory. Paul Gautier (1842-1909) had an international pool of clients and built instruments for observatories spanning from Brazil to Japan. Troughton & Simms emerged as the main instrument makers for observatories within the British empire. Their reputation was solidified by taking part in the decades-long re-instrumentation of the Royal Observatory, Greenwich during the first half of the nineteenth century.

3.  The Place of Meridian Circles within the History of Science

5Although scientific instruments became objects of analysis for historians of science during the second half of the 20th century, meridian circles remained relatively underrepresented. Most often, they appear in general histories of observatories, telescopes, and graduated circles, as well as in inventories of astronomical instruments (King, 1979; Howse, 1986; Bennett, 1987). These histories were often written by curators exploring the histories of the collections of their institutions (Alberti, 2019). More detailed and isolated histories of the instrument were written by astronomers themselves. E. G. Martin (1949) offered a brief overview of the history of meridian circles in 1949, induced by his curiosity about the variations in the size of their apertures. Although it makes the intriguing claim that ‘there are too many transit circles in the world’, he explains this by the instrument being the foundational element of any observatory. His article finished with a useful (though relatively inaccurate) list of meridian circles. The next notable published work was R. H. Tucker’s (1969) examination of observing programs with meridian circles at various observatories. Although it was prefaced with a brief history, its survey of various meridian circles and their associated research programs around the world remains a valuable source of information. Erik Høg’s (1974) paper is similarly notable for the technical assessment of the automatisation of meridian circles during the middle of the 20th century.

6During the 1980s historical studies began placing more emphasis on the materiality of objects as a document and a source that can provide a better understanding of practices (Hicks, 2010; Bernasconi, 2016). Despite this, meridian circles remained relatively absent from such studies. A renewed interest in meridian circles only emerged during the end of the 20th century. The first detailed historical examination of meridian circles was carried out by Klaus-Dieter Herbst, which traced the history of meridian circles from 1700 to 1850 (Herbst, 1996). It placed as one of its questions why it took more than a century for meridian circles to replace its predecessors. Incorporating questions like those asked by proponents of the material turn, the detailed study focused on the dynamics between instrument makers and their clients, as well as the availability of techniques and raw materials. Turning from overall histories to individual case studies, Gilbert Satterthwaite (2001) published an extensive overview of the history of the Airy Transit Circle. Claiming to be the last person to observe with that instrument, the work intertwined the tacit knowledge of an observer with historical technical reports about the instrument. Inventory operations have been carried out, particularly in France, to map meridian circles and to raise awareness of the importance of these artefacts. The work of Davoigneau & Le Guet Tully (2005) demonstrated the widespread presence of such instruments around France, their significant contributions to the history of the country and science, and prepared the grounds for more detailed historical analysis of individual instruments. The creation of this inventory was followed by the examination of individual histories of meridian circles. Poppi & al. (2008) provided an overview of German meridian circles at the Bologna Observatory. Granato (2008) examined the history of a Gautier meridian circle used in Brazil as part of the instrument’s restoration. Bonifacio & al. (2009) examined the dissemination of new meridian circle designs by the instrument makers Troughton & Simms through the instrument at the Coimbra Astronomical Observatory. Following the focus on international exchange, Chinnici (2009) explored the connections between meridian circles at the Dunsink and Palermo. The meridian circle at Dunsink Observatory came under further investigation through an examination of the history of its object lens by Meurs (2010). These works provided the fundamental groundwork for another wave of studies that increasingly responded to the detailed exploration of interplay between science and society. Sanhueza-Cerda & Valderama (2020) explored the key role of international communications in establishing the scientific credibility and technical stability of meridian circles. Gressot & Jeanneret (2022) showcased the reliance on the meridian circle at the Neuchatel Observatory in the development of the history of the institution and the region. Berard & al. (2020) applied digital tools to create a digital version of the Toulouse-Jolimont meridian circle and to remodel it within its surrounding environment. Belteki (2020a; 2020b) examined the negotiations over the purchase of the objects glass for the Airy Transit Circle, and explored the story of a model made of the same instrument.

7Surveying the elite instrument makers of meridian circles reveals their concentration within European empires. Their close connection to maintaining the material culture of observatories shows how instruments like meridian circles were inseparable from imperial observatories. As Schaffer (2012) has highlighted, the manufacturing and the major alterations of instruments remained within the hands of these central instrument makers, even if they were installed at colonial observatories. This special issue provides further insights into how the dynamics between eminent instrument makers and major observatories operated. Meanwhile, as Sen (2016) also remarked, the minor alterations and the routine maintenance of scientific instruments remained in the hands of local artisans. Their stories have largely been neglected from historical accounts of meridian circles, and it is a potential avenue to explore for future researchers. Such accounts would help with reversing the centralised and Eurocentric history of scientific instruments. Although this special issue does not engage with such questions, its further focus on the eminent instrument makers reveals the gaps within the historical scholarship that decentralised approaches can fill.

8Meridian circles supplied fundamental data about the position of astronomical objects, which helped in the construction of tables predicting their future positions. Such information was incorporated into nautical almanacs to aid navigation. This close connection reveals the ties of observatories to naval departments and institutions (e.g. US Naval Observatory, Royal Observatory at Greenwich etc.). Through the example of navigation we see that meridian circles connected the sky with the earth. The same observations helped with determining the exact geographical position of the meridian circles themselves. Given the precision attained with these instruments, they were the essential starting points of surveying and cartographic ventures. Projects such as the Ordnance Survey and the French triangulation maps used the positions of meridian circles as key reference points for their work. To ensure the accuracy of the positions of observatories, the advent of telegraphy allowed for the remeasurement of their longitudes. This prompted several observatories to carry out observations while being connected via a telegraphic network. Keeping our focus on the ground, with the increased precision of meridian circles during the 20th century, auxiliary instruments like seismographs also played an important role to account for the effects produced by seismographic activity that could jeopardise the precision of scientific instruments. Even though the instruments were gazing at the sky, they remained grounded on Earth. Among the public applications of meridian circles, the most often emphasised has been the production of time. Yet, the role of the instrument is rarely highlighted in histories of time distribution. The previously mentioned repeated measurement of the motion of astronomical bodies served as the basis for determining time, which was later disseminated across regions. Accurate time was important for the maintenance of commerce, trade, and transport at increasing speeds. This same regular re-determination (or keeping) of time also allowed for the more reliable testing of the quality of chronometers. As a result, sites like the Royal Observatory, Greenwich carried out regular trials for evaluating the performance of the devices. Thereby, meridian circles were positioned at the center of the key tenets of public life.

4.  Themes Explored in this Special Issue

9A key overarching theme emerging from the historiography related to meridian circles is the “boundaries” of the instrument. In a literal sense, what were the material boundaries of the instrument? Was it confined to its internal parts or did its auxiliary parts (e.g. meridian marks, chronographs etc.) also form part of the instrument? This inclusion and exclusion raises questions about the historical expertise applied. For instance, if we include the building as part of the instrument, one has to rely on architectural expertise (Tirado & Tirado, 2019). In a more conceptual sense, if meridian circles produced time and space for regions and empires, then did these temporal and geographic zones also act as part of its boundary? To tackle this question, we asked contributors to this special issue to deploy the term assemblage (DeLanda, 2019; Buchanan, 2021). Although as Buchanan (2015) highlighted, different scholars tend to deploy the term differently, which can lead to different approaches, in the case of meridian circles it is a useful conceptual tool to highlight 1) the significance of the multiple parts of the instrument, 2) that the state of each component actively affected the observation, and thereby, 3) the products of a meridian circle were assembled by its parts.

10A key advantage of using the term assemblage is the emphasis placed on interactions between the different parts of the assemblage. When limiting the scope of the analysis to the material components of the instruments, the approach demonstrates that the components are in a constant state of flux (Baker, 2012), they undergo constant repair and maintenance (Schaffer, 2011; Margocsy, 2017; Werrett, 2019), and are regularly adjusted or modified for the needs of their users. When focusing on the interactions between material components of an instrument, the hierarchies of significance among the material components also seemingly disappear. This is especially heightened in the cases of precision instruments (like meridian circles), where the effect of each material component can have a measurable effect on the observation. Studies focusing on a singular component of an instrument provide new information on how a type of component can affect the observation (Chapman, 1983; Brooks, 1991). Thereby, using the term assemblage not only positions a component among multiple others, but also liberates its significance as a potential object (or subject) of analysis. This approach makes it also possible to see that the data produced by the meridian circle is at the centre of different networks (instrumental, actor, spatial), which must be investigated in order to understand how precision is forged, what it means and when it is reached or exceeded.

11This focus on the active shaping of the instrument allows for enlisting as part of the assemblage seemingly unexpected entities. Coupled with the different ways in which contributors deploy the term assemblage showcases the various ways in which historians attempt to reassemble the material and historical context within which meridian circles operated (Latour, 2005). For example, Issenmann explores how the layout of the urban environment formed part of this larger assemblage. Belteki highlights how the air surrounding the instrument was monitored and brought into the assemblage of the instrument. Gressot & Jeanneret describe the various components of the instrument distributed around an observatory that made up the larger assemblage to produce, keep and distribute time which is built in close relationship with the instrument maker. Bell considers how meridian circles across North America created an assemblage for astronomical work. The deployment of the term assemblage in this way becomes a powerful tool to analyse the physical boundaries of the instrument.

12This special issue also brings together a variety of expertise from curators, historians, and astronomers. What emerges from these accounts is that meridian circles have a variety of meanings to different individuals. For Wolfschmidt, the history of the instruments embodies the dynamics between the instrument makers and the astronomers. In Leppik’s paper, the meridian circle becomes the central point around which the life of an astronomer can be explored. Meanwhile, in the articles of Schmidt & Dick and Jeanson & Davoigneau meridian circles become symbols for the histories of astronomical institutions. Exploring the idea of symbolic instruments further, Aubin argues that meridian circles were not only symbols for observatories. Instead, meridian circles were deployed as rhetorical and cultural symbols at different locations and at different historical periods. In the papers by Lomb and by Soulu, meridian circles did not only define the history of an institution, but also that of larger geographical regions. The variety of these different starting points demonstrate that if meridian circles were the foundations for producing time and space for society, then exploring their histories possesses the ability to re-assemble the histories of the regions and entities that were defined by those temporal and spatial regimes.

13It is also important to emphasise the gaps in this special issue and the ways in which we consider those gaps to be the necessary next steps for examining the histories of meridian circles. Although our initial aim was to tell a global history of the instruments, the final selection of articles remained Western in its focus. By combining the information presented in this issue with more recent research about the dynamics between empires and their colonies, it would be possible in the future to further explore the role of meridian circles in imperial ambitions and practices. The special issue also remains relatively silent about interactions between observatories. Astronomers discussed between each other how to coordinate their research, the latest astronomical publications, and the quality of the latest instruments. As the historical scholarship also highlighted, a similar discussion between astronomers and instruments makers were essential for the eventual uptake of meridian circles. Thereby, future research should examine such interactions between the individuals that surrounded meridian circles. In relation to the human component, there is no examination of the gendered nature of work with meridian circles. Were women trained to use these instruments, or were they trained on other types of astronomical instruments? Finally, there is little discussion in these papers about the role of funding institutions and scientific societies in relation to the history of meridian circles. How did funding bodies see meridian circles to contribute to their aims? And what was the role of scientific societies in the emergence of meridian circles as an essential astronomical instrument? To answer any of these questions, one must better understand the collective history of meridian circles. It was the aim of the editors of this special issue to bring together through these papers such a collective history of the astronomical instrument. As a result, we consider the gaps and questions mentioned above as our future steps for exploration.

5.  Meridian Circles Today

14Before concluding this introduction, it is worth returning to the question of what happened to meridian circles during the 20th century and where they are now. During the 19th century timekeeping devices gradually attained better and better precision. This put increased pressure on the precision attained with meridian circles too. In addition, the size and shape of the instrument imposes mechanical constraints, making the determination of errors more complicated as the level of precision sought increases. Despite the attempts to develop accessories and auxiliary instruments to improve the precisions attained (e.g. the impersonal micrometer), meridian circles were unable to keep up with the precision demanded by the scientific community. In addition, new types of astronomical instrument, like the Photographic Zenith Tube (PZT) and the Danjon astrolabe were devised, which attained higher precision than meridian circles did. These new instruments improved the precision by a factor of ten, up to a thousandth of a second. By 1947 the International Astronomical Union considered the PZT to be the future of practical astronomy (Gressot, 2023). Although meridian circles were gradually abandoned, it was the paradigmatic shift followed by the introduction of atomic time, which made the astronomical time determination obsolete. Therefore, meridian circles today are more likely to be used for astronomical observations (e.g. for mapping the sky) rather than for time determinations.