The many ends of the Airy Transit Circle

As outlined in the previous section, pinpointing the end of the life of the Airy Transit Circle depends on how we define its end. Focusing on instances of breakdowns and major repairs highlights the instances of temporary ends. There were very few of these occurrences, as the instrument underwent constant maintenance. The main maintenance of the instrument was undertaken every Monday, when the telescope tube was lifted and turned around on its East-West axis while the various moving parts of the instrument were lubricated. This task was so important that when John Green (the person responsible for carrying out the regular Monday maintenance) asked for a week’s leave, Airy requested that he start it after he had finished with the Monday maintenance and to return by next week as not to miss the essential task (Belteki Reference Belteki2019, 163). Similarly, when the astronomical assistant of the Royal Observatory at the Cape of Good Hope visited Greenwich, it was Green who was asked to demonstrate how to care for the instrument. Besides Green, the Chief/First Assistant of the Observatory was asked by Airy to provide monthly reports on the state of the instruments. Smaller problems with the instrument (e.g. making squeaking noises and requiring cleaning) were noted in these reports, though almost none of these minor problems were mentioned in Airy’s annual reports on the state of the Observatory (Belteki Reference Belteki2017). Green’s specialized task and the reports on the state of the instruments show that the maintenance of the Airy Transit Circle was considered an essential task, designed to avoid the instrument reaching a state of disrepair, and therefore its end.

This critical attention to maintenance as a crucial task did not, however, prevent the instrument from suffering occasional major breakdowns. For instance, in 1864 the chains holding up the East-West axis of the telescope tube broke, and operations with it had to be halted until repairs were carried out (Airy Reference Airy1865, 7). Similarly, when the wires in the eyepiece of the telescope broke, observations had to be discontinued until they were replaced.Footnote ¹ Such cases of repair practices demonstrate that the life of this instrument required maintenance by its users, without which the instrument’s life would have come to an end. In addition, such cases highlight the life of an instrument as characterized by a series of operational and non-operational states (Baker Reference Baker2012). This is important in highlighting that the life of an instrument does not end simply because it breaks. Instead, it continues in a non-operational state, in which its users decide whether to repair it or not.

It is important to note that the Airy Transit Circle was considered a precision instrument. As a result, it was used by constantly measuring its instrumental errors (Evans Reference Evans2012). This was due to the instrument responding to even the slightest of environmental changes and interactions with its users. Robert Main (the First Assistant of the Observatory from 1835 to 1860) highlighted this feature of meridian instruments by noting how even the breath of the observer could alter the measurements shown by an instrument (Main Reference Main1863, 23). The Transit Circle’s first official description highlighted similar aspects of the instrument. Almost a quarter of the entire description was devoted to an explanation of how the instrumental errors were measured, and how they were applied to observations (Airy Reference Airy1854, 17-23). Consequently, when the astronomical data was published in the Greenwich Observations, tables of instrumental errors were provided to help astronomers calculate the measured position of celestial bodies. In this light, the instrument was useful and reliable, but it was never perfect. Instead, as a precision instrument, its high quality rested on the extent to which its imperfections were made visible.

Such a design and approach towards working with astronomical instruments highlighted the hierarchy imposed by astronomers on instrument makers during the nineteenth century. The German astronomer Friedrich Bessel stated that “every instrument … is made twice, once in the workshop of the artisan, in brass and steel, and then again by the astronomer on paper, by means of the list of necessary corrections which he derives by his investigation” (Pannekoek 1961, 325). Similarly, John Herschel stated that “though we are entitled to look for wonders at the hands of scientific artists, we are not to expect miracles” (Herschel, Reference Herschel1849, 76-77). Instead, he emphasized that the instrumental errors can be overcome through the work of the astronomers who directed their attention “to the detection and compensation of [instrumental and observational] errors, either by annihilating, or by taking account of, and allowing for them” (ibid.). In relation to the end of the life of instruments, such quotes demonstrate that imperfections of instruments were not synonymous with their ends. Instead, the end of the credibility of a precision instrument arrived when its errors were identified as systematic, too large to account for, or when the material parts displaying the results were no longer useable.

It was precisely the credibility of the data produced by the Transit Circle that brought the instrument close to its end for the first time. Towards the end of the 1850s, Albert Marth began criticizing the processes that were implemented for the reduction of the observations made with the instrument (Marth Reference Marth1860; Belteki Reference Belteki2019, 244-253).Footnote ² By attacking the calculating processes, Marth was not questioning the materiality of the instrument, but rather the processes that governed the data it produced. In doing so, he attacked the Airy’s organization of the observing procedures and his new system of Observatory governance (Schaffer Reference Schaffer1988; Smith Reference Smith1991). Marth’s criticism also highlighted possible further sources of errors from which the instrument suffered, thereby attacking the credibility of the data produced with the instrument, and consequently the production process that defined space and time for the British Empire. Though Airy downplayed the importance of this criticism, it still prompted him to write a more than 30-page long, point by point refutation of Marth’s paper to the members of the Board of Visitors overseeing the work of the Observatory (an unprecedented occurrence under Airy’s directorship).Footnote ³ Despite facing criticisms of its materiality and use soon after its installation, the Transit Circle continued to be used and hailed as a reliable instrument by the astronomical community. However, “the Marthiad episode” (as it was labelled by Airy’s friend, William Henry Smyth) demonstrated that questioning the reliability of the instrument and the credibility of the data it produced had the potential to bring the instrument to its end.

We can also measure the end of the instrument by looking at the number of observations that were carried out with it. Fortunately, such data is available for historians of science, since each observation was published in the Greenwich Observations, and also summarized in the Annual Reports of the Astronomer Royal.Footnote ⁴ These numbers show that observations fell dramatically on three occasions: 1906, 1919, and 1942. The dip in the numbers in 1906 was due to two reasons. First, regular observations with the Transit Circle to create the Second Nine-year Catalogue of Stars, which previously had required additional observations on a daily basis, were completed. Part of this catalogue was intended to provide reference data for the larger international Carte du Ciel project that began in 1887. With the observations finished, the use of the instrument became less frequent. The second reason for the dip in the number of observations related to the materiality of the instrument. The Annual Report for 1906 notes that the object glass of the Transit Circle required repolishing that year, but, the optician in charge of the task being absent, regular observations were interrupted for two months (Christie 1906, K5-K9). In brief, the year 1906 is a useful example for demonstrating two of the reasons that can lead an instrument to come to a temporary end: either because its materiality requires alteration (e.g., repolishing of object glass) or because it is no longer required to be used for its previously intended purpose (e.g., observations for a catalogue finished).

The year 1919 highlighted a different reason for the large fall in the numbers. As a consequence of the war, the number of employees of the Observatory significantly decreased, which led to a reduction in the number of observations. In addition, the year witnessed exceptionally bad weather, which further limited the number of possible observations (Dyson Reference Dyson1922, F6). This example shows that the operations of an instrument can further depend on environmental forces (e.g. weather) and on the availability of qualified members of staff. Thereby, we observe that the readiness of the materiality of an instrument for use does not mean that it is being used. Even though the materiality of the Transit Circle was in good order, extra-material factors had the potential to bring about the end of its life.

With all these possible avenues for an instrument to reach the end of its life, we can now examine what happened when such problems accumulate over time. From the 1920s onward, systematic errors were gradually identified in the data produced by the Transit Circle, and the responsibility of making observations for time service was consequently transferred to another instrument in 1927 (Gething Reference Gething1954, 415). When this happened, the instrument lost one of the functions that had been assigned to it since 1851. In the following years, the quality of important parts of the instrument, such as the object glass and the divided circle, began to decay. By 1931, the object glass was badly stained, and considered to be too thin for further repolishing. This was one of the reasons that the Observatory sought the support of the Admiralty for funding a new transit circle to take over the work of the Airy Transit Circle (Dyson Reference Dyson1932, F16-F17). By 1936, a new transit circle (the Reversible Transit Circle) was installed at Greenwich, which gradually reduced the number of observations made with the Airy Transit Circle. Two years later, the poor state of the object glass of the Airy Transit Circle was once again highlighted, with the additional note on the impossibility of repolishing it further. The decaying state of the instrument’s divided circle, used for measuring the altitude of celestial bodies, was similarly noted. Years of regular cleaning had eroded the graduations on the divided circle’s surface, to the point where many of the divisions became unreadable. Even though the circle was re-graduated as part of a larger repair operation, the collective number of accumulated problems (i.e. systematic errors, stained and thin object glass, and the installation of a new transit circle) provided justification for abandoning the Transit Circle’s regular use (Spencer Jones Reference Spencer Jones1951, F38-F39). Yet, the same report stated that due to the instrument’s historical value and its close connection to the Prime Meridian, the instrument would remain at its present site (Ibid.). This decision ended the life of the instrument as a working scientific instrument, reframing it as a historical object. In other words, its significance was no longer derived from its potential contributions to future observation programs, but from what it had achieved throughout its history.

On 12 September 1940, observations with the Transit Circle came to an end (Spencer Jones Reference Spencer Jones1953, E3). Despite this, the ongoing war nevertheless had a major impact on its life. During a bombing of the Observatory, the building that housed the instrument was damaged and resulted in minor damage to the instrument itself. Fortunately, this did not lead to its destruction and the minor damage to the instrument was soon repaired. However, many other astronomical instruments around the world were destroyed in similar bombings. Among these were the large instruments of the Pulkovo Observatory, which, like Greenwich, devoted part of its research program to positional astronomy. When German troops began their advance towards Leningrad, the staff moved the optical parts of the telescopes and the smaller equipment to another location (Herbig Reference Herbig1945, 197).

Taking the necessary precautions proved to be a good preventive measure. During the siege of Leningrad, the German front lines were set up less than a mile away from the Pulkovo Observatory, which resulted in the heavy bombardment of the Observatory buildings and in the inevitable destruction of the instruments, mountings, and tools that remained there. Among these were the meridian instruments used for an ongoing observing program to create a catalogue of 2,957 geodetic stars (Mikhailov 1955, 31).

Due to the cessation of work at Pulkovo, it was decided to continue Pulkovo’s observation program with the participation of foreign observatories. Among these observatories was the Royal Observatory, Greenwich, which offered to revitalize its retired Airy Transit Circle. After the instrument was restored to an operational state, regular observations with it began again, and continued even after the war until 1954 (Spencer Jones Reference Spencer Jones1954, E3-E4).

This story of the Transit Circle highlights the fact that the end of a scientific instrument can occur multiple times, rather than being restricted to a single event. In this light, as long as the individuals responsible for the instrument are willing to continue with its maintenance, repair, and refurbishment, a scientific instrument can continue to avoid reaching its end. Moreover, the story shows that the instrument was considered to have reached the end of its life only in comparison to the other instruments available to the astronomical community. In this sense, the instrument itself did not end, but rather, its use was halted by the community. In addition, as highlighted by one of the Annual Reports of the Astronomer Royal, the instrument’s function was transformed from that of a scientific instrument to that of a historical object. According to such an approach, while the life of the Transit Circle as a scientific instrument ended, it continued to exist as a museum object.

The end of the instrument, the end of the Line?

The most renowned contribution of the Airy Transit Circle remains its association with the International Prime Meridian. Unlike the Equator, there is not a single Prime Meridian defined by the shape of the Earth. Instead, meridian lines can be established at any local position. The major obstacle in establishing such a line is defining it when there are very few points of reference available to the observer.

The history of astronomy and navigation are defined by the search for methods and techniques for the ever more precise measurement of longitude. Meridian instruments (such as the Airy Transit Circle) emerged as key tools within astronomy to make astronomical observations connected to local longitudes. These instruments not only measured the declination and the time of transit of celestial bodies, but also contributed by defining, with ever more precision, the meridian lines. In this sense, meridian lines and meridian instruments have always had symbiotic relationships: a line is required to set up the instrument, but then the instrument is used to define the line with greater precision. Through such an approach, the materiality of the instrument is inseparable from any of the abstract or material representations of the line. As a result, any history of a meridian is part of the biography of the instrument that defined it, and the politics of the instrument has an impact of the history of the line.

This approach to the relationship between instrument and meridian is actively present within histories of observatories and current displays at museums. Howse’s history of the Royal Observatory, Greenwich, for example, emphasizes that whenever a new transit instrument was installed, the Greenwich Meridian was accordingly shifted (Howse Reference Howse1980). Similarly, observatory buildings themselves were built with the intention to be the very instruments defining the meridian. For example, one of the major historical features of the Paris Observatory is the Meridian Room, where the meridian line on the floor was used to measure the local transits of the Sun. Despite this, histories of the measuring of longitude and time have tended to de-emphasize the concerns about the materiality of the instruments, which downplays the importance of the symbiotic relationship. By contrast, this paper argues that the symbiotic relationship between the Airy Transit Circle and the Greenwich Meridian was a defining feature of the instrument’s life.

This symbiotic relationship was regularly represented in articles about the Observatory that appeared in periodicals during the nineteenth and early twentieth century. One of the most popular nineteenth-century accounts of the Observatory was written by Edwin Dunkin (Fig. 3). He was employed at the Observatory for 36 years (1838-1884), gradually climbing from the lowest ranking computer to the role of Chief Assistant. He was also one of the members of staff responsible for making observations with the Transit Circle and determining its errors. Therefore, he had firsthand experience of how the instrument was used on a daily basis. His description of the Transit Circle emphasized the close relation between the instrument and the line by writing that “the middle wire in the eye-piece of the telescope is the meridian line from which arcs of longitudes on British maps are reckoned. In illustration, we may notice that one half of the observing room is in the eastern hemisphere, and the other half is in the western” (Dunkin Reference Dunkin1862, 23). According to this description, the line was not an independent or separate entity, but one that resided within the materiality of the Transit Circle.

Figure 3. Illustration of the Airy Transit Circle used in Edwin Dunkin’s book The Midnight Sky (1891) © National Maritime Museum, Greenwich, London.

Dunkin was not alone in describing the relationship between the line and the Transit Circle in this manner. James Carpenter, another member of the Observatory staff, noted the same relationship in a similar way:

As we see in this quote, Carpenter’s description can be characterized as even more embedded within the materiality of the Transit Circle and its components than Dunkin’s description was. For Carpenter, the line and the instrument, as well as their histories, were entangled and inseparable from each other. In fact, taking Carpenter’s approach even further, every time the middle crosshair or the spider’s web broke in the eye-piece, the observer broke not only the instrument, but the Greenwich Meridian too. Similarly, when instrument makers replaced the wires, they not only repaired the instrument, but also inserted a new Greenwich Meridian.

This type of representation gradually began to fade after the Greenwich Meridian was “selected” as the Prime Meridian in 1884. Instead, the line was thought of as an almost independent entity, going beyond the limits of the Transit Circle. A good example of this is Walter Maunder’s history of the Observatory. Like Dunkin and Carpenter, Maunder was a member of the Observatory staff. His description of the relationship between the instrument and the Transit Circle differs from theirs in linking it to the “optical axis” and other instruments resting on the continuation of the line, rather than to the middle wire in the eyepiece: “the optical axis of this telescope marks ‘Longitude Nought,’ which is further continued by a pair of telescopes [i.e. collimators], one to the north of it, the other to the south” (Maunder Reference Maunder1900, 148). In Maunder’s description, the line was no longer thought to be confined to the instrument itself, but was instead an entity that went beyond its limits and encompassed whatever was placed on it (e.g. collimators).

Maunder’s book was the last major popular description of the Observatory produced by one of its former members of staff, but discussions about the symbiotic relationship between the line and the instrument re-emerged once the Airy Transit Circle was turned into a museum object. The guidebooks produced by the National Maritime Museum (the institution that ultimately took over the Royal Observatory at Greenwich) and by members of its curatorial team presented mixed interpretations of the instrument-meridian relationship. In the first guide to the Observatory as a museum, Philip Laurie noted that “the Transit Circle performed its duties for more than a century and still stands defining the Prime Meridian” (Laurie Reference Laurie1960, 16). By doing so, Laurie continued to connect the materiality of the instrument to the Prime Meridian. Frank Carr, the director of the National Maritime Museum, maintained this emphasis on the material connection by stating that the “prime meridian has been the centre wire in the Airy transit circle” (Carr 1965, 14).

The next guidebook to the Observatory marked a small shift away from the materiality of the instrument towards a more abstract presence of the line. This guidebook defined the Prime Meridian as the “longitudinal centre line of this instrument [i.e. the Airy Transit Circle]” (National Maritime Museum 1969, 12), rather than as an item within the materiality of the instrument itself. A contemporary article by Derek Howse similarly highlighted this abstract definition of the Prime Meridian as “a meridian passing through the centre of Airy’s Transit Circle” (Howse Reference Howse1970, 208). This was a definition that preserved the wording of the resolution at the 1884 Washington Conference, where the instrument was chosen to define the Prime Meridian (“the meridian passing through the centre of the transit instrument at the Observatory of Greenwich”), and a definition that Howse reused in his book on the history of the Observatory (Howse Reference Howse1980, xiv).

Basil Greenhill’s guide to the National Maritime Museum continued the material link between the instrument and the Prime Meridian, by characterizing “the Line” as being defined by the “optical axis” of the Transit Circle (Greenhill Reference Greenhill1982, 110). Kristen Lippincott went even further by describing “the crosshairs of the great Transit Circle telescope” as the entity that defined Longitude Zero (Lippincott Reference Lippincott1994, 2). As these examples show, the history of the Greenwich Meridian has been connected to the materiality of the Airy Transit Circle. Despite this, however, the imagination of the public remains captured by standing on both sides “the Line” rather than by taking photos of the line with the instrument (Fig. 4). It is this engagement with the product of a “dead instrument” that raises the question: why is the product not considered “dead” too? Or, perhaps it is? Perhaps the line defined by the instrument has also reached its end?

Figure 4. Two tourists on the Meridian Line, Greenwich, circa 1931 © National Maritime Museum, Greenwich, London.

Walter Maunder’s popular book about the Observatory was also important for noting that the Greenwich Meridian was not always marked by the Airy Transit Circle. Instead, the Greenwich Meridian moved to the East by a few yards every time a new meridian instrument was set up inside a different room of the Observatory. Such an approach emphasizes both the close material links and the tangibility of the Line.

This movement of the line has served as the focus of historical research over the past decades. The issue gained further interest in 2015, when international news outlets noted the publication of a scientific paper that described how the Greenwich Meridian moved with the development of global positioning systems (Malys et al. Reference Malys, Seago, Pvlis, Seidelmann and Kaplan2015). Since then, Michael Kershaw demonstrated how the emergence of wireless technologies contributed to the gradual transition from measurements made with a single instrument to incorporating simultaneous measurements made with multiple instruments at different locations (Kershaw Reference Kershaw2019).

The availability of simultaneous measurements contested the need to refer the observations back to the Greenwich Meridian as defined solely by the Airy Transit Circle. Kershaw argued that the simultaneous observations used to establish a universal standard of time resulted in the shift from the meridian defined by the Transit Circle to a “fictitious zero of longitude” that was calculated to be just a few meters east of the Greenwich Meridian (Ibid., 236). This was a crucial moment in the life of the Transit Circle, since it separated the materiality of the instrument from its product (the line) for the first time. In this sense, redefining the measurement of the Prime Meridian marked the end of the line as defined by the Airy Transit Circle. It is important to note here that this coincided with the gradual downfall of the credibility of the Airy Transit Circle. Only a year before the establishment of the fictitious meridian, the measurements of time service observations had been moved from the Transit Circle to another instrument due to the systematic errors being identified within the data provided by the Transit Circle.

What Kershaw and other historians demonstrated was that “Greenwich Meridian” was a term used to denote what were actually multiple successive meridians, derived from observations made at the Royal Observatory, Greenwich. Through this approach, the Line never ended, but always moved. By contrast, approaching the history of the line through astronomical instruments demonstrates that the different meridians were dependent upon the specific astronomical instruments that determined them. As a result, a meridian line appears to reach the end of its life every time the operations with the specific instrument that defined it are halted. In other words, we can interpret meridian lines reaching the ends of their lives in two different ways: either as the same line reaching the end of its life multiple times (whenever a new instrument is installed) or as multiple meridians reaching the separate ends of their lives (with each instrument giving birth to a different line).

The way in which the historical Greenwich Meridian is displayed today at the Royal Museums Greenwich successfully mixes the two approaches (Fig. 5). The line that crosses the courtyard (and over which the iconic photos are taken) marks the Greenwich Meridian as determined by the Airy Transit Circle. Therefore, it is a representation of the Line that was selected at the 1884 International Meridian Conference, as opposed to the exact location of the Prime Meridian in use today. The connection between the Transit Circle and the Line is made as clear as possible through the arrangement of the site. The representation of the Line across the Observatory courtyard not only leads directly to the Transit Circle Room where the Airy Transit Circle still stands today, but the northern wall of the same room is partially transparent in order to give the visitors the ability to visually connect the instrument and the line. Yet, despite these attempts to make the connection between the two entities, photos of the line very rarely feature the Transit Circle in their backgrounds. They are more likely to show the view of London from the top of the hill or the shoes of visitors. The reason for this can be traced back to its practical origins, namely the relatively large size of the instrument in comparison to the room that houses it. Returning to Maunder’s description of the Observatory once again, the presentation of the Airy Transit Circle appears as a problem that has been present ever since its installation: “this room is not well adapted for representation by artist or photographer” (Maunder Reference Maunder1900, 147).

Figure 5. Illuminated Meridian Line at night, Royal Observatory, Greenwich (1994) © National Maritime Museum, Greenwich, London.