The heart of the observatory: The operating chain of Stockholm observatory’s 3.5-foot Bird transit instrument

Introduction

The T[ransit] clock had stopped due to the bob line being stuck. The clock was wound and set approximately during the observation. ¹

This note on a clock malfunction is one of many entries in the observation journal of Simon Anders Cronstrand, from 1811 director of the Stockholm Observatory. It highlights two fundamental and interconnected aspects of transit instruments. First, the operation of these instruments depended on regulator clocks, which were essential for carrying out observations. Second, these clocks had to be correctly set and maintain accurate time. However, successful transit observations involved more than clocks alone. Like all telescopes, a transit instrument required an appropriate observatory setting: a purpose-built space capable of housing the apparatus and a stable foundation that minimised vibration and allowed the instrument to be firmly secured. Beyond these spatial and structural conditions, the instrument depended on a set of auxiliary components for its proper adjustment and use, including a quadrant, a meridian mark (mire) and a spirit level for calibration. Finally, this assemblage of technical elements had to be monitored and operated by the astronomer in accordance with established routines for calibration and observation.

The present study takes as its focus this interconnected system of material infrastructure and observational practice, centring on the 3.5-foot Bird transit instrument employed by Cronstrand and his colleagues at Stockholm Observatory in the late 18th and early 19th centuries. It investigates the instrument’s role within the observatory’s working regime and analyses the procedures through which astronomers sought to secure reliable observations.

The basic design of the modern transit instrument was introduced by the Danish astronomer Ole Rømer in 1690. It comprised a small telescope mounted perpendicular to a horizontal axis oriented east-west, the axis itself being carried in trunnion bearings by a rigid support. ² This configuration restricted the telescope’s motion to the plane of the meridian, enabling the observation of transits – that is, the precise moment when a celestial object crossed the meridian. After appropriate calculations, the recorded timestamp yielded the object’s right ascension, one of the two principal celestial coordinates (the other being declination). In 1704, Rømer added a vertical divided circle to the instrument, allowing right ascensions and declinations to be measured simultaneously. Although many astronomers were sceptical of the divided circle and preferred to determine declinations using a quadrant or similar instrument, the transit instrument nonetheless gradually gained acceptance. By the end of the 18th century, in the words of Jim Bennett, such instruments ‘came to define the primary meridian of most observatories’. ³ A few decades later, the divided circle re-emerged, and the more advanced meridian circle became the standard instrument for the century to come. The instrument discussed here, however, belongs to this earlier phase. ⁴

In an article that discusses the time service established around the meridian circle at Neuchâtel Observatory in the mid-19th century, Gressot and Jeanneret employes the concept of an operating chain which they define as ‘a complex of scientific instruments, technical equipment, mathematical calculations, and environmental and human factors involved in the production of these data, that is, the time signal’. ⁵ This concept can also be applied to our case. It emphasises the instrument’s interconnectedness, its reliance on other equipment. However, I would like to stress the importance of the human factors – going beyond mere techne, the astronomers, the institutional setting and the many practices involved in preparing and operating the equipment, becomes important. ⁶

Even when a functioning operational chain had been established, it remained vulnerable. The astronomer might fall ill, bringing observations to an immediate halt; the janitor might neglect to procure oil for the lamp illuminating the eyepiece threads, rendering transit sightings impossible. Mechanical failures posed further risks: the clock could stop, as suggested in the opening quotation; the transit’s axis might deviate from proper alignment or the meridian mark could be obscured by smoke or haze. As Simon Schaffer observes, ‘Faults are defaults, yet instruments perform’. ⁷ Scientific instruments operate not by virtue of inherent reliability but through the sustained maintenance of an intricate socio-technical arrangement. The effective functioning of a transit instrument – and of the operational chain that supported it – depended on well-rehearsed routines, systematic calibration and meticulous attention to technical detail.

Our exploration will commence with the acquisition process of the Bird transit and the financial and architectural challenges that delayed the installation of the instrument for many years. We will then delve into the astronomical clocks and subsequently examine the procedures undertaken by astronomers to maintain the instrument’s functionality and address various issues that arose along the way.

The Transit

When the Stockholm Observatory was inaugurated on 20 September 1753, the newly appointed director and Academy secretary, Pehr Wargentin, faced the immediate task of equipping the observatory with suitable instruments. ⁸ At the top of his list of priorities, presented to the Academy later that year, were a mobile quadrant and a transit instrument, instrument types that were the foundation of late 18th-century astronomy. ⁹ The quadrant had multiple purposes, including altitude measurements (declination), geodetic studies and was also, as we will see, used to calibrate the transit instrument. The transit instrument, in turn, was used for positional astronomy (right ascension), and particularly for observing the transit of the sun or different standard stars to set the regulator clocks. Initially, the commission was given to the esteemed instrument maker of the Academy, Daniel Ekström. However, due to his sudden and unexpected passing, Wargentin sought the Academy’s approval to turn his attention towards London in the search for suitable alternatives. ¹⁰

Wargentin initially approached John Ellicott, a clockmaker and foreign member of the Academy, who provided him with a price list from renowned instrument maker John Bird. ¹¹ Ellicott informed Wargentin that a 5-foot quadrant would cost £230, while a 5-foot transit instrument would cost £50. ¹² Unfortunately, these prices exceeded Wargentin’s budget, forcing him to temporarily settle for a more versatile 3.5-foot quadrant and postpone the acquisition of the transit. Bird began working on the quadrant, and it was delivered to the observatory in June 1758 (Figure 1). ¹³ Meanwhile, negotiations for the transit instrument continued. Wargentin updated the Academy and stated that Bird could take on the task ‘if he could begin soon, considering his advancing age and declining eyesight, as it was uncertain whether he would be able to undertake such delicate work later on’. ¹⁴ The Academy hesitated and although funds were set aside for the transit, the meeting remained undecided on whether to commission it from Bird or explore other options.

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Figure 1.

The Bird quadrant. The instrument is presently on display at the Observatory. Image by the author.

From this point onward, additional negotiations were carried out by Wargentin’s friend and younger colleague, the astronomer Bengt Ferner (later knighted as Ferrner). At the time, Ferner was undertaking a grand tour of northern Europe and spent a year in Britain, starting in the summer of 1759. ¹⁵ During a prolonged stay in London, he visited all the prominent instrument makers. He acted as an envoy, representing not only Wargentin but also other Swedish astronomers who were planning to commission instruments from London workshops. Although Ferner had other suggestions, his meeting with James Bradley at the Greenwich Observatory convinced him that Wargentin should proceed with having Bird construct the transit instrument:

When I was at Dr Bradley’s, and he showed me the instruments belonging to the observatory, he naturally came to speak of Bird, who had made all those now in use and said what he reckoned one of the chief circumstances contributing to the precocity of astronomy in our time, is that such a skilful workman as Bird fortunately exist. This must not be taken as boasting [. . .]. He has, moreover, set up the instruments at Greenwich, verified them in company with Bradley, and thus had an opportunity of noting the small inconveniences which in the application of astronomical instruments give rise to great errors, and of which another worker cannot have the slightest suspicion of, and thus in no way can try to prevent. ¹⁶

After several letters were exchanged, Wargentin was persuaded. In June 1760, the Academy officially acknowledged the matter, and Ferner was tasked with commissioning a 3.5-foot transit instrument from Bird. ¹⁷ Since Ferner needed to continue his tour on the continent, Ellicott took charge of monitoring further developments. In November 1761, Ellicott informed Wargentin that the instrument had been completed, packed and was ready for shipping. ¹⁸ The final payment was made, and in June 1762, the transit instrument was transported to Stockholm aboard the commercial vessel The Appearance (Figure 2). ¹⁹

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Figure 2.

The Bird transit instrument had a conical main axis featuring cylindrical end pieces. It rested on V-bearings attached to marble pillars that supported the instrument. The bearings could be adjusted, one in the vertical plane, and one in the horizontal, to level and align the instrument. The latter bearing held a semi-circular zenith distance scale, divided into one-thirds of a degree. The main tube held the objective and eyepiece tubes, fixed to a focal length of 3.5 feet. The eyepiece featured nine vertical threads in the focal plane. The image shows the instrument as displayed in the Stockholm Observatory Museum’s exhibition, inaugurated in 1991. The present objective lens and eyepiece are reproductions created for the exhibition. The museum was closed in 2014 and the transit is now kept at the Center for History of Science (RSAS). Image: Stockholm Observatory Museum/RSAS.

With the successful arrival of the instrument, further progress came to a halt. Despite the commissioning and delivery of marble pillars in 1765, there was no designated space in which to install them. The original floor plan of the Observatory did not account for a transit room, and only allowed instruments to be placed within a large circular observation hall on the ground floor (Figure 3). ²⁰ Unfortunately, this space did not provide suitable conditions for mounting the transit instrument. ²¹ One possibility would have been to construct a new wing to accommodate the transit, but given the recent expenses for instruments, this option was deemed unfeasible. Consequently, the transit instrument remained unused, confined within its wooden crate.

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Figure 3.

This undated pencil drawing depicts the southern facade of Stockholm Observatory and was created by artist Johan Abraham Aleander, likely in the 1830s. The main circular observation hall can be seen in the centre, and the transit instrument room in the short wing to the right (east). When Aleander made the drawing a new transit room had been prepared in the west wing, while the old room had been renovated and restored into an office space. Image: Uppsala University Library.

After several years, Wargentin raised the matter with the Academy, emphasising the urgency of making the transit instrument operational. It was decided to proceed, and a small group of fellows, including Bengt Ferner, who had returned from his European tour, was tasked with convening at the observatory to determine a suitable location for mounting the telescope. ²² A few months later, the inspectors provided their feedback, explaining that currently there was no appropriate room at the Observatory in which to house the transit instrument. They suggested that the small eastern wing, initially intended and utilised as office space, could be expanded to accommodate the instrument. The meeting reached a favourable consensus but decided to postpone the project until the following year. ²³ However, due to financial constraints, an extension was not feasible. Consequently, the Academy decided to reconstruct the existing office room to accommodate the transit instrument. This still required a significant endeavour: the removal of the floor to accommodate the construction of a stone pillar through the cellar below; the installation of the marble pillars on top of that; the creation of new openings, with shutters, in the roof and both the southern and northern walls, and the fitting of a new floor. ²⁴ The project was carried out during the summer of 1772 and in early autumn, a decade after the instrument’s delivery, it was finally mounted within what would henceforth be referred to as the ‘transit instrument room’.

The transit project represented a significant venture for the Academy, involving substantial financial investments. The reconstruction of the eastern wing incurred costs of approximately 2500 riksdaler, the transit instrument itself amounted to 4400 and the marble pillars to 2100. ²⁵ Altogether, the new instrument’s total cost reached 9000 riksdaler. To provide some perspective on the magnitude of the investment, the sum was equivalent to Wargentin’s annual salary. ²⁶ In addition to these expenses, it is worth considering the additional cost of 12,000 riksdaler for the quadrant. ²⁷ As mentioned earlier, although the quadrant had other purposes, it was also essential for calibrating the transit instrument, and thus an integrated part in its operating chain. However, one crucial piece of equipment was still missing: an astronomical clock.

The clock

The Academy acquired its first astronomical clock in 1748. Crafted by Stockholm clockmaker Gustaf Nylander it featured a deadbeat pendulum escapement. ²⁸ Alongside a few smaller refractors, Wargentin utilised this clock in the years preceding the establishment of the new observatory. One notable application was the observations made to support Nicolas-Louis de Lacaille’s parallax measurements at the Cape of Good Hope between 1751 and 1752. ²⁹ After a few years, another pendulum clock was procured from clockmaker Petter Ernst. The Academy had recognised Ernst’s expertise a few years prior, leading to an arrangement in 1752 for him to relocate his business from the small town of Växjö to a government-controlled workshop in Stockholm. Additionally, a deal was struck with the Office of Industry [manufakturkontoret], granting Ernst a premium of 300 riksdaler for each pendulum clock that he produced, subject to the Academy’s approval of its quality. ³⁰ This partnership firmly tied Ernst to the Academy, and he went on to manufacture several clocks used in its various departments.

In the historiography of the Academy, Ernst has been described as ‘our Swedish Graham or Ellicott’, primarily due to his mastery of temperature-compensated pendulum clocks – a ground-breaking innovation introduced by George Graham in 1721. ³¹ Ernst constructed several clocks of this type specifically for the Observatory. The pendulum clock mentioned above was replaced in 1759 with a deadbeat escapement clock ‘that shows seconds and runs for six weeks after winding and is made with a composed pendulum rod after Graham’s invention’. ³² Ernst proposed a fee of 800 riksdaler for this clock, with the condition that he received the old clock in return. In 1765, Ernst delivered a similar but more advanced pendulum clock, charging 1400 riksdaler for it. ³³ From 1772 onwards, this later clock (Figure 4) would be paired with the Bird transit instrument.

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Figure 4.

The bonnet of the Ernst clock. The clock, preserved at the Center for the History of Science (RSAS), bears the marks of time. The pendulum has been missing for many years, the hands on the clock face are bent and the case is cracked. Image by the author.

With Ernst’s pendulum clock added to the list, the basic technical components in the operating chain revolving around the Bird transit are accounted for. In the next section, we will introduce the most vital link in the chain, the astronomer working the equipment.

Wargentin

The reconstruction of the Observatory’s eastern wing was successfully finished in August 1772. Wargentin made an entry in his journal, writing in Latin, that the Ernst clock had been installed in the new transit room and was fully operational. ³⁴ The process of mounting the Bird transit, under the supervision of instrument maker Johan Zacharias Steinholtz, was underway, but it took another month before the instrument finally saw first light:

The centre of the Sun passes through the Meridian [at 12^h 3^m 57,25^s]. This is the first Observation made with the new Meridian Tube, or Transit Instrument, now at last, after so many years’ delay, in its proper place, and prepared with due care. On the same day, the true moment of noon, by 8 pairs of well corresponding altitudes of the Sun [. . .], was found by marking the clock - - - - 12.3ʹ.55,1ʺ. Therefore, the error of the Meridian Instrument, by which noon is delayed, is only 3 seconds. ³⁵

The note reveals a procedure that would be repeated time and again over the years. In proper alignment (east-west and horizontally), the transit instrument would deliver reliable time after just a couple of minutes’ work at the eyepiece. However, before enjoying this convenience, certain prerequisites had to be met. First and foremost, the transit needed to be levelled and correctly oriented. Levelling the main axis was a straightforward process accomplished using a spirit level and some plumb lines and if necessary, with minor adjustments made to screws on the supporting V-bearings. For the meridian alignment, Wargentin employed the Bird quadrant and what was referred to as the corresponding solar altitude method. Multiple pairs of solar altitudes were measured in the hours preceding and following noon, enabling the calculation of true noon – the exact moment when the sun culminated on the meridian. This method was more reliable but also more time-consuming. In this particular instance, the quadrant indicated that the transit was off by a mere three seconds, which Wargentin deemed an acceptable deviation. In subsequent weeks of using the transit instrument, these three seconds were consistently subtracted from all observed transit times.

According to his journal, Wargentin would repeat this procedure, on average, three times a year as long as he got to use the transit (Figure 5). In the intervals between, he would regularly observe transits of stars with well-defined right ascension, such as Sirius and Rigel, to assess the instrument’s and the clock’s performance. Accordingly, the operating chain of the Bird Transit also included information taken from different star catalogues – the catalogues mentioned in Wargentin’s journal are Flamsteed’s Stellarum Inerrantium Catalogus Brittanicus (1725) and Nicolas-Louis de Lacaille’s Catalogue des 515 Étoiles Zodiacales (1763). Another calibration method he employed was observing the upper and lower culmination of circumpolar stars. If the alignment was accurate, the time interval between these transits should be equal. However, based on the journal entries, it appears that Wargentin only used this method once, showing a preference for the corresponding solar altitude method. ³⁶

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Figure 5.

There were different ways to check the alignment of the Bird transit, but Wargentin seems to have preferred the corresponding solar altitude method. The figure lists all such measurements in his journal, starting from the installation of the transit 1772, and shows the transit deviations in seconds. A positive value indicates that the instrument marks the transit a few seconds late (and vice versa for negative values). S indicates that the transit stood due south. Altogether, the list indicates that the transit was slowly turning towards the west. In 1777 Wargentin seems to have relied on star transits for the calibration, and the hiatus in 1782 coincides with Wargentin’s preoccupation with the newly discovered planet Uranus. During this time the instrument was calibrated by the transit of the nearby star Propus. Information from Wargentin, Observation Journal, 1767–1783 (Note 34), corresponding dates.

For the first few years, the results of Wargentin’s calibration efforts at the transit instrument remained relatively stable. The solar transits observed with the instrument were either spot on or had only minor deviations from the culminations determined with the quadrant. However, in the autumn of 1778, these discrepancies suddenly increased, indicating that something was amiss. This is also the first mention of a meridian mark in Wargentin’s journal. There is no further documentation, but it seems likely that it was installed in the summer months earlier in the same year (we will return to the mire later).

Wargentin first observed the deviation in September 1778. When looking through the eyepiece of the transit instrument, he noticed that the central thread in the field of view stood somewhat to the west of the meridian mark, causing a slight delay in transits. This misalignment was further supported by corresponding solar altitude measurements using the quadrant, which revealed a deviation of approximately ten seconds. ³⁷ A month later, Wargentin repeated the procedure and obtained the similar result. ³⁸ In an attempt to validate these findings, Wargentin observed several star transits (e.g. Altair and α Capricorni) over the following months. However, the results were inconclusive:

I do not know, indeed, in what manner it should be explained, why the Transits of the Fixed Stars seem to indicate an error in the Meridian Instrument of only one or the other, as the largest, of four seconds, when, nevertheless, the corresponding altitudes of the sun reveal an error of 9 or 10 seconds. I think it is more trustworthy to rely on fixed stars until more reliable information is available. ³⁹

Continuing his investigation throughout 1779, it was not until May the next year that Wargentin decided to act. ⁴⁰ Cautiously he made small adjustments to the azimuth of the transit, carefully evaluating the impact through new measurements. This iterative process allowed him to gradually shift the transit slightly towards the east. By September, he was able to confirm that the transit instrument was once again providing reliable and accurate results. ⁴¹

The reason for the transit coming out of alignment remains an unanswered question –Wargentin, if he harboured any suspicions, did not document them in the journal. However, the most likely contributing factor was subtle settling in the foundation on which the instrument rested, potentially aggravated by wear and tear on the transit’s bearings. It should be noted that Wargentin was well aware of how seasonal changes in temperature impacted the performance of the transit and that these minute changes were accounted for in his investigation. Naturally, the temperature changes also affected the performance of the Ernst clock, something that was meticulously recorded by Wargentin. Both the Ernst and the older Nylander clock’s performance can be traced in a dedicated clock journal, where he documented every adjustment made to the clocks. ⁴² The journal reveals that the Ernst clock operated flawlessly in warmer weather but tended to accelerate slightly during the cold winter months from December to February.

Wargentin’s meticulous evaluations of the Bird transit instrument and his unwavering monitoring of the clock’s performance illuminate the equipment’s constant state of flux. Observations made at the transit could not be taken at face value. Writing of a 19th-century context, Daniel Belteki likens astronomers to ‘detectives’, approaching instruments like ‘criminals’, methodically investigating every systematic (or irregular) error associated with the equipment. ⁴³ Belteki’s metaphors can also be applied to Wargentin’s situation of a century before. The erratic behaviour of the transit instrument cast uncertainty on the crucial time determinations essential for his astronomical work, demanding his immediate attention. Some uncertainty – plus minus a couple of seconds – was acceptable, but when deviations increased measures had to be taken. Consequently, valuable observation time was devoted to checking the transit and promptly addressing any issues that arose.

Although Wargentin occasionally employed the transit for planetary observations when they were favourably positioned, such as Saturn in the spring of 1774 and Mars in the spring of 1779, its primary purpose was to set the regulator clock. For his actual observations, Wargentin turned to other instruments, notably a cutting-edge 10-foot achromatic refractor crafted by John Dollond and installed at the observatory in 1764. ⁴⁴ Using this refractor, Wargentin engaged in a variety of astronomical studies, including observing lunar and solar eclipses, lunar occultations, occasional comets, as well as his preoccupation throughout his whole career: the immersions and emersions of Jupiter’s moons. In all these endeavours, particularly regarding the Jupiter satellites, the accuracy of the time obtained at the transit instrument determined the quality of all subsequent observations. A ten-second error in transit readings brought observations to a standstill, and they could not commence until the error was thoroughly investigated and eliminated.

Svanberg

Wargentin passed away in 1783, but the Bird transit lingered on. It was used in turn by his successors as Academy astronomer, Henrik Nicander, Jöns Svanberg and, as previously mentioned, Simon Anders Cronstrand. Unfortunately, there are no preserved observation journals from Nicander’s tenure. For the first few years he published some short observation notes in the Academy’s proceedings, but after that his responsibilities as Academy secretary, a role he also took over after Wargentin, limited his involvement in astronomical pursuits. Svanberg, who took over from him in 1803, was more inclined towards observational work, particularly with the transit instrument.

Svanberg’s meticulously maintained journals, which besides the actual observations (mainly solar and star transits) provide a minimum of additional information, will offer little contribution to our narrative. ⁴⁵ However, his opinion of the Bird transit instrument is articulated in an observation report he published in the Academy proceedings a few years before assuming the role of Academy astronomer. Then in the capacity of assistant astronomer, he initiates the exposition by detailing his approach to the transit instrument.

The almost daily change to which a transit instrument is subject thus first attracted my attention; in the examination of such an instrument, there are three circumstances to be observed. 1:o Does it describe a great circle? 2:o Is this great circle a Vertical? And 3:o Is this Vertical also the Meridian? ⁴⁶

Svanberg explains that the first two questions pertain to the instrument’s internal integrity: The tube and the main axis need to be at 90 degrees, and the main axis must be level – both aspects are easily verifiable. However, the third question necessitates an astronomical approach, involving either observations of corresponding solar altitudes or the upper and lower culminations of circumpolar stars. After a thorough investigation, using both methods, Svanberg concluded that the Bird transit instrument anticipates observed transits by two seconds, indicating a slight eastward deviation from the meridian. ⁴⁷ Adjusting the transit’s azimuth was, according to Svanberg, not a viable option. Such a procedure demanded a suitable meridian mark, and apparently, Wargentin’s mire had gone missing. In later sections of the paper, when presenting various observations, Svanberg therefore appears to distrust the time produced at the transit. Instead, he validates the observations by referencing corresponding star transits. ⁴⁸

Although Svanberg had a new mire installed in the summer of 1800, he seems to have lost interest in actual observations. ⁴⁹ When, a few years later, he took up the post of Academy astronomer, his focus was on geodetics and theoretical astronomy and he did not publish further observations. However, in 1832, many years after Cronstrand had taken over his position, he published a comprehensive paper on transit instruments and how to apply the necessary corrections when analysing the data. ⁵⁰

Cronstrand

Cronstrand assumed the position of Academy astronomer in 1811, taking over from Svanberg, who had transitioned to the role of Academy secretary. Cronstrand’s activities at the Observatory, including his utilisation of the Bird transit, are documented in his preserved observation journals, covering the period from 1813 to 1828. ⁵¹ As an astronomer with a particular interest in geodesy, Cronstrand’s focus was not primarily on the Observatory’s stationary instruments. Consequently, the Bird transit saw limited usage and was primarily employed for timekeeping. Nevertheless, it still required periodic calibration. Unlike Wargentin, Cronstrand seldom relied on the Bird quadrant for the purpose. Instead he used the new mire, which, as we will see, presented its own set of challenges. ⁵²

As mentioned above, the details of the installation of the first mire, including its contruction, are not documented. ⁵³ We know that it was attached to the handrail of Kungsholmen Bridge, some 2 km south of the Observatory and close to the city centre (Figure 6). Subsequent documents suggest that the mire was intended as a temporary solution, and there were plans to replace it with a marble pyramid. The pyramid, cut at the Kålmården marble mill, had been gifted to the Academy by admiral and county governor Theodor Ankarström in 1747. More than 20 years later it was finally transported to the slope of Observatory Hill. ⁵⁴ After an additional 20 years laying idle, the pyramid became a matter of concern for the Academy’s Treasury Inspection:

[Permanent secretary] Mr Nicander reported that a buyer exists for the large pyramid-shaped Marble stone, which for many years lay unused on the Observatory slope, and was intended to either be set up in Lapland as a commemoration of the geodetic survey, or on Södermalm [a hill south of the city] as a Meridian mark for the Transit-instrument and Quadrant, but which the costly transport and obstructing houses where it ought to be erected, had hindered; so that the Stone now not only lies in the way but is also spoiled by rain and wind, especially if it has some cracks. ⁵⁵

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Figure 6.

Kungsholmen bridge with the Eldkvarn steam mill in the background. This photo was taken in the 1890s, many years after the observatory mire had been removed from the bridge railing. Exactly where the mire was attached is impossible to say, but the transit’s meridian cuts the railing somewhere to the far right in this image. Image: Tekniska museet, unknown photographer.

The meeting decided to inspect the stone, ensuring its proper protection from the elements. However, they did not grant permission to sell it or take any immediate steps to utilise it.

With the absence of a more permanent mire, the astronomers had no choice but to continue using the original one. Reinstated by Svanberg in 1800, its location on the handrail of Kungsholmen Bridge still made it susceptible to damage due to heavy traffic, be it accidental or intentional. ⁵⁶ After Cronstrand assumed the role of Academy astronomer, in 1812 he brought up the issue in the Treasury Inspection. According to Cronstrand, the mire had been bent or displaced on several occasions and, at times, even removed without authorisation. Given the mire’s vulnerability, he proposed that a new mire should be installed on the roof of a steam mill (Eldkvarn, literally: fire mill), which was situated next to Kungsholmen Bridge. Abraham Niclas Edelcrantz, the owner of the mill and a notable figure in politics, invention and Swedish steam engine development, had a positive stance regarding this proposal. Additionally, Cronstrand advocated for a second mire to be placed north of the Observatory, reminding the attendees of the marble pyramid still present. He emphasised that these two mires are ‘important for perfecting transit observations, which is the only thing you can do with current equipment’. ⁵⁷

Despite the good intentions behind Cronstrand’s proposal, no progress was made. Consequently, he had no choice but to continue relying on the mire on Kungsholmen Bridge when calibrating the Bird transit. For the most part, it fulfilled its purpose. However, in June 1814, Cronstrand recorded in his journal that the mire’s position was compromised due to significant repairs being made to the bridge. ⁵⁸ As time went on, the city’s development presented a recurring issue. Cronstrand frequently lamented the obstruction of the mire caused by chimney smoke, making it difficult for him to obtain clear and accurate readings. ⁵⁹

Cronstrand faced additional challenges during his tenure. One of these was the Ernst regulator clock, which had served Wargentin well for many years without significant issues. The passage of time had taken its toll and, in 1814, the clock began to malfunction, ceasing to keep accurate time and repeatedly stopping. Recognising the need for repairs, it was sent to a clockmaker to address the malfunctions and in addition, a new winding mechanism was installed. ⁶⁰ After repairs and recalibration, the clock once again performed as it should. ⁶¹

However, the clock continued to experience malfunctions, and in 1816, it was once again sent to the clockmaker, this time for complete disassembly and thorough cleaning. ⁶² The journal entries suggest that the process took more than 2 years to complete. During this period, Cronstrand had to rely on the old Nylander pendulum clock, previously in the Observatory library. A modern chronometer, crafted by London clockmaker John Arnold, was borrowed from cartographer and naval officer Carl Peter Hällström, who also participated in some of Cronstrand’s observations. ⁶³ Unfortunately, due to the lack of audible ticks in the chronometer, the eye-ear method used during transit observations proved to be ineffective. ⁶⁴ Once the Ernst clock was fully operational, the chronometer was returned.

As we have seen, Cronstrand faced several challenges in maintaining the operating chain associated with the Bird transit. Eventually, the transit itself became an issue. In a memorandum presented to the Academy in 1819, asking for funding for new instruments for the Observatory, Cronstrand explains:

The first thing needed is a new Transit instrument. – What the Royal Academy currently possesses is, for several reasons, almost useless. – Besides the fact that its tube is non-achromatic, the objective glass is damaged, and the entire apparatus for its fine-tuning is failed, it also lacks sufficient clarity and magnification to observe any of the recently discovered planets [i.e., asteroids Ceres, Pallas, Juno and Vesta]. – Only with a Transit instrument can the time, which constitutes the most essential element in most observations, be determined. – If I have succeeded, at times, even with the current instrument, in achieving the desired accuracy, it has required calculations that could deter even the most tenacious of patient persons. ⁶⁵

The Bird transit was worn down and, more importantly, out of date – in terms of both technical developments and scientific demands. It no longer delivered time at the required accuracy, and maybe more importantly, its lesser aperture did not allow Cronstrand and Stockholm Observatory to partake in the asteroid frenzy. ⁶⁶ Cronstrand’s plea was received positively, and a few years later a new transit was commissioned from Reichenbach & Ertel in Munich. However, until the new transit finally was installed in 1829, Cronstrand had to make do with the ageing Bird transit.

Despite these numerous faults, the operating chain did for the most part deliver the necessary time and with an acceptable precision. As noted above, Cronstrand conducted few observations apart from using solar and star transits to set the clocks. In addition to his role as an Academy astronomer, he also served as a professor in the Topographic Corps, which consumed a significant portion of his time as he mapped various regions of Sweden. ⁶⁷ When he employed the observatory equipment for purposes other than timekeeping, it appears to be linked to his interest in geodesy. For instance, in anticipation of the solar eclipse on 19 November 1816, Cronstrand took meticulous measures to calibrate the equipment: several corresponding star transits, observed a day apart, determined the mean daily acceleration of the Ernst clock (+7.631ʺ s); and observations of some upper and lower culminations of circumpolar stars determined the transit’s deviation from the meridian (+2.90ʺ). Corrected for these deviations, the data collected from the observations of the eclipse was utilised to calculate the longitude differences between Stockholm, Uppsala and Åbo (in Finland). ⁶⁸ Additionally, he utilised the instrument to evaluate the accuracy of different chronometers – a crucial preparatory step for his geodetic expeditions. ⁶⁹ Naturally, the chronometers intended for field use required thorough examination before deployment.

Concluding remarks

Cronstrand’s memorandum not only prompted the acquisition of a new transit instrument but also initiated a comprehensive modernisation of the Observatory’s equipment. In 1826, a new regulator clock by Molyneux & Cope was installed, followed in 1829 by an achromatic refractor with an altazimuthal mounting by Fraunhofer. Finally, in 1834, a state of the art meridian circle, manufactured by Ertel, was introduced. Together, these strategic investments marked the end of a technical era, and its associated practices, that stretched back to the days of Wargentin and entailed the definitive retirement of the Bird transit.

However, before these upgrades, the Bird transit instrument had a crucial role in a rather complex operating chain (Figure 7). While it served other purposes, its primary function was to generate precise time, kept on the Ernst regulator clock. To ensure the transit’s accuracy, it required continual monitoring, which could be achieved in various ways. To check the alignment of the transit, Wargentin relied primarily on the corresponding solar altitude method, employed every 3 months or so. Additionally, during observation nights, he verified the transit and the time indicated by the Ernst clock by observing standard stars, using right ascensions from different catalogues. Although he had a mire installed, it appears to have been more significant for his successors. Svanberg and Cronstrand used the mire for all their calibrations and meticulously calculated its azimuth – the precise deviation from true south as observed from the central thread in the transit’s eyepiece. Instead of attempting to adjust the azimuth, as Wargentin did, they corrected for the deviation in their reductions. The different approaches of Wargentin on one side, Svanberg and Cronstrand on the other, is an example of how the astronomers themselves, with their training, experience and views on best practices, form part of the operating chain.

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Figure 7.

The main links in the Bird transit’s operating chain.

As we have seen, the role of the Bird transit extended well beyond the narrow task of timekeeping. The sidereal time maintained by the transit clock was indispensable for observations with other instruments – such as the aforementioned Dollond refractor – and for testing chronometers intended for field use. The transit operating chain was therefore not an isolated arrangement but a tightly integrated system, embedded in a broader network of instruments and observational practices. Any malfunction or deficiency in the transit or its associated components, weather technical or human, would propagate through this network, compromising the observatory’s programme of work. In this sense, the Bird transit was the true hearth of the observatory. Moreover, because the meridian defined by the Bird transit served as the fundamental reference for Swedish geodetic surveys, these interdependencies extended far beyond the walls of Stockholm Observatory, linking its internal operations to the spatial ordering of the nation itself.