Achieving Continuity: A Story of Stellar Magnitude

Abstract: Scientists tell a story of 2000 years of stellar magnitude research that traces back to Hipparchus. This story of continuity in practices serves an important role in scientific education and outreach. STS scholars point out many ways that stories of continuity, like many narratives about science, are disconnected from practices. Yet the story of continuity in stellar magnitude is a powerful scientific achievement precisely because of its connection to practice. The historical development of star catalogues shows how specific recording practices connected past and present in a useful way. The narrative of continuity in stellar magnitude, however else it might be subject to STS critique of narrative, maintains its power because of its connection to practice. I suggest that more attention be paid to connections between practice and narrative in STS, and in particular to the ways that historical practices sustain narratives by connecting past and present.

Author: Michael S. Evans

Keywords: astronomy, narrative, practice, constitutive, credibility

Notice: This is the author’s version of a work that was accepted for publication in Studies in History and Philosophy of Science. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Studies in History and Philosophy of Science 41(1):86-94, 2010, doi:10.1016/j.shpsa.2009.12.007. For quoting or citing, please refer to the published version.

1 Introduction

The stellar magnitude scale is one of the oldest scientific standards of measurement still in use, dating back to observations by Hipparchus in 130 BC and the publication of a stellar magnitude reference table in Ptolemy’s Almagest almost three hundred years later. Astronomy textbooks and most popular accounts tell the story of how the magnitude scale in use today is not only a direct descendant of the original magnitude scale produced by Hipparchus, but is essentially the same scale as when Hipparchus visually catalogued star positions and brightness in the second century BC (Birney, 1969; Carroll and Ostlie, 1996; Roy and Clarke, 2003). Introductory astronomy courses teach a consistent stellar magnitude story, with Hipparchus at one end and modern magnitude systems at the other (Lester 2003). Even official government science policy and outreach organizations perpetuate the origin story of the Hipparchus system of magnitude to lay persons interested in learning more about scientific practice (NASA, 1995; NASA, 1998; CUAD, 2003).

But is it really plausible that any scientific standard has remained unchanged for 2000 years? What astronomy claims about stars today is different even from what it claimed 50 years ago (see Burbidge, Burbidge, Fowler, and Hoyle 1957). The meaning of ‘magnitude,’ or of ‘star,’ or of ‘astronomy’ could not have been the same because of language differences, or cultural factors, or changing understandings of celestial mechanics over a period of 2000 years. Instruments ranging from the human eye to optical telescopes to advanced charge-coupled devices (CCD) require dramatically different methodologies in their use and interpretation. With so many changes over time and space, the story of stellar magnitude could easily be told in terms of its discontinuities.

For many scholars in Science and Technology Studies (STS), such discontinuities illustrate how narrative can never be about what scientists really do. Practices are messy. Narrative provides order and continuity where such order and continuity do not exist. On this view, scientists tell stories to achieve some goal, such as getting institutional support (Latour, 1988), emphasizing the novelty of discoveries (Gooding, 1990), resisting outside influence (Traweek, 1992), or attracting popular interest (Curtis, 1994). Such narratives do not depend on, or describe, the complexity of scientific practices. Rather, they present or represent a particular version of what happened to a particular audience for particular reasons. A story of continuity in stellar magnitude, then, is not a story about what astronomers have done for 2000 years, but an attempt by astronomers now to claim authority and credibility for themselves.

In this paper I argue that practices matter to narrative, particularly when disciplines have use for past data. Specifically, I argue that historical recording practices in the creation of star catalogues justify a narrative of continuity in stellar magnitude. In this case, continuity is not just a story deployed for strategic purposes, but a powerful scientific achievement tied to historical practice. Practitioners of astronomy took specific and deliberate actions to connect the present and the past in a useful way. To be clear, my purpose here is not to claim that there are really such things as ‘magnitude’ or ‘stars’ that have persisted throughout the centuries, nor to claim a necessary or inevitable progression or accumulation of astronomical knowledge. Rather, I show that practitioners took action at different times to connect the past to the present, and that this history of practice justifies the story of continuity in stellar magnitude as a scientific achievement.

This paper has three major parts. First, I review and engage STS literature on narrative and credibility, and agree that such stories can be powerful discursive resources, both in general and specifically in scientific disciplines. However, like many other scholars, I question the presumed disconnection of narrative and practice that underpins much (though not all) STS work on narrative and credibility. Second, I move on from criticism and consider stellar magnitude as an historical case for the view that practice can justify narrative. I examine several historical star catalogues, looking at sources of discontinuity and how practitioners developed responses to problems both experienced and anticipated as they published these reference works for future practitioners. Finally, I evaluate the historical case in light of my original proposition. I find that practices matter in important ways to the story of continuity in stellar magnitude. In this sense, continuity is not just a narrative constructed to gain credibility. It is a powerful scientific achievement. However, I caution that this case is only suggestive rather than conclusive, given astronomy’s unusual and specific need for historical data.

2 A Story of Stellar Magnitude

Students of astronomy, whether in a classroom or in a library, typically encounter a common story of stellar magnitude, which I will recount here in basic form.

Stellar magnitude is the measure of how bright a star appears to an observer. The measurement of stellar magnitude has its origins in observations made by Hipparchus around 130 BC. Hipparchus established a scale of apparent brightness ranging from 1 (brightest) to 6 (faintest) and recorded values for the celestial objects that he could see. Though no writings of Hipparchus survive, it is generally accepted that Ptolemy published the Almagest, the oldest surviving star catalogue, partly based on Hipparchus’ observations. Subsequent astronomers across the world built on this knowledge and recorded additional magnitude information as they encountered it. With the development of telescopes, astronomers extended the scale to include even fainter magnitudes. In 1856, Norman Pogson discovered a logarithmic fit for the gradations of the stellar magnitude scale, allowing the extended scale to be standardized beyond the original six magnitudes. Today, we extend the scale and record it precisely with better instruments. Hipparchus’ scale has remained essentially unchanged throughout the present day, and astronomers are part of a 2000-year continuous research tradition in measuring the brightness of stars.[i]

In this story of stellar magnitude, the central claim is a claim of continuity. This story of continuity connects astronomy of the past to the present, while also making important implications about those who practice astronomy today. In this particular case, the story of continuity contributes to scientific credibility, which Steven Epstein (1996, p. 3) defines as the ‘capacity of claims-makers to enroll supporters behind their arguments, legitimate those arguments as authoritative knowledge, and present themselves as the sort of people who can voice the truth.’ Astronomers are reliable, since they have been consistent in their practice for 2000 years. Astronomers are trustworthy, since they draw on 2000 years of evidence and analysis. Astronomers speak the truth, because conclusions that survive 2000 years of research are robust. The story of continuity implies that astronomy and astronomers are more credible because they have successfully linked the past and the present. In this sense, the story of continuity is a powerful discursive resource.

In general, it is largely uncontroversial to say that stories are powerful discursive resources. Stories are uniquely important to making sense out of experience (Bruner, 1991). Stories can be deployed instrumentally to gain, control, and mobilize resources, or to exclude others from those resources. In abstract terms, narrative is part of the symbolic arsenal available to participants in an agonistic discursive field, of which science is a notable example (see Bourdieu, 1975). Of course, not all stories are equally powerful, nor are all resources gained through story-telling equally useful. The key point is that stories are a discursive resource whose validity is not necessarily dependent on ‘empirical verification or logical requiredness’ (Bruner, 1991, p. 4).

A persistent theme in STS is that narrative has discursive power, but that this power is derived from how narrative is constructed for a particular audience, rather than its connection to practice. This idea comes from Ludwik Fleck’s 1935 (1979) work on the social production of scientific facts. Fleck distinguished between ‘journal science’ and what he called ‘vademecum science,’ or handbook science. Journal science is closer to practice, and reflects the messy, personal, provisional aspects of science. Vademecum science emerges as such knowledge moves through the scientific community, where either it is made clean, impersonal, and certain, or it is discarded (Fleck, 1979, pp. 118-120). The complexities of practice, once evident in journal science, are effaced as a community of scientists constructs a simplified narrative. Messy practices become sanitized facts.

Drawing on this intellectual tradition, later scholars in what became STS emphasized the disconnection of narrative from practice. Laboratory studies found that scientific narrative did not at all reflect the daily activity of scientists (e.g. Latour and Woolgar, 1979; Knorr-Cetina, 1981). Rather, stories served to translate scientific practice in a way that accomplished particular goals, obtained particular resources, and gained the support of particular allies (Latour, 1987; 1988). While not always sharing the same theoretical grounding as laboratory studies, studies of scientific controversy also supported the disconnection between narrative and practice. Historical case studies showed how practitioners often settled debates by using narrative to appeal to the right kind of allies at the right time (Shapin and Schaffer, 1985), or by using narrative to draw exclusive and durable boundaries around people and resources (Gieryn, 1983).

Some recent work in STS continues this focus on the instrumental use of narrative to gain resources from particular audiences. This is perhaps most obvious in the dissemination of scientific research to a broader audience, whether scientific or general. For example, Gooding’s (1990) historical case study of Michael Faraday’s electromagnetic motor shows how Faraday minimized the many failed research strategies he had pursued. By constructing a narrative of discovery, Faraday gained credibility as an innovator from other practitioners. In a more recent case, Hedgecoe (2001) finds that genetic scientists studying schizophrenia construct a narrative of ‘enlightened geneticization’ to avoid politically-charged extremes of genetic or behavioral explanations, thereby attracting more funding for their research program. Ron Curtis (1994) has demonstrated that scientific journalism is most appealing when it is structured in familiar narrative forms, such as the detective story where the intrepid scientist leaves no stone unturned in tracking down the culprit, even though such forms necessarily leave out important details of scientific practice. And Turner (2001) has shown how government regulatory agencies not only construct narratives to appeal to target audiences, but reconstruct such narratives as necessary to achieve public support for regulatory policies. Again, the discursive power of narrative is not linked to practices, but to practitioners, who deploy stories instrumentally as part of the ongoing struggle to gain and maintain credibility (and its attendant benefits).

One response to this view has been to claim that narrative actually drives practice. In this view, while narrative remains distinct from practice, it is not disconnected. For example, Rees (2001) notes in her study of the primate infanticide controversy that practitioners in the field regularly construct and reconstruct narrative accounts not only to create new meaning, but to manipulate and control research subjects. Mialet (1999) further identifies how narrative drives performativity in practice, giving an example where the story of the lone inventor is reinforced through retelling and redistribution of intellectual credit by other practitioners. Mialet finds that people who hear the story of the lone inventor do things to make the story true, even when they are aware of practical complexity. In these cases, narrative not only changes the meaning of practice, but also generates new practices.

A more basic and comprehensive response has been to demonstrate how the analytical opposition of narrative to practice in STS is unhelpful at best and damaging at worst. In this view, narrative is an integral and constitutive part of science that cannot easily be distinguished from (other) practices. For example, Joseph Rouse (1996) argues that activities are only recognizable as scientific practices at all because of their place in a contentious narrative field. Through ‘narrative reconstruction,’ scientists constantly negotiate their uncertain position between precedent and possibility by ‘enact[ing] a narrative in the midst of which their present activities (and those of others) would be intelligibly situated’ (Rouse, 1996, p. 27). Similarly, Bono (1990) argues that narrative, as the currency for exchange between scientific discourse and broader social and cultural discourse, provides the common ground for anything like a scientific community to exist at all, even though such exchange also imports particular commitments and produces tensions that may render scientific discourse unstable.

More specifically, Rouse points out that narrative provides a framework for understanding how some practices differ from others (historical or contemporary) in ways that produce and reproduce scientific research and knowledge. For Rheinberger (1997), this constitution of difference is what makes narrative integral to science. To the extent that scientific activity is generative, it is generative precisely because of the ‘differential reproduction’ (p. 82) of epistemic entities, a process sustained in part through narrative. But even as narrative constitutes difference, the differences thus constituted become part and parcel of scientific research. Therefore any activities constituted and recognized as scientific practices contain, in Rheinberger’s (1997, p. 186) terms, ‘remnants of older narratives’ and ‘shreds and traces of narratives that have not yet been related.’

The claim that practices cannot be separated from narrative is especially powerful because it demonstrates how STS claims about the disconnection of narrative and practice are subverted by their own enactment in STS writing. In her broader criticism of STS approaches to narrative, Traweek (1995) recognizes that the disconnection of narrative from practice is a rhetorical strategy that some STS scholars engage in order to make sense of their own research, but points out that that such strategies only make sense within the framework of narratives about “great men, great machines, great laboratories, and great ideas” (p. 431) that figure prominently in stories that scientists tell about themselves. The practice of writing the claim about disconnection does not make sense without reference to a prominent narrative. This is not an abstract criticism. As both Traweek and Haraway (1989) argue, such rhetorical strategies are not just unhelpful in terms of understanding how science works, but also actively damaging, as they can reinforce particular power relationships that reproduce broader narratives about prestige and hierarchy related to cultural categories such as gender, race, and national identity.

I present the case of stellar magnitude to stake out a moderate position between the earlier STS claim about disconnection and the responding claim that narrative and practice are constitutively linked. In contrast to some STS claims about disconnection between narrative and practice, I claim that the narrative of continuity in stellar magnitude is connected to a history of practice. In this sense the current paper is complementary to the broader critique of earlier STS work. But in contrast to the claim that narrative constitutes practice, I argue that historical and current practices in stellar magnitude sustain the narrative of continuity. While I do not dispute that practices are constrained or made sensible by the contentious narrative field in which they are constituted and recognized, I emphasize that in some cases, and in particular those cases that have prominent historical dimensions, practices can make particular stories possible.

Studies of laboratory practice and of scientific controversies are often historical in the sense that they study past activity, but not always historical in the sense that they see events as part of an historical trajectory (see also Rheinberger, 1997). This slicing of time into case studies, which is necessary for ethnographic work and analytically convenient for controversy studies, can efface important historical connections. I argue that in the case of stellar magnitude, history matters. Astronomy has need for the past. This is not to say that practitioners were aware of their place in an overarching narrative of progress and merely did their part. Nor is it to say only that practices only made sense within a particular narrative field at a particular historical moment, which necessarily incorporated elements of previous narratives and practices. My claim is that, at various times and places, practitioners took deliberate action to connect the past to the present in a useful way. The result is a story of continuity that is not just a rhetorical strategy at one particular historical moment or an artifact constructed from narrative remnants, but a powerful scientific achievement.

In the next section I discuss how the story of continuity is tied to a history of effort. I show how observers took specific actions to achieve and maintain continuity, in particular through the formal recording and reproduction of stellar magnitude data in star catalogues. Before proceeding to the historical examples, however, I should specifically recognize two points. First, I recognize that in reproducing the categories of ‘narrative’ and ‘practices’ in this paper that I am subject to some of the same criticisms as earlier STS scholars. What counts as ‘scientific practice’ or ‘historical narrative’ is part of what is at stake in contentious discourse about these issues. To be clear, I do not seek to make a universal claim about distinctions or provide a reference definition. My point is about what kinds of stories are made possible by the past, and about what makes those stories possible. The most useful way to situate this point in the existing scholarly literature is to speak of ‘narrative’ and ‘practices,’ and I accept the requisite challenges directed at this rhetorical strategy.

Second, I recognize that in linking narrative to practice, I may appear to be claiming that continuity in stellar magnitude is ‘what really happened,’ thus taking sides in a long-standing methodological debate between STS and more traditional history of science and technology over analytical uses of narrative (see Buchanan, 1991; Jasanoff, 2000; Croissant, 2003). To be clear, my point here is not that STS analysis of narrative and practice is wrong-headed or inaccurate. Sometimes practitioners deploy stories to get things for themselves. Sometimes practitioners make sense out of their activities by framing them in narrative terms. And sometimes, people hear a story and do things to make that story true. But also, I argue, a narrative can emerge as the result of a diverse set of historical practices. In the case of stellar magnitude, these historical practices, however different and disconnected from each other, justify the narrative of continuity that students of astronomy encounter in the classroom and the textbook.

3 A History of Stellar Magnitude

The biggest challenge in studying the history of stellar magnitude is how to select good evidence from over 2000 years of history. My research question is simple. Do historical practices in stellar magnitude justify a story of continuity? To answer this question, I chose to investigate a particular type of object that embodies the results of practice in stellar magnitude. Specifically, I looked to star catalogues, where practitioners record observational data.[ii] The star catalogue, as a genre, is the formal presentation of the results of observations of celestial objects published in book form, as distinguished from specifically mathematical treatises on celestial movement, from star tables used for chronological or astrological purposes, or from observing reports written in a narrative style.[iii] As with any genre, star catalogues display a range of forms within the bounds of the category, and many individual star catalogues do not represent the same information in the same way.

But which star catalogues make good evidence? Even focusing on star catalogues as sources, how could I investigate a 2000-year history in a meaningful way? For this investigation I forged a path bounded by a combination of textual significance and availability of source material. For textual significance, I looked at references within star catalogues to see how these catalogues referred to one another over time. By significance I mean that a star catalogue remained an important reference to other star catalogues, not that it is or was more accurate or correct in some anachronistic sense. In terms of availability of source material, I had access to archives containing either original documents or faithful reproductions of star catalogues by Argelander, Bayer, Bayer, Flamsteed, and Pickering. In other cases I had access to partial reproductions such as scanned images, or to translations of originals, as with catalogues by Ptolemy and al-Sufi. I supplemented these primary sources with information from biographies (e.g. Thoren, 1990), historical monographs (e.g. Grasshoff, 1990; Hearnshaw, 1996), contemporary correspondence (e.g. Forbes, Murdin and Wilmoth, 1995), analyses of catalogue data (e.g. Knobel, 1885; Eichhorn, 1974; Schmidt, 1994), and other scholarly analysis of practice in stellar magnitude (e.g. Mayer, 1992; Zissell, 1998).

The result is not a comprehensive or exhaustive list of star catalogues or related practices. I cannot support claims about all star catalogues or all practices, nor do I intend to make such claims. However, I can and do point out examples of specific practices in the production of star catalogues that connect past and present. I note that these practices, while sometimes similar across practitioners, responded to local concerns and contemporary challenges. Discontinuities of language, culture, climate, and resources across 2000 years of history are multifarious. The point is not that these practitioners all followed the same script, but that in responding to the challenges they encountered, they connected the past and the present in a useful way. Such practices sustain the narrative of continuity in stellar magnitude.

In this section I first look at the origins of stellar magnitude in the work of Claudius Ptolemy. I then emphasize how the formal precedent of Ptolemy’s Almagest was produced and reproduced in practice, and how al-Sufi, Tycho, and Bayer took specific steps to ensure the continued connection of past, present, and future. Finally, I look at how observers using telescopes extended the stellar magnitude scale, and in particular how they connected the past and the present to maintain continuity with observational data going back to the Almagest. Again, while I offer examples in roughly chronological order, this is not intended to suggest a teleological or accumulative trajectory of astronomical knowledge. To the contrary, I suggest that the reason continuity is a significant achievement is precisely because these practices occurred across time and space, regardless of the many sources of discontinuity.

3.1 Origins: Hipparchus, Ptolemy, and the Almagest

According to astronomer Ronald E. Zissell, ‘Any historical survey of stellar magnitudes begins with the work of Hipparchus of Nicea’ (Zissell, 1998, p. 151). The Carroll and Ostlie textbook An Introduction to Modern Astrophysics (1996, p. 66) claims that ‘[t]he Greek astronomer Hipparchus was one of the first skywatchers to catalog the stars that he saw’ and that ‘[i]n addition to compiling a list of the positions of some 850 stars, Hipparchus invented a numerical scale to describe how bright each star appeared in the sky.’ Annemarie Mayer (1993:283) writes that ‘[a]lthough there are some discrepancies in the astronomical literature, the Greek astronomer Hipparchus…generally is considered to be the first one to have produced a star catalogue which indicated the positions and comparative brightness of several hundred naked-eye stars.’ Jay Pasachoff (2003:1) points out that ‘the magnitude scale is thought to have come from the work of Hipparchus in about 150 B.C., though the original references are lost.’ In short, the majority of scholarly, popular, and educational sources agree that Hipparchus, born in Nicaea, created the first star catalogue around 130 B.C., in which he recorded the positions and magnitudes of approximately 850 stars visible from his observatory in Rhodes (Pannekoek, 1961; Schmidt, 1994).

Yet the oldest published star catalogue that remains relatively intact is contained in the Almagest, a massive work by Claudius Ptolemy originally written sometime around A.D. 150 (Toomer, 1984, p. 1; Grasshoff, 1990, p. 1) Among other items of mathematical and astronomical interest, the thirteen volumes of the Almagest contain a star catalogue in tabular form, listing descriptions, locations, and magnitudes for 1028 celestial objects organized by constellation. The documentary evidence, at least, seems to point to Ptolemy as the creator of the first star catalogue, and therefore the founder and innovator of the genre.

But, as Tycho Brahe discovered in the late sixteenth century, celestial positions recorded in the Almagest do not match up with the positions one would have expected to see if observing from Alexandria around AD 137 (Brahe, 1987 [1602], p. 145). This discrepancy is the source of the origin story of stellar magnitude. Ptolemy’s claim that ‘we observed as many stars as we could sight down to the sixth magnitude’ in Almagest VII.4 is a straightforward declaration of personal observation (Toomer, 1984, p. 339), but the later discovery of Ptolemy’s error opened the door to questions of authorship. Who was the observer, or observers, whose work appeared in the Almagest?

Relatively few texts from the period have survived. However, there is a body of evidence suggesting that Hipparchus, astronomer at Rhodes, is that observer. Pliny referred to Hipparchus in his work on natural history, and claimed that Hipparchus had created a record of star positions and magnitude. In Almagest V.2, Ptolemy declares ‘[w]e were led to awareness of and belief in this [second anomaly] by the observations of lunar positions recorded by Hipparchus’ (Toomer, 1984, p. 217). The Almagest refers to On The Length of the Year, On Intercalation of Months and Days, and On the Change of the Solstices and Equinoxes as important works of Hipparchus, and Ptolemy offers full credit to Hipparchus for the information provided in those texts (Pannekoek, 1961, p. 125). While not everyone agrees that Ptolemy borrowed from Hipparchus, the official story as promulgated through science education is that Hipparchus is the observer whose data formed the basis for the Almagest’s star catalogue (but see Evans, 1987a; 1987b for a summary of the debate).

The Almagest set two important precedents of practice for stellar magnitude. First, the original magnitude scale, as published in the Almagest, rated the brightness of stars using the numbers 1 through 6. It is an inverse value scale, with a lower number indicating greater brightness, making it something of an anomaly in the history of scientific measurement. Second, the scale was not simply expressed as whole numbers, but in some cases as whole numbers with indicators that express ‘greater than’ and ‘less than’ relationships. For example, in modern mathematical notation, the Almagest magnitude for the first star in Cygnus, the ‘star on the beak’, would be ‘3’, while the magnitude for the third star in Cygnus, the ‘star in the middle of the neck’, would be ‘>4’ (Toomer, 1984, p. 350). The stellar magnitude scale did not attempt to provide an absolute measurement based on more precise fractional quantities, instead expressing categories in whole terms while providing relative flexibility within a categorical slot.

Magnitude data was therefore distinct and identifiably different from other data published in the Almagest, and provides an excellent basis of comparison with other star catalogues in order to determine the long-term significance of the observational choices possibly made by Hipparchus and certainly published in Ptolemy’s star catalogue.

3.2 Reproducing Magnitude: Al-Sufi, Tycho, and Bayer

From the middle of the first millennium AD through the early sixteenth century, further activity on Hipparchus and Ptolemy’s observations occurred primarily in the Arab world, by which I mean both the Arabian peninsula and later the Islamic tradition that traveled through the vehicle of the Arabic language. Around AD 964, Abd Al-Rahman ibn Umar al-Sufi published his star catalogue, entitled The Book on the Constellations of the Fixed Stars, or simply Book of Fixed Stars. Al-Sufi undertook a direct observation of the stars listed in the Almagest. For stars that he could find, he recorded updated positions and magnitudes. He also added stars that he could see that were not listed in the Almagest.

Al-Sufi connected past and present by following the formal precedent of the Almagest, recording in tabular layout 48 constellations by name, order within constellation, latitude, longitude and magnitude. As in the Almagest, al-Sufi also preserved the flexibility of the magnitude scale by providing comparisons across stars, both to reference new observations against known bright stars and to provide order within a constellation where two stars were not exactly the same brightness but would fall into the same numeric “bin” in the magnitude scale. His solution, similar to that of the Almagest, was to record questionable magnitudes as ranges, with the first number indicating the higher probability magnitude and the second indicating the trend. For example, a star with a probable magnitude of ‘3’, but which was slightly fainter than another star of magnitude ‘3’, might be recorded as magnitude ‘3-4’ (Zissell, 1998, p. 152). This allowed useful comparison against Almagest data.

But al-Sufi also recognized additional challenges related to the transmission of astronomical data, and incorporated two significant practices to make the Book of Fixed Stars a useful reference for later practitioners. First, in addition to the tabular presentation of data, al-Sufi provided illustrations of each constellation, superimposing on the star map the figure of the mythical object traditionally represented by the constellation names. In these illustrations, the size of the dots indicating celestial objects roughly corresponds to their relative magnitude as he observed it (al-Sufi, 1965, Plate 22B).[iv] Thus a later observer could use either, or both, textual and graphical information to inform their own work.

Second, al-Sufi recorded his magnitude numbers not as letters or numerals, but as words, in order to minimize future copy errors by those less familiar, or less careful, with numerals. By taking both textual and graphical precautions, al-Sufi not only connected past to present, but also anticipated possible challenges to future practitioners wanting to use his work as a reference for their own. Al-Sufi’s Book of Fixed Stars remained the standard reference in Arab astronomy for the next several hundred years, and his magnitude numbers were recopied in Ulugh Beg’s Book of Fixed Stars for epoch AD 1437 (Knobel , 1885; 1917).

In the late sixteenth century, Tycho Brahe independently observed many hundreds of previously observed celestial objects from his observatory in Denmark. Tycho used as references his own copy of the Almagest as well as copies of Arabic star catalogues (Grasshoff, 1990, p. 23), and produced the first star catalogue in Europe. Tycho’s catalogue of approximately 780 stars, titled Astronomiae instauratae Progymnasmata, was printed in full in 1602 after his death (Swerdlow, 1986, p. 190; Thoren, 1990, p. 421).

Tycho’s contribution is notable precisely because it did not break significantly from formal precedent. Even while presenting the first real explication of Ptolemy’s major error in the Almagest,[v] Tycho went to great lengths to preserve the same form as the Almagest. For example, though he had directly measured star positions in equatorial coordinates (right ascension and declination), he published his findings in the form of ecliptical coordinates (latitude and longitude). This practice of converting his observations into ecliptical coordinates connected his observations to the Almagest and other previous star catalogues, which also used ecliptical coordinates. Also, as with the Almagest and the Book of Fixed Stars, Tycho’s star catalogue employed a system to indicate greater and lesser gradations for comparison of like magnitudes, using dot notation to grade magnitudes in thirds. This practice enabled relative comparison of magnitudes against older catalogue data, making variation over time visible to practitioners.

One year later, Johannes Bayer published the Uranometria, using Tycho’s position and magnitude numbers for the original 48 constellations and adding observational data of his own for 12 more constellations (Brahe, 1969 [1602]; Bayer, 1987 [1603]; Swerdlow, 1986). Like al-Sufi, Bayer recognized the challenge of transmitting star catalogue data across different languages and cultures, and incorporated recording practices not only to connect to past catalogues, but to minimize transmission and communication errors to future practitioners. First, Bayer presented a new way of visualizing celestial objects in the form of accurate star maps, allowing readers to locate objects by position as well as appearance, based on the latitude and longitude numbers that served as grid axes for the sector maps and the relative brightness of the stars in a given sector. Bayer’s illustrations are pictorial representations of Tycho’s observational data (which drew from Ptolemy and al-Sufi) plus his own. Second, Bayer created a systematic practice (later to be called the ‘Bayer number’) for identifying the relative hierarchy of magnitude within a constellation grouping. In addition to the magnitude number established by Tycho, Bayer assigned a Greek or Latin alphabetic character to each star in a constellation to indicate where it landed in that constellation’s magnitude hierarchy. Thus Aldebaran, the brightest star in Taurus, became alpha Tau, the next brightest beta Tau, and so forth (Bayer, 1987 [1603]).

3.3 Extending Magnitude: Flamsteed, Herschel, Pogson, and Pickering

With the introduction of the telescope, the practical possibilities for stellar magnitude measurement outpaced its formal framework. Objects invisible to the naked eye became visible when viewed through a light-amplifying telescope. John Flamsteed, the first holder of the office of Astronomer Royal of Britain, used his own telescopes to observe and record positions and magnitudes for newly observed objects as well as objects noted in previous texts. His Historia Coelestis Brittanicae, published in full in 1725 after his death, contained a “large and compleat catalogue of about 3000 fixed stars, with chartes of all the Constellations here visible” (Forbes, Murdin and Willmoth, 2001, p. 22).

Flamsteed’s Historia is significant in two ways for the story of continuity in stellar magnitude. First, he spent significant time and effort to create a comparative set of tables containing the observation data from Ptolemy’s Almagest, Ulugh Beg’s catalogue (which contained al-Sufi’s magnitude data), Tycho’s Progymnasmata, Bayer’s Uranometria, and some published observations by Hevelius. The large format of the Historia allowed for many columns of data, and Flamsteed took advantage of this by comparing different pairs of observers on position and magnitude, often including his own data for comparison or noting the differences as corrections. As with the Almagest, the Historia orders its tables by constellation, and by classical sequence within the constellation, though Flamsteed also included the Bayer numbers for reference (Flamsteed, 1725). This table not only connected past data to the Historia, but also connected other star catalogues to one another in a direct and useful way.

Second, Flamsteed did not confine magnitude measurements for his own observations to the six magnitudes in the Almagest scale. In a method similar to that of al-Sufi (and later of Hevelius), but applied to fainter stars, Flamsteed often recorded questionable or intermediate magnitudes in ranges, with the first number as primary and the second number as the trend, for example ‘6-7’. But other astronomers did not take up this practice unproblematically. Though Friedrich Argelander’s Uranometria Nova, published in 1843, illustrated only those stars of sixth magnitude and brighter (Argelander, 1843), several other astronomers after Flamsteed continued to estimate magnitudes of fainter stars using telescopes. Francis Baily, the first president of the Royal Astronomical Society, compiled observation data from Flamsteed, Bradley, Lacaille, and many others into The Astronomical Society Catalogue, including position and magnitude for approximately 2880 stars. He followed shortly thereafter with another compilation, the Catalogue of Stars of the British Association, which incorporated even more observations to cover approximately 8370 stars. These catalogues made clear that for magnitudes fainter than 6, practitioners varied widely in their assessment of stellar magnitudes as observed through telescopes.

Different systems of extending magnitude presented a challenge to recording practices. For earlier catalogues based on naked-eye observations, newly noted stars could still be compared to existing stars with magnitude measurement, and recorded using established practices of star catalogue production. But without a standardized practice for identifying and recording fainter magnitudes, the connection to past data, and the possibility of noting important variations over time, could be lost. William Herschel proposed a mathematical solution. In his own observations and in his serially published star catalogues, William Herschel attempted to formalize the magnitude system beyond the six categories of the Almagest, first by listing “sequences” of relative star brightness within constellations using printer’s punctuation marks (Herschel, 1796), and then by proposing a mathematical relationship where the magnitude number of a star (m) was also the multiplier for its distance from the observer (Herschel, 1817, pp. 308-313). William Herschel’s geometric relationship was directly challenged by other observers, notably Carl von Steinheil of Munich, who was the first to propose an alternative, logarithmic relationship between magnitudes (Hearnshaw, 1996, 59-61).

In 1856, as a response to the different proposed mathematizations of magnitude, Radcliffe Observatory assistant Norman Pogson used magnitudes from the star catalogues of Argelander, Bessel, Piazzi, Lalande, and Groombridge to set up an experiment using the method of reduced apertures to determine a precise mathematical relationship between magnitude categories. Instead of an observational comparison between stars, however, Pogson simply reduced the aperture on a single telescope until total extinction occurred, which allowed him to take a given star’s magnitude from a catalog and record a corresponding aperture measurement for it, then compare the various measurements to determine if a systematic relationship existed (Pogson, 1856). Pogson engaged in this practice not only to evaluate fainter magnitudes, but to evaluate and link historical star catalogue records to newer telescopic observation.

Pogson found that the magnitude scale yielded a logarithmic relationship between its categories. A star of first magnitude was brighter than a star of second magnitude in the same ratio by which a star of second magnitude was brighter than a star of third magnitude, and so forth throughout the scale. Though the exact value for this ratio was not certain to multiple decimal places, Pogson nonetheless selected a ratio of 2.512 to represent the true value. While this number may seem arbitrary, it had the redeeming attributes of being relatively close to measured values from other historical and contemporary observers, and of being easily calculable, for if the ratio is 2.512, then a first magnitude star is exactly 100 times brighter than a sixth magnitude star (Jones, 1968, p. 4).

Pogson’s ratio provided the missing element in the magnitude scale that allowed it to extend beyond the limits provided in the Almagest, and allowed a unification of extended telescopic magnitudes based on a common system of derivation. It is perhaps less appreciated, though no less important, that the association of magnitude numbers with a mathematical scale provided a systematic apparatus for error detection and correction in existing star catalogues, and for error avoidance in future catalogues. There is little dispute that the Pogson ratio became the de facto standard for magnitude by the beginning of the twentieth century, due mostly to the efforts of E. C. Pickering of the Harvard Observatory and his use of the Pogson ratio in the 1884 Harvard Photometry.

Pickering published the Harvard Photometry with new observations of 4260 stars for epoch 1880 using the large meridian photometer at the Harvard Observatory. In form, the Harvard Photometry does not deviate significantly from the established star catalogue genre, but Pickering did make a special effort to connect Harvard Photometry to historical star catalogues. Harvard Photometry lists stars by catalogue number (with cross-references to Bayer and Flamsteed), constellation, equatorial position, number of observations, reference magnitudes from Argelander (1843), Heis, and the Durchmusterung, both photometrically measured and optically estimated magnitudes, and range of probable error (Pickering, Searle, and Wendell, 1884).

Pickering’s work also provides a systematic comparison of his newly observed magnitudes with those of Ptolemy, al-Sufi, William Herschel, Argelander, and more contemporary work by Gould, Peirce (1878), and others. Specifically, Pickering compares those works that have any system of indicating range within a magnitude, such as Ptolemy’s ‘greater than,’ or Herschel’s printer’s punctuation system, with his own observations, and summarizes the conversions in a table in addition to listing specific magnitude cross-references for every identifiable common star (Pickering, Searle, and Wendell, 1884, p. 93). As with Flamsteed’s work, Pickering’s catalogue maintains the link to past observations through side-by-side comparisons, while reproducing the form of the star catalogue genre with new observations. By assigning a numeric value to other notation schemes, such as 3.7 for al-Sufi’s ‘3-4–4-3’, Pickering could support systematic and mathematical analyses of relationships between and among historical star catalogue magnitudes.

By accepting the Pogson ratio as the basis for an comprehensive treatment of stellar magnitudes, Pickering’s Harvard Photometry star catalogue extended the magnitude scale published by Claudius Ptolemy. While this is important by itself, its importance is amplified by Pickering’s concerted effort to connect magnitude back through previous star catalogues all the way to Ptolemy’s Almagest. By providing both a systematic link to the past and a formula for connecting to the future, Pogson’s mathematization of the magnitude scale, as expressed in Pickering’s Harvard Photometry, preserved a sense of continuity in the measurement of stellar magnitude, and made it possible for the formal framework of magnitude to align with the practical possibilities presented by technology.

4 Continuity as Achievement

In the historical examples above, I have noted several practices that connect past and present. Many of these practices are similar over time, and astronomers today engage in similar recording practices. Like their predecessors, recent star catalogues report visual magnitudes alongside constellation number, position, and proper motion (e.g. Hoffleit, 1964). And Eichhorn (1974, pp. 103-106), for example, provides a checklist of pertinent questions for referring to older star catalogues. The checklist is designed to help an astronomer make all of the corrections necessary to account for problems unknown at the time a particular catalogue was published. Such ongoing connection of past and present enables analysis that is simply not possible in many other scientific disciplines.

At several points in the history of star catalogues, for different reasons at different times, astronomers engaged in specific practices to connect their past to their present. Al-Sufi used words rather than numbers to avoid the possibility of future copy errors. Tycho reported star positions in ecliptic rather than equatorial coordinates to remain comparable to the Almagest and Arab star catalogues. Flamsteed assembled a massive compendium of comparison data to connect star catalogues of the past to the Historia and to one another. Pickering employed the Pogson mathematization to connect naked-eye magnitudes of the past to telescopically-observed magnitudes of the present and future, providing conversion tables for older catalogue data.

Each of these examples of practices highlight the deliberate consideration of connecting past and present in the historical practice of celestial observation. These efforts were extended to the magnitude scale, as astronomers measured and reported brightness in the terms established in the Almagest. Because of these practices, stellar magnitude of the past is connected to stellar magnitude of the present in useful ways.

It would be a mistake, however, to point to the history of star catalogues and claim continuity simply because of formal similarity in the production of published documents. Those familiar with the stress and strain involved in the actual achievement of consensus, with the process of negotiation that leads to the production of knowledge, will certainly question the progressive and teleological march of magnitude as the aegis of knowledge under which more and more celestial objects are gathered. It would be naive to suggest that magnitude for Hipparchus was the same thing as magnitude for Flamsteed, or that Bayer’s Aldebaran is the same as Argelander’s Aldebaran. Magnitude as a recorded result may be consistent across the genre, but this does not necessarily imply that the path to magnitude was consistent, or that ideas of what magnitude represented constituted any sort of common sense.

Yet it is entirely possible that astronomers could have stopped using the magnitude scale of the Almagest for measuring the brightness of celestial objects, or that they could have abandoned such measurements as unimportant. Even today some astronomers question the utility of stellar magnitude measurement (Pasachoff 2003), identify it as a source of confusion (Schulman and Cox, 1997, p. 1006), and refer to it as ‘a complicated mess’ (White no date). Nevertheless, the genre of the star catalogue was respected through continued reproduction, and each star catalogue reinforced the set of practices represented in its published results, linking past to present. The results are a scientific discipline that has access to its past in useful ways, and a story of continuity justified by practice.

The historical examples I have provided here demonstrate that the story of continuity in stellar magnitude, as told to aspiring astronomers, students, and public audiences, is not simply a story disconnected from practice, but a story made possible by practice. Certainly there is discursive power in such a story, as many scholars in STS would suggest. Certainly there are performative elements driven by narrative, as some of the ‘narrative drives practice’ scholars suggest. And certainly, at any given historical moment, narrative mattered to recognizing activities as relevant, different, and significant practices, as many scholars would claim.

But it is important to remember that in some cases, practices matter to narrative, and this is particularly true when scientific disciplines have need for the past. In this case, I find that historical practices in the production of star catalogues justify a narrative of continuity in stellar magnitude. It is still possible to gloss over 2000 years of problems and present a simple story of continuity from Hipparchus to the present day, not as a narrative disconnected from practice, but as a narrative made possible and sustained by historical practices linking past and present. This is a powerful scientific achievement. The story of continuity in stellar magnitude can be seen as true, not just because of what science is or who scientists are, but because of what scientists did (and do).

But it would be optimistic at best to characterize this achievement as the outcome of a designed system. Many stories fail. In many ways stellar magnitude is a scientific success story, but for every stellar magnitude there is a phlogiston, or Philosopher’s Stone, or æther, where the narrative was not, or could not be, maintained in the same way through practice. Though the very sources of variation that problematize the idea of continuity can resolve, in some cases, into a story of continuity, such outcomes are not predetermined by a deliberate process. Rather, they are only recognized after the fact. In the case of stellar magnitude, continuity is an achievement resulting not from a common dedication to a particular meaning or cosmology, but from a contingent and ultimately successful reproduction of practice over time and space.

The concept of continuity as achievement has broader implications for STS, which I will only briefly engage. As a discipline, STS is grounded in sustained criticism of the stories that science tells about itself. However, the focus on criticism may blind STS to the ways in which stories can be true, despite robust STS critique. Put another way, a robust critique should not be the stopping point of STS analysis. Rather, it should be the beginning of a more sustained analysis. I do not argue against STS critique, as it is useful and indeed necessary. However, the story of continuity in stellar magnitude suggests that some stories do not lose their power in the face of STS critique. This in turn suggests that STS scholars might craft critique in way that matters to the discursive power of stories, not just within STS, but to a broader audience. How this might be accomplished remains an open (and empirical) question, but it is clear that there are important ways that stories, and their discursive power, persist through practice. To the extent that STS criticism does not engage this connection between narrative and practice, it misses an important analytical opportunity.

5 Conclusion

There is a productive lesson to be learned from the case of stellar magnitude. Stories are not just discursive resources that lose their power in the face of robust criticism. Rather, they also exercise and generate discursive power through their connections to scientific practice. Continuity, in this sense, is not an attribute of science or scientists, but a powerful scientific achievement. It is something that scientists do, whether or not the claims of discontinuity are valid in other ways.

In the case of stellar magnitude, the reproduction of practice and the construction of the stellar magnitude scale through the stabilizing genre of the star catalogue resulted in a story of continuity that can be told by astronomers both to new pupils and to those interested in finding out more about celestial observation. Stellar magnitude is a scientific success story, able to admit the criticisms of discontinuity while continuing as a scientific standard. This suggests some important directions for STS analysis. But while it is tempting to take the historical achievement of continuity in stellar magnitude as a template for understanding scientific knowledge production, there is nothing in this particular story to suggest that such an achievement can be repeated.


I thank the staff at the Ernst Zinner Collection, Special Collections and University Archives, San Diego State University for archival assistance and John M. O’Meara for expert technical guidance. I also thank James Evans, Martha Lampland, Naomi Oreskes, Leigh Star, Robert Westman, and an anonymous reviewer for helpful comments on earlier versions of this paper.


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[i]This is not a direct quote from any single source, but a composite from several sources, including personal conversations with physics and astronomy teachers. For examples of these claims in textbooks, see Birney 1969, Carroll and Ostlie 1996, Roy and Clarke 2003. For an example of an introductory course, see Lester 2003. For outreach programs, see NASA 1995, NASA 1998, CUAD 2003.

[ii]Though the term “star catalogue” is employed here, there is no deliberate implication that any observer knew or understood that what he observed was a “star” in the same sense as modern astronomers understand it. The term “catalogue of celestial objects” is perhaps more appropriate, but also a bit unwieldy.

[iii]The notion of “publication” is not particularly useful for describing the process of creating any given star catalogue, as the meaning of the term is highly variable with regard to specific historical cases. For the purposes of this paper, “publication” describes the result of a historical process of dissemination, not the process itself.

[iv]I note here yet another source of discontinuity in the stellar magnitude story. For Greek, Arab, and early European astronomers, stellar magnitude corresponded to size. Bigger stars were brighter stars. For later astronomers, particularly after the introduction of the telescope, stellar magnitude corresponded to distance. Closer stars were brighter stars.

[v]Tycho ([1602] 1969, p. 145) was the first to note the systematic error, since his observations were independent of the previous numbers rather than adjusted for precession (Duke 2002).

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