Hellenistic Astronomy


Aristarchus’s 3rd-century BCE calculations on the relative sizes of (from left) the Sun, Earth and Moon, from a 10th-century CE Greek copy / Library of Congress Vatican Exhibit

By Dr. Alan C. Bowen
Institute for Advanced Study


In the interval from Aratus and Aristarchus of Samos (third century BCE) to Claudius Ptolemy (second century CE), there was a dramatic restructuring and reorientation in what was called astrologia—a term typically rendered by “astronomy,” though “celestial science” or “science of the heavens” would be better. The primary cause of this change was doubtless the transmission of Mesopotamian astrology to the Greco-Roman world and its subsequent, creative reception. Yet, the details of how this change actually occurred remain elusive (see Bowen 2009). There are from the third century on clear instances in which we can see Hellenistic Greeks adapting Babylonian theory—P. Hibeh 27 (third century BCE), for example, adapts an arithmetic, linear zig-zag scheme for the increase/decrease of the length of daytime throughout a year of 360 days in order to structure an Egyptian festival calendar of 365 days (see Bowen 2008b). But, while there are much later reports of Greek astrological activity after the late third century, the earliest horoscopes in Greek date only from the late first century BCE (see Neugebauer and Van Hoesen 1959). The problem, however, is not just a lack of documentary remains; it is also one of how historians have read the texts surviving from this period.

This problem of reading is actually one of blindness induced by two very wrong-headed assumptions and one questionable mode of argumentation. The first assumption begins with the undeniable truth that astronomy is an exact or mathematical science and then adds that historians should, therefore, isolate and focus on the development and transmission of numerical parameters, mathematical models, and techniques of calculation. Thus Otto Neugebauer, after remarking on the great variety of the notions of astronomy in antiquity, proposes (1983, 35–36, with my italics):

We shall here call “astronomy” only those parts of human interest in celestial phenomena which are amenable to mathematical treatment. Cosmogony, mythology, and applications to astrology must be distinguished as clearly separate problems—not in order to be disregarded but to make possible the study of the mutual influence of essentially different streams of development. On the other hand, it is necessary to co-ordinate intimately the study of ancient mathematics and astronomy because the progress of astronomy depends entirely on the mathematical tools available.

The second is the thesis that historians are duty bound to produce narratives, stories that “make sense” of the extant texts and artifacts.

Both assumptions miss the mark. The first is a false reduction. Granted, it is true that Hellenistic astrologia embarked on a process of increasing technical sophistication which is greatly enriched in the second century CE, notably, in Ptolemy’s writings, although Tihon (2010) provides indications that Ptolemy’s immediate context was more complex than hitherto suspected. The reasons for this development, however, were never purely technical but lay also, for example, in considerations of the relations of this science to other intellectual disciplines and to broader philosophical and cultural concerns about how one should make decisions in the course of living one’s life. Indeed, such considerations put into question the very idea of what astrologia is and who its practitioners are. Thus, the assumption that historians should isolate and concentrate on the mathematical apparatus of this science is no more compelling than the contention that historians of the automobile today should bracket political questions raised by global warming, financial questions raised by an emergent global economy, and cultural aesthetics in favor of accounts of the automobile restricted narrowly to matters of its engineering. To the contrary, in both instances, the technical and the non-technical interpenetrate one another to such an extent that their separation in principle is not an act of clarity but of folly.

As for the second assumption, even if one holds that historians are to produce narratives and is comfortable with the narcissism that this usually entails, the problem is that the documents extant in Hellenistic astrologia either come to us without indication of their authors and provenance or they are typically works by authors who are otherwise unknown and difficult to contextualize. In other words, what survives to this day does not permit a credible narrative. Moreover, the persistent effort to impose on this period Simplicius’ narrative of the development of astronomical theory from the time of Plato to that of Autolycus are predicated on misreadings of what Simplicius writes and are thus nugatory as well (see Bowen 2013b).

Allied with both these assumptions is a curious kind of argumentation in which late citations (quotations, paraphrases, reports) of earlier works are taken to convey historical fact. Plainly, there is no problem when such citations are used to understand the source making the citation and his times. Otherwise, however, these citations have probative value only when there is direct, supporting evidence from what is cited and its historical context. Regrettably, most citations bearing on Hellenistic astrologia are about matters for which there is no such supporting evidence. Yet, rather than refrain from using these citations in reconstructing Hellenistic astrologia, scholars defend their readings of them by claiming that they are not just possible but probable. Yet, our sense of what is probable is largely determined by our culture and the history it tells itself and is hardly a reliable guide to the history of other cultures, especially when so much historical fact is wanting. Indeed, to rely on such a sense in reconstructing Hellenistic astrologia is to project one’s expectations into the past and thus to impede any account that honestly seeks to interpret on its own terms what has survived from Hellenistic times. This kind of argumentation, then, is a method of blindness, a way of failing to perceive the extant documents in favor of what others say about them. The situation is hardly improved when interpreting citations involves mathematical analysis, since the certainty of mathematics still cannot bridge the gap to historical truth. As for the idea that citations by scientific writers should be trusted because they are part of an activity that is by nature objective and free from corrupting passions, this may have passed as solemn truth in the time of Hermann Diels; but today it is derisible.

Accordingly, in what follows, I shall put aside readings of Ptolemy which, in spite of what he actually says, infer that Apollonius discovered the equivalence of the epicyclic and eccentric hypotheses of planetary motion as well as the existence of stationary points in epicyclic motion—almost 100 years before these matters were demonstrably relevant in astrologia—and supports them fallaciously by drawing mathematical consequences from theorems in Apollonius’ treatises (see Bowen 2013b, 244–246). Moreover, I shall not construct a narrative situating the mathematical details of astrologia during the period from Aratus to Ptolemy. Instead, I shall examine several texts from this period to determine what they tell us of the broader intellectual issues and ideas which in fact constrained and inspired the technical development evident in theorizing about the heavens during this time. Since the impact of Babylonian astrology is only first recorded in texts written in the latter parts of the second century BC and later, I shall attend primarily to treatises written in the following two centuries.

Each of the core texts to be discussed is well known. The problem lies in understanding and connecting what they say about astrologia. To date, the scholarly response has been limited to generally negative comments about the worth of the particular claims made. It should come as no surprise, then, that these texts have been relegated to the margins in modern accounts of ancient astrologia. Yet, one of their striking features is that the author typically undertakes to demonstrate the great importance and value of what he writes and contends that he is best qualified to convey this information to the reader. Thus, in the texts to be considered, what is written is to be interpreted not in some artificially imposed (narrative) context nor in a vacuum: these texts have a purpose and the meaning of the sections in which they present astrologia must be grasped by reflecting on these assertions of personal authority and any claims of benefit to the reader.

As I will explain, these texts, when read accordingly, show a very fundamental disagreement about astrologia and why it should be studied. But it would be wrong to suppose that the fundamental question was simply “What is astrologia?” because there was already in place a loose collection of activities and theoretical writings apparently constituting this discipline.[1] In fact, it is more useful to say that the question “What is astrologia?” was taken up in two closely interrelated and ultimately inseparable questions:

(a) What precisely is to be included in astrologia?, and
(b) How does astrologia stand in relation to philosophia, that great body of reflection traditionally focused on the explanation of what there is and of how we should act or, at least, of how we should make decisions?

These concerns for the articulation and demarcation of astrologia, respectively, constitute the framework in which the writers to be considered staked out their positions and advocated their views.

Diodorus Siculus and His Bibliotheca historica

Bust of Diodorus of Sicily, Roman, third quarter of the first century BCE / Vatican Museums, Rome

It is in the proem to the Bibliotheca historica (30 BCE) that Diodorus defines himself in relation to his reader. The first step in his assertion of authority (1.1.1–1.2.8) consists in praise of historical writing and historians. The basic thesis is that, since we all learn from experience, we should be grateful to those who write universal histories because they offer instruction that extends our experience without any of the attendant danger. Granted, the utility of a universal history, that is, of a history rooted in the kinship of all mankind, is primarily political: as Diodorus makes clear, such histories enable us to learn from the successes and failures of others, especially in matters concerning the polis. Still, he does include cultural and intellectual matters that are to be of use to his readers as well. In any event, for Diodorus, the basic value of universal history lies in its enabling decisions about the future. Thus, he concludes, “one must, therefore, acknowledge that the acquisition of this (subject) is in fact most useful for all the circumstances of life” (1.1.4).

The second step is an argument that he is the first to undertake writing a proper, universal history (1.3.1–1.4.5). As he says, previous writers have failed because they have focused on single, self-contained wars fought by a single people or polis. Indeed, he continues, few have tried to write a history running from the earliest times to their own day; and, of those that have, some have done so without dating the events, others have ignored mythic times or the barbarians, some again have not been able to complete their work, and none has carried his account beyond the mid second century BCE. Thus, in spite of the fact that a universal history would be of the highest benefit to mankind, no one before Diodorus has tried to encompass all events and times in a single systematic treatise (1.3.2–3).

After reiterating the shortcomings of current historical writing and emphasizing both the great difficulty of writing a universal history and the promise of its supreme utility (1.3.4–8), Diodorus takes the third and final step in his claim to authority by asserting his special qualifications. First among them is that he has been working at his account for 30 years, much of it spent traveling throughout Asia and Europe (at no small risk to himself, so he says) in order to familiarize himself with the important sites and thus to avoid error.[2] Next, he cites his great passion for this project and the fact that he has had access to documentary materials assembled from around the world in Rome; and last, he notes that he has a good working-knowledge of Latin (1.4.1–5).

Thus, Diodorus would have his readers understand that he is the leading authority and guide to a subject of vital importance to them, and that they may rest supremely confident in the truth of what he has to say and its benefit in their decision-making.

It is worth noting that, for Diodorus, astrologia is an ethnic undertaking: rather than present a single or universal account of astrologia, he describes this science as it is found in Egyptian and Chaldaean culture. Thus, in book 1, after accounting for the Egyptian rulers prior to the Trojan War, he eventually comes to the Egyptian claims of priority in the discovery of writing, the observation of the stars, the theorems of geometry, most of the arts, and the best laws (1.69.1–7); and then to the related matters of priestly education and astrologia. As he tells the story, the sons of priests were trained in the techniques of land measurement (geometria) and arithmetic reckoning (1.81.1–4). The latter, he says, was very useful for work in astrologia, given that

the position and motions of the celestial bodies have received careful observation among the Egyptians (if anywhere else) and they have preserved records concerning each for a incredible number of years, since the study of these matters has been valued from ancient times. (1.81.4)

Moreover, he adds,

they have observed most avidly the motions of the wandering stars, that is, their circuits and stations, and, further, the powers of each relative to the births of animals, of which good effects and which bad effects they are productive. And while they are often successful in predicting to men what is going to befall them in life, frequently they announce upcoming destructions of crops or the opposite, bumper crops, and further, plagues for men and cattle; and from observation over a long time they have foreknowledge of earthquakes, floods, the risings of comets, and all the things which to the many seem impossible to know. (1.81.4–5)

In sum, Diodorus affirms that the Egyptians have expertise in astrologia deriving from long study and observation of the heavens, and that the main thrust of their astrologia lies in the prediction of harm and benefit on the basis of the positions of the celestial bodies.

In book 2, after describing the Assyrian kings and the destruction of their empire by the Medes (2.1.3–28.8), Diodorus turns to those whom he calls Chaldaeans, a priestly caste, known, so he says, for their expertise in astrologia, the mantic arts, and the rituals for warding off evils and securing benefits (2.29.1–3).[3] After discussing how Chaldaean children were educated, a process arguably superior to education in Greece (2.29.4–6), Diodorus outlines Chaldaean astrologia. He specifically states that the Chaldaeans’ long-term interest in observing the motions and powers of the heavenly bodies and in predicting the future is underwritten by their
convictions that the cosmos is by nature both uncreated and indestructible, and that its order is governed by Providence (2.30.1–2). He next mentions their doctrines regarding the planets or Interpreters (2.30.3–5), the 30 zodiacal stars or Counseling Gods (2.30.6–7), how the planets are ominous (2.31.1–3), the 24 non-zodiacal stars (2.30.4), the Moon (2.30.5–6), eclipses (2.30.6), and the Earth (2.31.7). As he indicates, there is much more to tell; but since it would be alien to his history—he presumably does not deem it useful to the reader—he concludes by noting (2.31.8–9) that the Chaldaeans have the highest standing in astrologia of all men and have pursued it most diligently, though he finds incredible their claim to have observed the heavens for 473,000 years prior to Alexander’s crossing into Asia (334 BCE).

Diodorus and His Readers

For Diodorus, Egyptian, Chaldaean, and Greek astrologiae are but varieties of a concern with the heavens. What draws his attention in the Egyptian and Chaldaean versions is their utility in predicting the future—he has no reservations about this (see 1.1.3, 1.81.5, 2.31.1–3, 17.112, 19.55). The lesson to be learned by his Greek readers is, apparently, that the Greeks would do well to appropriate this science as a means of making any decisions about the best course of action. But how this is to be done he does not say. So far as Greek astrologia is concerned, what survives of the Bibliotheca indicates that it is limited to matters of timekeeping (see 1.98.3–4) and geography. The Bibliotheca thus leaves open whether astrology is to be yet another subject within Greek astrologia or whether it is to supplant it.

Vitruvius and De Architectura 9

A 1684 depiction of Vitruvius (right) presenting De Architectura to Augustus / From Vitruvius on Archtitecture by Thomas Gordon Smith

Within a decade of the publication of the Bibliotheca, Vitruvius wrote a treatise on architecture in which he presents his own ideas about astrologia. The rhetorical setup is, however, different and points to a new conception of this science. Unlike Diodorus, who promises benefit to any and all readers, Vitruvius offers to but one reader, Caesar Augustus, the great employer of architects, a book explaining the technical terms and principles of architecture in a way that will allow Caesar to teach himself how to evaluate works built for him (1.praef.1–3). Granted, Vitruvius does expect others to read his book as well (see 1.1.18) and it is clear that they are to include other architects (see 7.praef, 1.1.18). Indeed, by prescribing to the authority the details needed for judgment in matters architectural, Vitruvius makes his book compulsory reading for anyone else in the field.

What is new in Vitruvius’ account is the idea that architecture is not just a matter of building walls, it also one of determining that a wall is well built. So, the question becomes, How is the architect to acquire this capacity of judgment? (1.1.1–2). As Vitruvius sees it,

to be educated, [the architect] must be skilled in drawing and learned in geometry; he should know many histories, pay careful attention to the philosophers, understand [the science of] music, and not be ignorant of medicine; he should know the responses of legal experts and comprehend astrologia and the recognized principles of the heavens. (1.1.3)

In the course of explaining why such subjects are necessary (1.1.4–10), he remarks that

from astrologia are ascertained east, west, south and north, and, further, the theory of the heavens (the equinox, solstice, and the courses of the stars). If one is not acquainted with these matters, he will not in any way be able to understand the theory of sundials. (1.1.10)

Further, he insists that

since [architecture] is such a great discipline adorned and abounding in diverse and numerous branches of learning, I think that only those can rightly declare themselves without qualification to be architects who, by making their ascent by way of the steps of these disciplines from childhood, have been nourished in the knowledge of a great many literary works and arts, and who have arrived at the very lofty temple of architecture. (1.1.11)

Of course, the architect is not to be expert in all these disciplines; as Vitruvius quickly points out, a general education in them will suffice (1.1.12–16).

Thus far, Vitruvius has included astrologia as part of an educational program needed for judging architects and architecture because it is needed for the design and construction of sundials (1.1.10). However, book 9, which is intended to explain this in detail, shows that there is much more to Vitruvius’ appreciation of astrologia than its theory of sundials. The first clue is that, before reiterating in book 9 that he will give

an account of the principles of sundials, that is, of how [these principles] are found in the world from the Sun’s rays through the shadows of a gnomon, and for what reasons [these shadows] are lengthened and contracted, (9.praef.18)

he inserts an encomium of intellectuals—he mentions Pythagoras, Democritus, Plato, and Aristotle—since they benefit mankind with the everlasting utility of their works and are thus so much more to be prized and honored than mere athletes (9.praef.1–4,15–17). Curiously, he does not mention any astrologi in particular: still the encomium does frame what follows with the question of its everlasting utilitas.

The second clue lies in the subjects that Vitruvius includes in his account of astrologia. Vitruvius begins by asserting that the divine intelligence has arranged the behavior of shadows in such a way that equinoctial shadows cast by gnomons vary in length depending on the terrestrial location of the gnomon, and that the lengths of these equinoctial shadows indicate the shapes of the analemmata on the basis of which the hour lines are constructed (taking due account of the locations and the shadow of the gnomons). He next defines the analemma as

the configuration searched out in the course of the Sun and discovered by observation of its shadow as [this shadow] lengthens towards winter solstice. By means of this configuration, through architectural principles and drawings by compass, (the Sun’s) work in the world is discovered. (9.1.1)

Then, he describes the continuously rotating celestial sphere with its axis and pivots (9.1.2), introduces the zodiacal belt with its 12 constellations of equal size (9.1.3), and adds a fundamental principle of celestial timekeeping—that, while exactly which of these constellations are visible will vary with the season, at any point in time there will be six above the horizon and six below (9.1.3).

All this is tolerably relevant to the theory of sundials. What follows, however, is not. For Vitruvius turns now to the motions of the seven planets through the zodiacal constellations (9.1.5–10), an account generously supplemented with a demonstration that Venus and Mercury do indeed make stations and retrogradations (9.1.6–7), and with an explanation of the stations and retrogradations of Mars, Jupiter, and Saturn (9.1.11–13). Even more unexpected is that the chapter ends with a statement of the order of the planets (9.1.14–15) and of how this ordering explains their differences in temperature (9.1.16). Granted, there follow chapters on the lunar phases (9.2) and on the correlation of the Sun’s position in the zodiacal constellations with the seasons and the length of daytime (9.3) that are arguably relevant. But the next chapters on nonzodiacal constellations to the north (9.4) and south (9.5) are not.

Vitruvius does try to pull everything together in chapter 9.6:

I have given instruction so that there is an overview concerning the revolution of the world about the Earth and the arrangement of the 12 zodiacal constellations and of the arrangement of the stars in the northern and in the southern part. For the tracings of the analemmata are found from this turning of the world, the contrary course of the Sun through the zodiacal constellations, and the equinoctial shadows of gnomons. (9.6.1)

But he immediately interrupts himself again by remarking on the effects that the 12 constellations, the five planets, the Sun, and the Moon have on human life. He then concedes to the Chaldaeans their prowess in casting nativities, a discipline which, he says, Berosus first brought to Cos and taught to Antipater and Achinapolus (who reportedly cast nativities based on the moment of conception and not on the moment of birth) (9.6.2).

At this point, it begins to become clear that this “digression” which introduces Chaldaean astrology by explaining some of the underlying concepts of planetary motion is really preliminary to a re-interpretation of Hellenistic astrologia that emphasizes the mastery of time. For, in contrast to the Chaldaeans, Vitruvius maintains, certain Greek theorists (Thales, Anaxagoras, Xenophanes, and Democritus) have well thought-out theories explaining “on the basis of what things the nature of things is governed and how [this nature] has its effects” (9.6.3). And, he continues, others who have followed their findings—he mentions Eudoxus, Euctemon, Callippus, Meton, Philippus, Hipparchus, and Aratus—have discovered the risings and settings of the stars, and the signs of changes in the weather, on the basis of astrologia through their teachings of the parapegma (9.6.3). Such men, Vitruvius says, merit our admiration and deference “because they were of such great dedication that they seem with divine intelligence to declare in advance the upcoming signs of changes in the weather” (9.6.3), thus putting them among the intellectuals praised in the preface to book 9.

The upshot is that, for Vitruvius, there are two types of astrologia, both concerned with divination or the interpretation of celestial signs: the Chaldaean and the Greek (cf. 9.6.1–3)—“Greek” in that Vitruvius does not name any Latin astrologi. Both make predictions on the basis of the configuration of the heavens at a given time. But, whereas Chaldaean astrologia looks to the day of someone’s birth in order to determine his life and so forth, Greek astrologia looks to any given day of the year to determine the weather. What is worth noting, however, is that Vitruvius unifies both types of astrologia by construing them causally—not only does he supply his own causal explanation of the retrogradation of planets above the Sun (9.6.11–13), he casts the parapegmatists as students of the theorists who propound causal explanations of natural phenomena. By interpreting the configurations of the heavens mentioned in either instance as causes of what is signified, Vitruvius provides a foundational unification of the two types of astrologia that subsumes them under the general rubric of Greek physical theory (physica). But he goes no further in this: his subsuming astrologia under physica seems not to entail, as it did for others, that astrologia is part of philosophia. Thus, for Vitruvius, astrologia is neither propaedeutic to philosophia nor are its starting points established only by philosophical argument. For him, the relation between astrologia and philosophia is very different. As he makes clear, astrologia is a science of divination enabling decisions about what one should do; whereas philosophia, with its ruminations on the good and so forth, is not the (sole) arbiter of correct action. Indeed, the job of the philosopher in some instances at least is simply to warrant the claim that decisions about what to do reached through astrologia are in accord with nature.[4]

Vitruvius’ presentation in 9.7–8 of the analemma, the species of sundial, their inventors, and the waterclock drives home that the special contribution of astrologia is the ability to make decisions at any moment throughout the year that are in accord with a proper philosophical account of the world. At the same time, Vitruvius envisages a class of astrologi who can actually make a living by casting horoscopes, developing parapegmata for given locales, and designing (if not making) sundials. Thus, like Diodorus, he conceptualizes a profession of astrologia as opposed to a mere intellectual discipline.

New Definitions of Astrologia

Concordantia astronomiae cum theologia, printed in Augsburg, 1490, by Erhard Ratdolt. / Courtesy of Universitäts- und Landesbibliothek Darmstadt.

The idea that “traditional” Greco-Roman astrologia should include or even adapt Chaldaean astrologia certainly found favor with Manilius. His didactic poem, written in the last years of Augustus’ reign (AD 9–14), offers to Caesar a detailed account of celestial science that elaborates astrology and its conceptual apparatus in accordance with what Diodorus and Vitruvius would have viewed as Greek astrologia and physical theory. Yet, though the Astronomica is the earliest extant text that actually develops a unified astrologia, it does not follow Vitruvius’ plan: there is no mention of parapegmata, sundials, and waterclocks.

Others, Geminus and Pliny the Elder among them, held that astrologia should not include astrology at all, but for different reasons. So let us turn now to them.

Geminus and his Introductio astronomiae

Unlike Diodorus and Vitruvius, Geminus, who was writing at roughly the same time, does not present himself to his reader as an authority in something that will be of use. Instead, he stands before the reader as a teacher whose lessons are, for the most part, cast impersonally and so, presumably, meant to be taken as objective and true. Geminus does, of course, address his reader to introduce the solution to the problem of how the Sun, moving smoothly as it must in a circle, still travels equal arcs in unequal times (1.22) and to report on lexical matters (6.5, 16.19–20).[5] He also interjects himself to assist the reader in the course of an arithmetic (8.29–30, 8.38–39, 8.47,18.7) and a geometrical argument (1.40); and he further enhances his standing as teacher and authority when distinguishing what belongs in more advanced studies and what belongs in a first introduction to astrologia such as the present treatise evidently is (5.14–15, 5.17). But, overall, the authority of the Introductio derives not from its author but from its context and content.

As for his reader, there is reason to think that Geminus’ “lessons” are directed in good measure at (would-be) philosophers or those with philosophical interests. In the first place, we learn from Simplicius that, as Geminus would have it, concern with causation and causal explanation are characteristic of philosophia qua physical theory, not astrologia:

It is for physical theory to inquire into the substance of the heavens and of the celestial bodies, into their power and quality, and into their coming into existence and destruction. Through these [investigations], it can certainly offer demonstrations concerning size, shape, and ordering. Astrologia, on the other hand, does not attempt to speak about anything of that sort. Instead, it demonstrates the order of the celestial bodies after declaring that the heavens really are a cosmos, and speaks about the shapes, sizes, and distances of the Earth, the Sun, and the Moon, about the eclipses and conjunctions of celestial bodies, and about quality and quantity in their movements. It follows that since astrologia deals with the theory of quantity, duration, and type of shape, it is reasonable for it to need arithmetic and geometry for this. And concerning these matters, which are the only ones about which it undertakes to supply an account, it has the authority to make inferences through arithmetic and geometry.

Now astrologi and physical theorists will in many cases propose to demonstrate essentially the same [thesis] (e.g., that the Sun is large, that the Earth is spherical), yet they will not follow the same procedures. For whereas [physical theorists] will make each of their demonstrations on the basis of substance, or power, or “that it is better that it be thus,” or [the processes] of coming into existence and change, astrologi [will do so] on the basis of the [properties] incidental to shapes or to sizes, or on the basis of the quantity of motion and of the time interval appropriate to it. And physical theorists will in many cases deal with the cause by focusing on the causative power; whereas astrologi, since they make their demonstrations on the basis of extrinsic properties, are not adequate observers of the cause in explaining that the Earth or the celestial bodies are spherical, for example. Sometimes they do not even aim to comprehend the cause, as when they discourse on an eclipse. At other times [astrologi] make determinations in accordance with a hypothesis by setting out some modes [of accounting for the phenomena]; and, if these are the case, the phenomena will be saved. (Kidd 1988–1999, F18.5–32)[6]

Second, not only does Geminus mention causation in presenting the theory of aspects and their influence (ch. 2), he attacks the view that the celestial phenomena listed as signs in parapegmata are causes of the correlated changes in the weather as well as the even more prevalent belief that Sirius’ rising with the Sun is the cause of the intensification of summer heat (ch. 17).

Further, that Geminus includes students of philosophy among his readers is certainly consistent with his criticizing philosophers on several occasions (cf. 12.14–19, 16.21–23, 17.32–35). And it does make sense of the fact that, while he mentions numerous instruments, he does not include their design or construction in his articulation of astrologia: such inclusion would have been inappropriate given that philosophia is, for him, the contemplative activity indicated in Simplicius’ report and presupposed in his own reference to the celestial bodies as constituted of either fire or aether (17.15, 17.33), a point of debate in his time among Peripatetics as well as between Stoics and Peripatetics.

Regrettably, there is no way to say whether Geminus is addressing those of like philosophical mind or even to identify Geminus’ own philosophical allegiance, because designation at the time as a “Stoic” or “Peripatetic”, say, was more a matter of what one read than of doctrinal agreement. So, all we can do at this point is to explicate what he conveys to his readers.

To begin, Geminus’ Introductio, viewed against the background of Hellenistic astrologia, clearly defines and articulates the subject in a way that no earlier or contemporary (extant) text does. That is, even if one sees Geminus as responding to Diodorus and Vitruvius, it is important to keep in mind that he does not respond with an outline or an agenda but with a worked-out solution to the problems of the demarcation and the articulation of astrologia. Next, in marshaling the technical detail—a process of selection and omission that effectively defines this discipline—it is noteworthy that Geminus marginalizes the Chaldaeans. For, though he mentions them twice (2.5, 18.9), only the first reference is astrological in that it concerns the theory of planetary aspects. Moreover, unlike Diodorus and Vitruvius, Geminus does not acknowledge Chaldaean expertise. Instead, he simply describes the aspect of opposition, states that the Chaldaeans allowed its significance in casting nativities, and supplies a reason that has authority not because it derives from the Chaldaeans but, as he says, because it is based on traditionally recognized powers of the celestial bodies (2.4–6).

At the same time, Geminus’ Introductio plays down the role of prediction in astrologia in general and, as one might expect given Simplicius’ report, brings to the fore instead the idea that astrologi should be concerned with making sure that their accounts are in accord with proper causal explanations. Thus, although he does allow that the configurations of the planets may serve as the basis for predictions, not only are his examples meteorological rather than genethlialogical—e.g.,

For if the North Wind blows when the Moon is in some one of the three zodiacal signs [scil. of the first trigon], the same condition will persist for many days. Wherefore, the astrologi, starting from this observation, predict northerly conditions (2.8: cf. 2.9–11)

—what he emphasizes is their underlying causal mechanism (2.13–14). In the same vein, though Geminus does allow that stellar risings and settings may serve in making predictions about the weather (17.6, 17.23), his concern is likewise with what warrants these predictions. Thus, he criticizes the “peculiar apprehension among the uninformed” that the significations in the parapegmata, that is, the connections of sign (the stellar event) and signified (change in the weather) are causal, and maintains instead that they are just generalizations from experience or rules of thumb. The parapegma thus turns out for him to be a “pretty unscientific part of astrologia” (17.26–45). Indeed, in his view, it would be far better to use signs given by Nature—solar and lunar risings and settings, lunar haloes, and so forth—to predict the weather, thereby following, so he says, Boethus, Aristotle, Eudoxus, and many other astrologi (17.46–49).

The inescapable conclusion is that, for Geminus, astrologia is not a science of divination but a discipline dependent on philosophia in the guise of physical theory: its value lies in its promoting contemplation and not because it warrants decision-making. (This is actually quite reasonable: knowing that there will be a change in the weather because there is a lunar halo, for example, does not predispose to any particular course of action.) Thus, while astrologia may include in its conceptual apparatus astrological notions rooted in a causal account, it does not include astrology per se. In short, where Vitruvius uses causal theory to include astrology in astrologia and to deny effectively its subjugation to philosophia, Geminus uses it to exclude astrology from astrologia and ratify its one-sided dependence. The only form of prediction that Geminus’ astrologia admits is meteorological. One will not find in the Introductio any claim that astrologia is useful in the sense that Diodorus and Vitruvius maintain.

A caveat: the preceding analysis is predicated strictly on what we have in the Introductio. As for the works promised in the Introductio, the most one is entitled to expect is a causal account of why the planets (other than the Sun) appear to have a unsmooth motion in longitude, given that their real motions are all circular and uniform, and a more detailed description of the celestial sphere and how a globe should be marked out.

Pliny and his Naturalis historia

Pliny the Elder, as imagined by a 19th-century artist. No contemporary depiction of Pliny is known to survive. / Wikimedia Commons

Pliny’s great treatise on Nature was completed in 77 CE and is dedicated to Titus Flavius Vespasianus, elder son of the Emperor Vespasian, some two years before Titus’ accession in 79 CE and Pliny’s death a few months still later. It is a massive work in 37 books, which his nephew and adoptive son, Pliny (the Younger), describes as “diffuse, learned, and no less diverse than Nature itself” (Epist. 3.5.6). This is fair enough, if one does not then treat the Naturalis historia as a mere encyclopedia.

In his prefatory letter, Pliny presents his treatise as a lighter work—lighter in literary values—about a subject that is barren from a literary standpoint, to wit, the nature of things or, as he glosses it, life (praef. 12–13). Pliny’s focus, then, will not be on Nature in isolation but on man’s mundane interaction with it. In fact, were this treatise not dedicated to Titus, it would, he writes, be offered to the lowly crowd of farmers and artisans as well as to the leisure moments of students (praef. 6). After pointing out that Naturalis historia is the first ever of its kind among the Romans and Greeks (praef. 14), he admits that writing it was difficult; so, if the result fails in giving

novelty to what is ancient, authority to what is new, brilliance to what is old, light to what is dark, charm to what is disdained, credibility to what is doubtful—indeed, Nature to all things and all her due to Nature (praef. 15)

the attempt is still fully honorable and splendid. But, he adds, such concern with literary niceties is beside the point: the value of the Naturalis historia lies in its utility (praef. 16). This work, then, is neither a literary text nor one in philosophia qua physical theory.

Pliny concludes the letter by making two points. The first goes to the question of authority. He claims (and this is an underestimate) that the Naturalis historia comprises 20,000 noteworthy facts drawn from roughly 2,000 volumes (typically unread because of their abstruseness) written by 100 authors, with many things added which were unknown before or discovered subsequently (praef. 17). This claim to authority by virtue of the sheer quantity of factual detail is supplemented by Pliny’s rooting it largely in tradition. As he notes, the most trustworthy (iuratissimi) of modern authors transcribe the ancients word for word, albeit typically without acknowledgement and for unacceptable reasons; and so he advises Titus that rather than steal credibility he will list (in what is now book 1) all the authors whom he has consulted (praef. 21–23). (We should observe, however, that the massive factual detail putatively drawn from past authorities that warrants Pliny’s claim to authority does not always sit well with his assertion that what he writes is also to be useful: there are occasions when the utility of the facts related seems non-existent or, least, highly attenuated.)

The second point concerns the nature of Pliny’s project. He remarks at some length that the Naturalis historia is in many respects incomplete. This recognition of incompleteness is not, I think, just an admission of personal limitation in accord with literary convention, though there is much of that in his flattery of Titus. It derives, I suggest, from Pliny’s awareness that the human engagement with Nature changes over time; and so, to be useful, his work will require updating every now and then. Again, it is clear that the utility in question is mundane: Pliny advises Titus that he includes a table of contents which will enable others to look for particular topics without reading the treatise through.

Book 2, that is, book 1 by Pliny’s reckoning (see praef. 17), starts by describing the mundus (world: cosmos in Greek), that unique, bounded, and eternal divinity which is both a work of nature and the very nature of things (2.1–2). As one might expect, given Pliny’s explication of “the nature of things” by “life” (praef. 13), he is not about to embark on an account of the mundus simpliciter in what was called natural philosophy at the time. In the first place, he indicates that he will pay no heed to the traditional concerns of philosophia. Thus, he peremptorily dismisses debates about the size of the mundus, its uniqueness, and whether there is anything outside it (2.3–4). Moreover, in arguing that the mundus is spherical, he adduces only considerations of language and of our common experience of rotating shapes and the sky’s curvature (2.5): he makes no mention of the arguments found in Aristotle’s De caelo, for example; and he shrugs off the “Pythagorean” question of whether the rotating mundus makes a noise and, if so, why we do not hear it (2.6). Likewise, he flatly affirms that there are four elements in the mundus (fire, air, water, earth), and that the Earth lies at its center in a system defined by the dynamic opposition of light and heavy (2.10–11), thus overlooking the debate about Aristotle’s fifth element (aether)—which Geminus noticed—and the whole theory of natural place.

But, if the Naturalis historia is not a philosophical work on the mundus, what is it? The answer comes in a digression on god (deus) that is occasioned by Pliny’s mention of the planets situated between the Earth and the constellations engraved on the surface of the mundus. For, after maintaining that in assessing the Sun’s works, we should accept as true that

it is the soul or, more clearly, the mind of the entire mundus, that it is the primary governor of Nature and a divinity. It supplies its light to things and takes away shadows; it conceals the remaining stars and fills them with light; it governs the changes of the seasons and the year which is always reborn in accord with Nature’s custom; it disperses the sadness of the heavens and even calms the clouds of the human soul; it also lends its own light to the other stars; [it is] splendid, exceptional, all seeing, and all hearing too, as I see that Homer, the foremost of writers, held in the case of [the Sun] alone (2.12)

he pauses to consider the nature of divinity and its role in Roman life.

In his view, there is but one god and it is either the mundus as a whole or some part of it. He rejects as imbecilic any attempt to represent the likeness of god and as an unseemly projection from our fears and frailty, the view that there are many gods (2.14–16). For the oft-told stories of divine marriages, adulteries, and so forth, he has nothing but contempt (2.17). To him, the fundamental truth is that “God it is for a mortal to help a mortal; this is the way to eternal glory.” Such, he says, was the course that the Roman princes took and that Titus’ father now takes (2.18).

This truth, which goes to the heart of Pliny’s idea of proper Roman life, is supplemented when he asks whether one should accept or doubt the idea that god (whatever it might be) is concerned with human affairs and is not defiled by such involvement (2.20). As he says, it is hard to decide which alternative is more advantageous to the human race given what people actually do. Moreover, the question is complicated by the almost universal practice of postulating Fortune as an intermediate deity responsible for the good and bad in human life (2.22). True, he says,

another group also banishes her and attributes events to its own star and to the laws of birth, I mean, [the law that,] for all future men, once and at any time there is ever a verdict from god, it is given for the time remaining. This view has begun to take hold; and both learned and unlearned people go to it equally at the run. (2.23)

These adherents of astrology are just like those who rely on any other form of divination (2.24); and together the consequence of their beliefs and practices is that for man, lacking foresight as he does, “the only certainty is that nothing is certain and that nothing is more wretched than man or more arrogant” (2.25). And so, it would follow, belief in divine solicitude is harmful and debilitating. Yet, Pliny will still allow that there is some advantage in accepting it, if only as a political ploy to curb evildoers (2.26).

However, as Pliny reflects further, a solicitous god is himself limited. He points out that such a god cannot commit suicide, make mortals immortal, recall the dead, or make one who has lived not to have lived. Indeed, such a god has no power over the past except forgetfulness—and he certainly cannot change the truths of arithmetic, and so on. Indeed, for Pliny, these very limitations of a solicitous divinity testify indubitably to the power of Nature and that this is ultimately what we mean when we speak of god (2.27).

The Naturalis historia will plainly not accommodate astrology or any activity predicated on the belief that there is a god mindful of, and concerned with, human affairs. Next, it is also clear that this massive work instantiates the dictum that divine being consists in helping others. This is, after all, the principle underlying Pliny’s writing in the first place and his excusing its literary infelicities: as he explains to Titus,

So, for my part, I think that there is in learning a special place belonging to those who in overcoming difficulties have preferred the value of helping (utilitatem iuvandi) to the prestige of giving pleasure. (praef.16: cf. 17–18)

Yet there is more to this instantiation. Like Diodorus and Vitruvius, Pliny presents his work as useful. But, for Pliny, this utilitas is coupled with, and constrained by, the idea of service. And this service is understood, above all, to be a service to the Roman people (2.18: cf. 2.19–20). This means that the selection of what is recounted in the Naturalis historia is guided by consideration of whether it will help the Roman people and their Emperor. This is what motivates, for example, the books on geography, ethnography, human biology, zoology, agriculture, botany, and pharmacology. At the same time, the idea of service is a means by which heroes are identified. Pliny’s praise of Hipparchus is an instructive case in point, as I will explain.

When Pliny resumes (2.28), he returns to the account of the mundus in 2.7–9 and begins a “journey” which proceeds from the outermost reaches of Nature to its innermost core, Earth, and ultimately to man. But, again, at the outset, he affirms that his account will contain nothing to support the various beliefs underlying astrology or divination (2.28–31). This does not mean, of course, that his description of what lies between the celestial sphere and the Earth (the planets, comets and meteors) excludes the idea that they have power (vis) and influence events and living things on Earth. Pliny holds, for example, that the planets contribute to the weather (2.105–106, but see 18.352) and that the Moon influences the growth of shell-fish (2.109). Still, as he says in his account of meteors,

I think that these (like all other natural phenomena) occur at fixed times, not, as the many think by virtue of the diverse causes which the ingenuity of clever men contrives. True, they have been omens of great evils. But I do not think that the [evils] happened because these [omens] took place; rather, [I think] that [the omens] therefore took place because those [evils] were going to happen, and that the explanation of [the omens] is obscured by their rarity and, therefore, not known in the way that the [stellar] risings mentioned above, eclipses, and many other things [are known]. (2.97)

Likewise it does not mean that Pliny will avoid astrological concepts in explicating the celestial motions (cf., e.g., 2.65).

Nevertheless, Pliny’s “itinerary” will not address astrologia and the abstract issues that exercised his sources, Diodorus and Vitruvius, for example, directly. His aim is to provide rules (leges) that are useful (18.321: cf. 18.207–210) in such very practical matters as locating, sowing, raising, and gathering crops (see 18.321–365). Moreover, these rules are intended in part for ignorant rustics (18.206). So, it is hardly surprising that he focuses on the subject matter of astrologia and says next to nothing of the science itself. Indeed, his reluctance to engage more abstract questions of the demarcation and articulation of astrologia is suggested by his avoiding the term almost entirely in favor of paraphrases such as the “theory of the stars” (ratio siderum). When he does mention astrologia, it is either the name of a book (18.213) or simply a science that someone has established (7.123, 7.203, 35.199).

With that in mind, I will conclude with Pliny’s discussion of eclipses (2.41–57). As he says, solar eclipses are “a wonder of the first rank in our entire viewing of Nature and like an omen” (2.46). After explaining how lunar and solar eclipses occur (2.47–48), Pliny remarks on what eclipses tell us about the relative sizes of the Sun, Moon, and Earth (2.49–52). Then, with this wholly naturalizing account of eclipses in hand, Pliny turns to the history behind this explanation. He starts with Sulpicius Gallus, thereby placing himself firmly in the tradition of a literary topos that emphasizes the great value, especially in time of war, of knowing that eclipses are not omens but naturally recurrent events, a topos taken up before him by Polybius, Diodorus, Livy, and Cicero. In Pliny’s variant of the story, Sulpicius Gallus delivered the Roman army from fear by foretelling an eclipse on the day before the Romans defeated the Greeks at Pydna in 168 BCE (2.53), thus bringing to an end the Third Macedonian War and solidifying Roman hegemony in the Mediterranean. The emphasis on prediction is noteworthy: for Pliny, Gallus did not calm the Roman forces by an explanation but by a prediction. And so, in his “history,” Pliny lists Thales’ prediction of a single eclipse—thus choosing one of the two interpretations of Herodotus, Hist. 1.74 available in his time (see Bowen 2002, 79–81; Bowen and Goldstein 1994)—and then Hipparchus’ prediction of eclipses in a cycle of 600 years (2.53: see Bowen and Goldstein 1995). But notice: not only is Pliny so impressed with Hipparchus that he later describes him as “a partner in the plans of Nature” (cf. 2.95), he rhapsodizes:

O mighty heroes, beyond things mortal, who ascertained the law of such great divinities and freed the wretched mind of man from fear as it shivers in terror of calamities or some death of the stars in eclipses―it is well known that the lofty words of the poets, Stesichorus and Pindar, were [uttered] in this fear of an eclipse of the Sun―or as mankind infers sorcery in the case of the Moon and on account of this offers assistance with jarring, clashing sound―owing to this terror [of lunar eclipses] Nicias, the commander of the Athenians, who was unaware of their cause, destroyed their resources because he was afraid to bring the fleet out of port―glory to your genius, you interpreters of the heavens able to grasp the nature of things, and discoverers of a theory by which you have vanquished the gods and men! For, who, after discerning these things and the fixed labors―for so it has pleased [mankind] to call [them]―of the stars does not accept that mortals are born to their own destiny? (2.54–55)

In sum, for Pliny, astrologia, the study of the region from the outermost mundus to the Moon is useful because it enables ordinary people to know in advance the changes in the weather and the occurrence of eclipses. Knowing when the weather will change is, of course, a great benefit. Knowing when eclipses are to occur is also a benefit but one that is different and, to Pliny, more important. For, as he would have it, such knowledge enables release from the superstitious fear that at critical moments can bring down armies and empires. This is what underlies the inclusion of astrologia, specifically, prediction, in the Naturalis historia as well as Pliny’s apotheosis of Hipparchus.


Hellenistic astrologia is very much a work in progress during the two centuries about the millennium. It was not a settled discipline. Different writers making diverse claims to authority urged in quite different contexts divergent views of what astrologia should be and why it is important. In this way, they helped to define the framework in which the technical apparatus of this science developed. Thus, the fundamental or precipitating cause of change did not lie within the astrologia that goes back to Plato and Aristotle (a study itself in transformation). Indeed, this cause was not intellectual at all: it was instead the embrace of Babylonian astrology at all levels of Greco-Roman society. That this cause lay outside astrologia licensed intellectuals of diverse backgrounds, not just experts in astrologia, to stand forth and to offer their own solutions to the social and intellectual problems of appropriating astrology. Curiously, however each viewed the astrologers’ predictions of events on Earth given the configurations of the heavens, they all accepted that astrologia should incorporate predictions of the positions of the celestial bodies that are derivable from a theory of their motions. And so, collectively they envisaged a new astrologia that was no longer content just to describe the celestial motions as Aristotle and others had. Thus emerged a new desideratum: a predictive astrologia rooted in a mathematical description of the celestial motions that was also a profession and not merely a discipline.


In observing that several texts from the first centuries BCE and CE present themselves as introductions to astrologia, we were able to assess them together in a way which reveals that what they include is dependent on the readers addressed and how their authors seek to persuade or inform these readers. Much more, of course, can be done in pursuing this type of contextualization, since it affords entry, for example, into questions of daily life, education, and social standing. But for now, the question is, Is this sort of inquiry still feasible when the documents at issue do not present themselves as introductions and may not show a guiding concern to articulate and demarcate the science of the heavens?

To begin, it is important to note that prior to, and concurrent with, these introductions, there were documents in astrologia that served to constrain and channel the development of this science from the first century on. To judge from what has survived, however, they did not do this because they constituted a fixed body of learning; rather, they seem to have made their impact by imposing not their results so much as their own questions and demands. Let me indicate briefly how one might pursue this.

There are numerous Hellenistic works that describe the celestial sphere. Their progenitor seems to have been Eudoxus’ Phaenomena, an account in prose of the constellations lying on, or in relation to, circles on the celestial sphere that are defined in relation to an observer and his horizon. Eudoxus’ description was subsequently set to verse in another Phaenomena by Aratus, who includes a list of signs of changes in the weather. Both of these works were together the subject of a commentary in prose by Hipparchus (second century BCE) that presents itself as the substance of a letter addressed to Aischrion (otherwise unknown). This mishmash of literary forms is remarkable and raises the question of why they were chosen, a question to be resolved in good measure by study of how Aratus and Hipparchus position themselves and their work in relation to their readers. But such issues of expository form also had historical significance in that they were taken up in debate about the form of writing appropriate for astrologia. That is, for Hipparchus at least, this question of form was inseparable from the question of expertise in matters celestial. To use his linguistic markers, the critical question was: Is the celestial scientist an astrologus (like Aratus) or a mathematicus (like Eudoxus)?[7] Thus, questions of expository form went directly to the question of the authority of the discipline and its practitioners; and they did not pass away after Hipparchus, as the subsequent tradition of Aratea bears witness. (Aratus’ Phaenomena, sometimes said to be the most widely read work in antiquity after Homer’s Iliad and Odyssey, was the subject of numerous commentaries and Latin translations.) At the same time, this debate over authority is a struggle to define the reader. After all, if poets are false claimants to authority in astrologia and their poetry is misleading, then the readers of poetical works in astrologia lack credibility as well.

Related to this nexus of expository form, authority, and suitability is the use of mathematical demonstration in Hellenistic treatises devoted broadly to the rotating celestial sphere—the De sphaera quae movetur and De ortibus et occasibus by Autolycus (dates uncertain: see Bowen 2013a); the Phaenomena by Euclid (dates also uncertain: see Bowen and Goldstein 1991, 246, nn. 29–30); Theodosius’ Sphaerica, De habitationibus, and De diebus et noctibus (second/first century BCE); and Hypsicles’ Anaphoricus (second century BCE: see Bowen 2008a). Whether any of these works was regarded as one in astrologia at its time of writing is a nice question. But for now, we should note that, however each was classified, they severally displayed very successfully the power of mathematical argumentation in establishing knowledge of the heavens, though at the same time each plainly raises a problem in demarcation—after all, astrologia is not mathematics. The same is true as well of Aristarchus’ De magnitudinibus, a determination of the relative sizes and distances of the Sun, Moon, and Earth that is cast in an axiomatic, demonstrative style (see Bowen 2013c).

In sum, there is, I contend, much to be gained by looking on documents in Hellenistic astrologia as species of literary production and by interpreting their content in relation to their their form and their intended readers. The promise in this is an understanding of the place of Hellenistic astrologia in its intellectual and social contexts that has hitherto been lacking. So far as one can tell, none of the issues driving this kind of literary production was settled in antiquity. Thus, it follows that historians today go astray by focusing solely on the positive claims made about the heavens in Hellenistic texts. Such “content” is, in fact, largely the outcome of the choice of an expository style by a writer who, if he does not seek explicitly to present himself as an authority, still aims to make what he writes authoritative to a select, or even self-selecting, group of readers.


  1. Regarding these earlier writings, one should note that, although Hipparchus’ commentary presents itself and its target texts—Eudoxus’ Phenomena and the poem based on it by Aratus—as works in celestial science, many surviving treatises such as P. Hibeh 27, Aristarchus’ De magnitudinibus, Hypsicles’ Anaphoricus, do not; and, thus, that inclusion of such treatises in astrologia is a matter of inference from earlier work and later reports.
  2. Although Diodorus does mention travels to Egypt and Rome, there is no evidence in the Bibliotheca to support the claim that he traveled to Europe and Asia: Sacks 1990, 161 and nn. 1–2.
  3. On the Chaldaeans of history and who Greek writers thought they were, see Eck 2003, 153, n. 3.Though Vitruvius does emphasize that philosophia explains the nature of things (1.1.7: cf. 1.1.17), he does not explicitly deny that philosophia has no independent contribution to moral discourse: see 7.praef.1 for his acknowledgement of the distinction between natural and moral philosophy. In general, he seems more concerned to emphasize that any art or science succeeds when it is in accord with Nature (something to be shown by philosophia) than to explain what philosophia is.
  4. Though Vitruvius does emphasize that philosophia explains the nature of things (1.1.7: cf. 1.1.17), he does not explicitly deny that philosophia has no independent contribution to moral discourse: see 7.praef.1 for his acknowledgement of the distinction between natural and moral philosophy. In general, he seems more concerned to emphasize that any art or science succeeds
    when it is in accord with Nature (something to be shown by philosophia) than to explain what philosophia is.
  5. Geminus speaks in the first person plural: such usage is plainly a fiction (cf. 1.22) and a mark of urbanity that at the least means “I” (cf. 17.5, 17.17).
  6. Cf. Bowen 2013b, 38–51. In general, Simplicius’ quotations are reliable and typically constitute valuable indirect testimony to the early history of the text quoted. In this instance, Simplicius takes pains to assure that reader that Alexander’s citation of Posidonius’ citation of Geminus is also reliable (see Kidd 1988–1999, F18.1–4). Whether Posidonius is equally reliable is an open question: the most one may say is that what he attributes to Geminus is consistent with what one finds in Geminus’ Introductio astronomiae.
  7. For an account of Hipparchus’ treatment of Aratus, see Mastorakou 2007.


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Forthcoming in Oxford Handbook of Science and Medicine in the Classical World (Oxford University Press, 2017)



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