Advances in Early Greek Astronomy

8/22/2019 Advances in Early Greek Astronomy

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1One effective criticism of Dicks is found in Kahn 1970. I have tried to emphasize thescientific character of Presocratic philosophy in Graham 2006, but with a focus on methodologyrather than empirical success.

Advances in Early Greek Astronomy

Daniel W. Graham

Contemporary views of early Greek astronomy seem to stand largely where they were left by D.

R. Dicks in his impressive study, Early Greek Astronomy to Aristotle (1970). In summing up the

Presocratics, he observed,

it is difficult to resist the feeling that their ideas on the physical universe olent

lucerna, they smell of the lampthey are the dream children of the speculative

thinker in his study intoxicated by the novelty and daring of the new intellectual

atmosphere, and intent on applying the new methods of thought . . . in the widest

possible field. . . . [The Presocratics] were not, however, primarily scientists,

much less astronomers, and observation of actual celestial phenomena seems to

have played a relatively minor role in their thinking. (60, Dickss italics)

His general assessment was not particularly novel even when he wrote it, but, backed up by a

solid investigation of the methods and accomplishments of the Presocratic thinkers, it was, and

still is, an authoritative conclusion. So much so, that little has been done to reconsider the

contributions of the early philosophers to scientific astronomy.1 Indeed, the phrase early Greek

astronomy is often used to denote the theories of the fourth century B.C. (sometimes even the


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2E.g. Neugebauer 1972; Kalfas 1990. Goldstein and Bowen 1983 recognize an earlystage of Greek astronomy, but give it a diminished role. A recent history of astronomy that isquite good on modern developments is poor on early Greek contributions: Wilson 1997.

3E.g. Grant 2007; Evans 1998. Pedersen 1993 praises the Presocratics but cites noastronomical advances for them.

third century and later), as if the Greek thought of the sixth and fifth centuries contains no real

astronomy, but only, as scholars sometimes put it, speculative cosmology or natural philosophy.2

Dicks licensed another generation of scholars to patronize rather than study seriously the efforts

of Presocratic thinkers to understand the workings of the heavens.3

It will be my contention that this view is myopic and false. The Presocratic philosophers

made major contributions to astronomy that are so fundamental that we tend to take them for

granted and overlook them in a search for a different kind of contribution which they did not

make. I shall first consider the challenges for attributing a scientific understanding of the

Presocratics (I), then examine one area in which they made an important conceptual advance (II),

which led to a powerful explanatory model (III) that was widely accepted as a scientific advance

(IV).

I

The brief against the Presocratics as scientific students of the heavens is invariably the one

mentioned by Dicks. They are too speculative, and not sufficiently empirical. Dicks gives them

credit, as do virtually all historians of science, for developing a method that had the potential to

solve the problems of science: by abandoning mythological traditions and subjecting external


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phenomena to a process of logical reasoning, untrammelled by religious dogma, and even by

investigating the actual processes of thought, they opened up a whole new field of knowledge

which is virtually inexhaustible (ibid.). To put the point in the discourse of science, they were

good at advancing hypotheses and at discussing them critically in an open forum, but bad at

testing them against observational data.

Failures of observation are not, however, the only problem Dicks sees. In some sense,

surprisingly, their theories were not abstract enough. Their theories are all qualitative in

conception (ibid., Dickss italics). The theories lacked a mathematical basis that could provide

a truly scientific view of the heavens. Dicks seems to think that a close observation of risings,

settings, and the like would actually provide the mathematical background needed for a robustly

scientific conception of the heavens. Philolaus innovative scheme in which the sun, moon, and

planets circle a central fire is an example of bold thinking without sufficient empirical

grounding: it is very much the product of the study and bears little relation to the facts of actual

observation (e.g. it takes no account of the latitudinal and longitudinal variations of the planetary

bodies) (70). Like many other historians of astonomy, Dicks sees the beginnings of scientific

astronomy only in the measurements of Meton and Euctemon (ca. 430 B.C.), who made

observations of the solstices and equinoxes and the length of the year and the month, in order to

solve problems of constructing a practical calendar. Out of studies like those of Meton and

Euctemon, one can build a theory of the motions of the sun, moon, and stars, whereas the general

speculations of the Presocratics lead to a scientific dead end.

Beside this traditional challenge to Presocratic science, from the perspective of scientific

inquiry, there is a very different challenge from the outside. In recent years it has become


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increasingly common for thinkers to reject the claims of science to offer a reliable and

progressive path to knowledge. Some philosophers of science have argued that there is no

standard by which to judge one scientific theory against another, but only a very human

conception, which can be replaced from time to time. Cultural critics also reject the claims of

science to represent secure knowledge that is somehow better or more reliable than what people

can arrive at by ordinary processes of negotiation and consensus. If this is so, then the whole

project of the history of science, to track and illuminate the path from ignorance to knowledge,

and from random discoveries to reliable methods, is an illusion. The history of science is no

more than a certain kind of social history of a field that in the past has made inflated and

unjustifiable claims to truth and social utility.

This is not the place to engage in a large-scale debate about methodology and

historiography. Yet I cannot proceed with the present study without at least taking a stand on

these broad issues. I share the assumptions of traditional historians of science and wish to

criticize the status quaestionis from within that framework. My disagreement with current views

of early Greek astronomy is not with the method of traditional historians, but with a shortsighted

application of that method. It is more difficult to reply to revisionist philosophers of science

who call into question the very possibility of scientific progress. The most influential theorist in

the area is Thomas Kuhn (1996 [1962]), who allows for a kind of progress in solving problems

relative to a given paradigm in normal science, but sees no possibility of judging progress

between incommensurable approaches to science, which compete during revolutionary episodes.

Precisely when change is most radical, rational grounds fail for choosing between competing

paradigms. Other theorists such as Imre Lakatos (1970) and Larry Laudan (1977) have


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suggested ways in which a more robust notion of progress within a research program or tradition

is possible, yet they still fall short of endorsing a notion of absolute progress in understanding

the world or getting at the truth of the matter. In the words of Xenophanes, even if [a man]

should completely succeed in describing things as they come to pass, nonetheless he himself

does not know (B34).

I could frame the present study within the bounds of a kind of local scientific progress,

and that would offer some kind of interesting result. Yet the long-term advances made by

science suggest that this notion of progress-within-a-tradition is unnecessarily weak. Science

has progressed not merely in solving its own parochial problems (just as, for instance,

philosophy has), but also in understanding phenomena in a deep way that allows their

manipulation and control in many cases. As Hilary Putnam famously remarked, realism is the

only philosophy that doesnt make the success of [a] science a miracle (Putnam 1975, Putnams

italics)*. In fact, I hope to offer the present study as a modest contribution to the cogency of

scientific realism.

I shall presume, in accordance with scientific realism, that it is possible to make scientific

discoveries. I assume the following minimal (necessary) conditions for scientific discovery: (1)

Scientist S puts forth hypothesis H to explain phenomenon P at a time t; prior to t no one had an

adequate account of P (as seen, perhaps, by the fact that there is no generally accepted account of

P). (2) S or a community C of his peers adduces empirical evidence E on the basis of theory T

that confirms H as an account of P. (3) After t and as a result of a consideration of E in relation

to H, C accepts H as an account of P. Now I do not claim that (1)-(3) constitute sufficient

conditions for a scientific discovery. Yet they seem to present at least a partial description of


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4Cf. the HippocraticAncient Medicine 1: I assert that [medicine] has no need of a vainhypothesis as do remote and bewildering phenomena, such as those in the sky or under the earth,concerning which it is necessary, if one is to explain them, to use a hypothesis. But if [readinga(\ ei/)fora)ei/with Littr] one should explain or claim to understand what these things are, itwould not be clear either to the speaker or to the audience whether his statements were true ornot. For there is nothing against which to test ones knowledge.

what we mean by a scientific discovery, and hence provide criteria for deciding in historical

cases whether an episode could count as a scientific discovery.

I began this section with recent objections to Presocratic science. But similar objections

go back to ancient times. Xenophon says about Socrates, he wondered why it was not plain to

[cosmologists] that it was impossible for men to discover [heurein] these things, inasmuch as

even the most advanced theorists did not agree with each other (Memorabilia 1.1.13).4 This

objection seems to point to the lack of evidence (2) as seen from the failure of consensus (3). It

seems that from the time of Socrates to the present, Presocratic theories have been rejected as

insufficiently scientific.

With this background, let us turn to early Greek theories of the heavens. Can we find

anything that would count as an astronomical discovery?

II

One phenomenon or set of phenomena that was recognized as an explanandum by early Greek

philosophers was the character of the moon, in particular its phases. Sixth century theorists

presented several theories to explain the changing appearance of the moon, but none of them

could be justified on the basis of scientific evidence. Thus the question of the moons light could


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5For Babylonia, see Hunger and Pingree 1999. For the cultural setting of Babylonianastrology and astronomy, see Rochberg 2004. For Egypt, see Neugebauer 1957, 80-96;Maravelia 2006, 134-35, 228-29, 428-429. For India, see Billard 1971. For China, see Needham1959; Heliophotism seems to have been known by the fourth century BC (227); by 20 BC someChinese astronomers had a correct understanding of solar eclipses (414).

6allotrion phs borrowed light ~ allotrios phs foreign man Od. 18.219, with Coxon1986, 245.

count as a problem of philosophy already in the sixth century, a problem that had not been

solved. Furthermore, a survey of astronomical lore of Babylon, Egypt, India, and China will

show that no culture had a scientific understanding of lunar light in this period, although

Babylonian astrologers, at least, had amassed extensive and detailed observations of the sun,

moon, and planets.5 At this time Greek philosophers seem to be unique in offering speculative

theories of heavenly phenomena often based on physical models of the world and its contents.

Our story begins with two fragments of Parmenides of Elea, found in the cosmological

part of his philosophical poem:

B14. [moon] shining by night, wandering around earth with borrowed light,

B15. ever peeking toward the rays of the sun.

Here we find a striking insight: the moon shines with borrowed light, as evidenced by the fact

that its shiny side is always facing the sun. There is no argument here. The first line presents a

clever word-play parodying a Homeric line.6 The second line recognizes an important

relationship, but does not elaborate. It is possible that Parmenides offered further considerations,

but we have no record of a scientific explanation, nor does the poetic setting lend itself to


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7Empedocles B45 with B43, B47, B48, B42, PlutarchDe fac. orb. lun. 929d-e.

8The learned essayist Plutarch also connects Parmenides B15 with Anaxagoras B18, Thesun imparts to the moon its brightness (De fac. orb. lun. 929a-b).

expositions such as one might expect from a prose treatise. Indeed, one might question whether

we are reading too much in to Parmenides remarks. Nevertheless, Empedocles virtually quotes

Parmenides line, and understands it as embodying a theoretical claim, one that he assents to and

elaborates on.7 If a successor, who had access to the whole poem and partook of its milieu, took

it as an explanatory hypothesis, we seem to be justified in reading it similarly. 8

The simple insight is that the moon gets its light from the sun, as seen by the fact that its

luminous face is always looking directly towards the sun. The insight gets the facts right, as

seen from hindsight, and marks the first explicit recognition of this relationship in the history of

the world. As presented, however, it is little more than an insight. It is embedded in no theory,

supported by no argument, justified by no further evidence than that stated in B15. Thus, by our

criteria, the insight does not constitute a discovery: there is something we can call a hypothesis

to account for a phenomenon, but no argument or evidence produced beyond what might support

an initial connection.

III

Of what happened next we have no detailed information. But consider what a follower of

Parmenides, who took his proposals seriously, as we know Empedocles did, might glean from

Parmenides insight. If the light from the moon is reflected from the sun, then the phases of the


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9DC291b17-23. Cf. Popper 1998, 133 n. 63.

10Cf. PlutarchDe fac. orb. lun. 929b-d.

moon are plays of light and shadow on the face of a body. The patterns presented in the phases

of the moon: thin crescent, thicker crescent, half-circle, gibbous shape, can be produced only by

a spherical or spheroid body, as Aristotle explicitly noted.9 Hence we can infer that the moon is

a sphere, or approximately so.

Furthermore, the moon must be opaque. Otherwise it would never appear dark, but

would always glow as a translucent body partially illuminated by the penetration of the suns

light, even on the parts not facing the sun. Since part of the moons surface is dark part of the

time, the body must block the suns rays where it does not reflect them.10

Furthermore, the moon must be closer to the earth than the sun, for otherwise it would

never be completely dark, as it is during the time of the new moon. For if the moon were farther

from the earth than the sun, then the sun should illuminate it when it is approximately in

conjunction with the sun, as its luminous face would be oriented toward the earth. As it is, the

moon is full only when it is in opposition to the sun, and dark when it is in conjunction.

These three features allow one to construct a model of the heavens in which the moon

moves around the earth relative to the sun. The interplay of position and illumination provide a

basis for anticipating the appearances of the moon. Indeed, the model provides a theory for

predicting in detail the phases of the moon. At the conjunction of the moon and sun, the moon

will be invisible. At a few degrees distant from the sun (at the latitude of Athens, for instance),

the moon will appear as a thin crescent, above and to the left (east) of the setting sun. At about

45 distant the moon will appear as a crescent occupying about one-fourth of the lunar disk. At


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11For the term, Mourelatos 2002, with reflections on Xenophanes influence; for the roleof Parmenides, see Whrle 1995; Graham 2002. Cf. O'Brien 1968. Popper 1998, 70-73, 84-88,96, 100, 136, sees Heliophotism as a great discovery of Parmenides, one which inspired both hisontology and his cosmology.

90 it will appear as a half moon. At 135 it will appear as a convex shape covering about three-

fourths of the disk. And at 180 a full moon will appear. After the full moon, the phases will

repeat in mirror image, with the shiny face directed towards the rising sun in the east rather than

the setting sun in the west.

Thus the model provides for detailed predictions that correlate the apparent shape of the

moon with its position relative to the sun, assuming only the features of the model and that the

moon is traveling westward more slowly than the sun. Nothing about these predictions is

striking to us now. Yet prior to Parmenides, none of the several theorists of lunar light and lunar

phases so much as noticed a correlation between the phases and the position of the moon relative

to the sun. Once we formulate the model, we have precisely correlated the position of the moon

with that of the sun. We can observe the progression of the phases night by night for lunar

month, and see all the predictions borne out faithfully. We can watch for another month and

observe them repeating day by day all over again. The model makes predictions which are

invariably confirmed. It reduces the question of lunar phases to a problem of geometry, and

solves it.

We have then an initial insight about the source of the moon light: the sun. This

explanation, which has been called heliophotism, seems to originate with Parmenides.11 It

gives rise to a model, which supports a general theory of lunar phases, which in turn can be

verified empirically. As I shall demonstrate later, this theory is espoused by both Anaxagoras


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12See Lakatos 1970.

and Empedocles, two near contemporary cosmologists who were a generation younger than

Parmenides. One of them, we have seen, tacitly acknowledges the influence of Parmenides, and

both of them clearly respond to philosophical doctrines of Parmenides.

Is this then a solid scientific advance? Let us take the story one step farther. The theory

of Heliophotism suggests explanations of other heavenly phenomena apparently unconnected

with lunar phases. If we can show a link between heliophotism and other explanations, we will

have a clear case of what Lakatos called a theoretically progressive problemshift; if it can show

how the theory was supported by new evidence, we will have an empirically progressive

problemshiftand perhaps more than that.12

The model of lunar light and its associated theory have implications for another problem

of Presocratic cosmology. What causes lunar and solar eclipses? The case of the lunar eclipse is

perhaps the easiest to address within Heliophotism. The source of the moons light is the sun. If

the moon ceases to shine, it must be because in some way or other the suns light is failing to

illuminate it. The model itself suggests one time when the suns light might fail to reach the

moon. At the time of the full moon the sun and moon are in opposition, which means that they

are on opposite sides of the earth. The earth, in most early theories, is relatively large. It is,

then, a feature of the model that the earth mightintervene between the sun and moon, causing the

suns light to fail and the moon to go dark. In effect, the moon would be passing into the shadow

of the earth. Here a potential explanation grows out of a model and its associated theory.

The model suggests another account for solar eclipses. Whereas the moon gets its light

from the sun and so is subject to being blocked by an intervening body somewhere between the


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sun and the moon, the sun generates its own light. Hence only a quenching of its light or a direct

interposition between the sun and the earth could explain a solar eclipse. But here the model

again suggests a possibility. According to the model the moon remains as a dark body even

when it disappears from view at the time of the new moon. Indeed, the theory dictates that

whenever the moon disappears, it is near the sun, in conjunction with it. We can observe at

times when both bodies are present in the sky, that they are approximately the same size,

subtending about of the ecliptic. We also know that the moon is below the sun, closer to

earth than it. Hence during every new moon, there is a dark body in the vicinity of the sun that

could, under the right conditions, interpose itself between the earth and sun. If this happened,

the moon would occult the sun.

I shall call the complex theory that accounts for both the darkening of the moon and that

of the sun by an interposition or antiphraxis (a term of art used by Aristotle and later

philosophers). Now in fact both Anaxagoras and Empedocles put forth the Antiphraxis theory.

They were (it will be argued later) the first cosmologists to do so. It is instructive to consider

Anaxagoras general theory of the heavens, as reported by Hippolytus.

(6) The sun and moon and all the heavenly bodies are fiery stones carried around

by the revolution of the aether. . . . The moon is below the sun and nearer to us.

(8) The sun exceeds the Peloponnesus in size. [H] The moon does not have its

own light, but gets it from the sun. The revolution of the stars carries them under

the earth. (9) [A] The moon is eclipsed when the earth blocks it, or sometimes

one of the bodies below the moon; the sun is eclipsed when the moon blocks it at


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13Hippolytus testimony is supported by Anaxagoras B18 (see n. 8* above).

14This claim was controversial in antiquity, as we shall see later. PlutarchNicias 23.2-3 =A18 attributes the first theory of Heliophotism to Anaxagoras; Plato Crat. 4091-b = A76attributes Heliophotism to him, but suggests an earlier discovery of the theorybased on playfuletymology. Dicks 1970, 59, accepts Anaxagoras as the first to give the true explanations of themoons phases . . . , and allows that Empedocles (54) and Anaxagoras (59) understandeclipseswithout managing to notice how these admissions undermine his general assessment ofPresocratic astronomy. Thoren 1971, 24, observes more generously, If there is anything thatcan truly be called a datum in pre-Socratic astronomy, it must surely be that Anaxagorasprovided, if not for the first time, then at least to the first general satisfaction, an explanation ofthe phases of the moon. Closely related to this tradition is the report that he also explained thecause of eclipses. See also Heath 1913, 78-80; O'Brien 1968; Curd 2007, 211-12.

the time of the new moon. The sun and the moon make their turnings when they

are deflected by the air. The moon makes frequent turnings because it cannot

overcome the cold. (10) He first correctly explained eclipses and illuminations.

He said the moon was earthy and had in it plains, and valleys.

(HippolytusRefutation of All Heresies 1.8.6-10 = DK 59A42)

The sentence marked [H] asserts Heliophotism.13 The heavenly bodies are stones, and hence

solid, three-dimensional bodies. The moon is earthy, and hence opaque. It lies below the sun

and nearer the earth. Anaxagoras gives, almost as a corollary, the correct account of both lunar

and solar eclipses, namely Antiphraxis [A], the first to do so according to 10 of the report. 14

We see from this report that Anaxagoras acknowledges all features of the model constructed

above, and treats the theory of eclipses as an extension of the model.

From Hippolytus and presumably his doxographical sources we get no indication of how

Anaxagoras came to his astronomical views or how they were connected in his mind. It seems

plausible, however, to see Heliophotism as the insight that inspires the whole theory. The model


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of lunar light provides the bridge from Parmenides original insight to an explanation of eclipses.

We find the same general account in Anaxagoras contemporary Empedocles, though not so

neatly juxtaposed as here. It does not matter for present purposes which of the two cosmologists

originally connected all the dots in the theory, though I believe it is Anaxagoras for reasons I

will allude to shortly. In any case, the model suggested by Heliophotism provides the basis for a

theory that explains not only the phases of the moon but both lunar and solar eclipses, and does

so correctly, as determined by subsequent research. There is then, a progressive articulation of

astronomical theory from Parmenides to Anaxagoras and Empedocles which can plausibly

understood as an instance of theoretical progress.

What of empirical evidence? This, according to critics of Presocratic theory, both

ancient and modern, is one of the weak links in the Presocratic program. The early cosmologists

are ingenious in their theorizing but quite unable to adduce empirical evidence to decide between

one theory and another. Yet as we have seen, Heliophotism suggests a model that makes precise

predictions for phases of the moon, which are confirmed infallibly. Furthermore, the model

makes predictions about lunar and solar eclipses. A lunar eclipse caused by the earths shadow

will happen only at the time of the full moon. Anaxagoras allows for lunar eclipses to be caused

also by other dark bodies. But as we now know, the earth causes all lunar eclipses.

Furthermore, the moon enters into the shadow of the earth from the right (west) and exits to the

left (east), just as the model would require, even with a moving sun and stationary earth.

A solar eclipse will, according to the model, occur only at the time of the new moon. A

circular disk must be seen to pass over the sun as the sun overtakes the moon and passes behind

it in its westward course. Today we would describe the moon as passing in front of a stationary


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15 Graham and Hintz 2007.

sun as the moon travels eastward in its orbit, but the anticipated effect is the same. Every solar

eclipse conforms to the model. In fact there were two solar eclipses visible in the region of

Greece during the period of activity of Anaxagoras and Empedocles. One in 478 BC passed over

the Peloponnesus and Attica and proceeded across the Aegean to Ionia. One in 463 passed over

central and northern Greece. Neither would have been visible (except as a partial eclipse) in

Acragas, Empedocles homeland in Sicily. The former could have been visible to Anaxagoras

whether he was his native Clazomenae in Ionia or in Athens (according to different accounts of

his life). Elsewhere I have argued that the otherwise inexplicable remark in 8 of the Hippolytus

passage quoted above (the sun is larger than the Peloponnesus) only makes sense if Anaxagoras

inquired into the extent of the umbra of the earlier eclipse, and the information from the

Peloponnesus would have been available to him only if he were in Athens at the time. 15 The

timing and location suggest that Anaxagoras was the first to understand the solar eclipse, and

that he also tested it empirically by establishing the extent of the shadow cast (assuming,

incorrectly but plausibly, that the shadow was about the size of the body that cast it).

If this is so, then Anaxagoras not only first hypothesized that solar eclipses are

occultations of the sun by the moon, but also tested his hypothesis empirically when an

opportunity presented itself. Thus (1) Anaxagoras offered an explanatory hypothesis for a

mysterious phenomenon, one that had been first offered by Parmenides but not defended by him.

(2) He adduced evidence for the hypothesis, which confirmed his hypothesis. That is, he was

able to confirm that the eclipse of 478 took place at the time of the new moon. A circular disk

was seen blocking the suns light (this was an annular eclipse in which a thin ring of fire was


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visible at the circumference of the sun). Finally, he seems to have been able to confirm that the

maximum eclipse was visible in a limited area about the size of the Peloponnesus. On no earlier

hypothesis was the eclipse a shadow with a limited radius, so the test would have effectively

eliminated all earlier hypotheses. Anaxagoras could have found out the extent of the eclipse by a

basic procedure consistent with the simplest form ofhistoriasking travelers at the port of

Piraeus what they had seen recently. What was important in this case was not a highpowered

scientific methodology, but an effective way of checking an otherwise meaningless piece of

information: was the eclipse seen over a limited area? The theory made the question relevant,

and the answer provided an important piece of empirical evidence in favor of a bold new

hypothesis.

IV

One large question remains: did Anaxagoras manage to convince any of his contemporaries and

successors? Did he achieve the kind of discovery that won over his peers? One of the major

criticisms of Presocratic cosmology, even in ancient times, was that its theories were

unverifiable. The heavenly bodies are remote and the true causes of their motions and actions

are thus inscrutable. One piece of evidence of the failure of early cosmological theories was the

very proliferation of such theories, without any apparent way to eliminate rivals and achieve

anything approaching consensus. In modern terms, the theories were, given the limited

empirical means available, not only unverifiable but (what is worse) unfalsifiable.

Here I propose to turn to a late account of cosmological theories, the doxographical


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tradition preserved in Atius. The very scheme embodied in doxography is adversarial: alternate

theories of the same phenomena are presented, often in contrasting groupings. Doxography

feeds on disagreement and often preserves such disagreement as a kind of fossil record, dissent

frozen in time. Three chapters from Book II of Atius Placita deal with topics of direct

relevance to the present inquiry: 28 On the Illumination of the Moon, 29 On the Lunar Eclipse,

and 24 On the Solar Eclipse. These chapters allow us to glimpse competing opinions and to

infer what sort of relation the several accounts had to each other.

We can survey opinions about the moons light in Atius:

Ch. 28. On the light of the moon

1. Anaximander, Xenophanes, Berosus [say] it has its own light (idion . . .

phs).

2. Aristotle says it has its own light, but it is somehow rather tenuous.

. . .

4. Antiphon [says] the moon has its own light, and that the light that is

hidden around it is darkened by the impact of the sun, as the stronger

naturally darkens the weaker; this happens to the other heavenly bodies

also.

5. Thales first said it is illuminated by the sun. Pythagoras, Parmenides,

Empedocles, Anaxagoras, Metrodorus likewise.

6. Heraclitus says the sun and moon manifest similar effects. For since the

heavenly bodies are shaped like bowls, when they receive rays from the


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moist evaporation, they shine to our view, the sun more brightly, for it

travels in purer air, while the moon travels in more polluted air, and for

this reason it appears dimmer.

As is typical in doxographical collections, the list is not laid out chronologically but according to

some dialectical scheme. The impression we are given is that there were two parallel traditions:

Anaximander, Xenophanes, Heraclitus, and Antiphon held that the moon generates its own light,

while most of the other early philosophers held that it is illuminated by the sun. It so happens

that the moon is illuminated by the sun, so some Presocratics got the explanation right. They

also got it right for the first time, since no other culture we know of ever came to this conclusion

independently. But was it a lucky guess, or a scientific discovery? Of course a scientific

discovery can begin with a lucky guess, but to count as scientific it must be confirmed in some

systematic way.

For the purposes of this inquiry, there are two contrasting opinions. Either the moon gets

its light from the sun or it generates its own light. The first theory is Heliophotism; the second I

shall callIdiophotism. Let us put our protagonists in historical order, as far as is possible. Then,

according to Atius, and abbreviating the two theories by their initials:

Philosopher Theory

Thales H

Anaximander I

Pythagoras H


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16For a more extended argument on this point, see Graham 2002, 352-58. See also Dicks1959; Stephenson 1997, 342-44. For Thales as originator of the theory, see Panchenko 1993;Panchenko 1994; Panchenko 1996. O'Grady 2002, 126-46, thinks Thales may have made somesort of prediction based on repeated cycles, but also thinks he may have understood the nature ofeclipses, based on a new fragment from Aristarchus in POxy. LIII. 3710. ii (c).36-43 (seeLebedev 1990). This may, however, reflect no more than a reconstruction by Aristarchus. Notethat one can make predictions on the basis of repeating patterns of eclipses with nounderstanding whatsoever of the cause of the events.

Xenophanes I

Heraclitus I

Parmenides H

Anaxagoras H

Empedocles H

Antiphon I

Metrodorus H

Aristotle I

No pattern emerges from this survey.

There are, however, good historical reasons for questioning several of Atius

attributions. In the case of Thales we actually have no reliable astronomical theory to go on. It

seems likely that he left no written record of his theories. We get a reference to the earth as like

a raft floating on a primeval seaa view that is probably incompatible with a correct

understanding of lunar light. One report ascribes earthy heavenly bodies to himbut on what

authority we do not know. Aristotle himself seems to have had no written record of Thales

views to consult.16 Although I cannot go into detail here, it seems likely that the legend of


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17 Burkert 1972, 299-350; Huffman 1993, 231-88.

18 Pendrick 2002, 296-97, claims that the impact of the suns light is meant to explainwhy the moon appears dim in daytime; but the suns light does not just fall on the moon duringthe daytime, nor is it plausible that Antiphon could have thought so in the late fifth century.More likely the matter concerns the secondary or residual light seen during a lunar eclipse.Anaxagoras seems to have had a theory about this earlier. See OlympiodorusMeteorology67.32-37, Plato Cratylus 409a-b, PlutarchDe fac. orb. lun. 933f, with Boll 1909, 2343-44;O'Brien 1968; Panchenko 2002. Grgemanns 1970, 36 n. 68, thinks this theory may originatewith Atius; but note the Plato passage.

19SeeA Po. 90a15-18, 93a29-b7;DC291b17-23, 292a-6, 297b23-30;Mete. 367b19-30;GA 777b24-26.

Thales prediction of a solar eclipse led to the uncritical attribution to him of a complete theory

of lunar light and solar eclipses, by astronomers writing after Aristotle.

As for Pythagoras, he left no writings and his society seems to have maintained a code of

silence for the sixth and early fifth centuries B.C. Recent scholarship has shown that much of

the cosmology and astronomy attributed to Pythagoras or the Pythagoreans originates from

Philolaus, who was a Pythagorean, but was active in the second half of the fifth century. 17 He

wrote a treatise containing cosmological and astronomical speculations. If Pythagorean views

originate with him, then they do not originate with Pythagoras, and they fall after many of the

other figures in the chronological list. In the case of Antiphon, a closer reading of the evidence

shows that he thinks the moon has some secondary light of its own, which is obscured by the

primary light of the sun falling on it; in other words, what we usually see is a reflection of the

suns light.18 As for Aristotle, Atius is just wrong, unless he refers to some missing account of

secondary light.19 So let us reexamine the list:

Philosopher Theory


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Thales ?

Anaximander I

Pythagoras ?

Xenophanes I

Heraclitus I

Parmenides H

Anaxagoras H

Empedocles H

Philolaus H

Antiphon H

Metrodorus H

Aristotle H

A different picture emerges. As far as we can see, Idiophotism does not alternate with

Heliophotism; first comes Idiophotism, then Heliophotism. The turning point is Parmenides, and

after him there is no going back.

We get another survey of opinions on the subjects of solar and lunar eclipses from Atius

(II.24 and 29, respectively):

Ch. 24. On the solar eclipse

1. Thales first said the sun is eclipsed when the moon moves directly under

it; and the moon is seen to be earthy . . .


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20See n. 16* above.

2. Anaximander [says it happens when] the mouth of the breathing-hole of

fire is covered.

3. Heraclitus, with the turning of the bowl, so the concave part is turned up,

the convex part down towards our sight.

4. Xenophanes, with quenching; another sun in turn appears toward the east .

. .

5. Some [say] a condensation of clouds coming over the disk [of the sun].

6. [The Pythagoreans say] when the moon comes under [the sun].

7. [Empedocles,] when the moon comes under [the sun].

Here we see that Thales, certain Pythagoreans, and Empedocles give the correct account of solar

eclipses. But we must remove Thales from the list for reasons similar to those given for the

theory of lunar light.20 As for the Pythagoreans, some of them wrote after Empedocles, so we

cannot on the vague reference given, attribute a breakthrough to them. The anonymous theory in

5 has been plausibly referred to Anaximenes, though the attribution is not certain.

For the lunar eclipse we get the following reports:

Ch. 29. On the lunar eclipse

1. Anaximander [says a lunar eclipse occurs] when the opening on the wheel

[of the moon] is blocked.

. . .


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3. Alcmaeon, Heraclitus, Antiphon with the turning of the bowl-shaped

container and its inclinations.

4. Some of the Pythagoreans according to Aristotles history and the

statement of Philip of Opus [say it occurs] sometimes by the blocking [of

the suns light] by earth, sometimes by a blocking by the counter-earth . . .

5. Xenophanes [says it and?] the monthly disappearance [occur] by

quenching.

6. Thales, Anaxagoras, Plato, the Stoics, in agreement with the mathematical

[astronomers, say] the moons conjunction with the sun and the fact that it

is illuminated by the sun cause the monthly disappearances of the moon,

and its moving into the shadow of the earth cause its eclipses (as the earth

lies between both bodies) . . .

7. Anaxagoras, as Theophrastus says, [holds that] sometimes bodies below

the moon cause its occlusion.

8. Chrysippus [says that] the moon is eclipsed when the earth causes its

occlusion and it enters into the earths shadow.

Those who understand lunar eclipses as the blocking of the suns light by the moon are Thales,

certain Pythagoreans, Anaxagoras, Plato, and the Stoics, including specifically Chrysippus. The

attribution of the correct eclipse theory to Thales is dubious for reasons similar to those for

denying to him a theory of heliophotism. As far as the Pythagoreans, the counter-earth seems to

be a unique feature of Philolaus cosmography, and Philolaus wrote one or two generations after


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Greek Astronomy 24

21Atius 2.7.7, 3.11.3, 3.13.2, Huffman 1993, 239.

22See n. 18* above.

Anaxagoras.21 Antiphon the Sophist is grouped with the Heraclitean theory; yet his account of

lunar light as reported in chapter 28 is incompatible with Heraclitus theory, so I omit the alleged

theory of Antiphon from chapter 29 as highly problematic.22 As for Plato, we have his complete

dialogues, which advance no theory of eclipses.

Thus we get the following scheme, with A for a theory based on Antiphraxis, O for

other:

Solar Eclipses Lunar Eclipses

Philosopher

Thales ? ?

Anaximander O O

Anaximenes? O

Xenophanes O O

Alcmaeon O

Heraclitus O O

Anaxagoras A

Empedocles A

Philolaus A

Chrysippus, Stoics A

Mathematical astronomers A


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23 Boll 1909, 2343-44; Bicknell 1967.

24 Toulmin 1967 views the theory as problematic and primitive. For other reports ofBerosus theory, see Lucretius 5.720-28, Vitruvius 9.2.1-2.

With both lunar and solar eclipses, we observe the introduction of a new theory, and then a shift

to that theory by all successors.

The reports of Atius assign Empedocles to solar eclipses and Anaxagoras to lunar

eclipses. They also point out that Anaxagoras admitted other bodies that could cause lunar

eclipses by blocking the suns light. I shall call these by their modern name, asteroids. There is

an reasonable explanation for his extension of the theory to asteroids, which I shall not pursue

here.23 Other ancient testimonies assure us that Anaxagoras understood both solar and lunar

eclipses and we have a fragment of Empedocles to demonstrate that he understood lunar eclipses.

There is no record of Parmenides having had anything to say about eclipses. Thus a corrected

reading of Atius, with figures put in chronological order, indicates the appearance of

Heliophotism with Parmenides and the appearance of Antiphraxis with Anaxagoras and

Empedocles. As we have seen Parmenides account of lunar light and Anaxagoras and

Empedocles theory of eclipses are closely related, as parent and offspring. Further, the present

exercise show that once these related theories were introduced, no other competing theories were

proposed.

There is one exception: Berosus offered an alternative account of the moons light

(Atius 2.28.2). But Berosus was a hellenized Babylonian priest writing in the late fourth

century BC. His theory is heavily influenced by the now-standard theory of Heliophotism and

seems like just a rather ad hoc variation of it, by (dare we say?) a barbarian.24

The Epicureans


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25Lucretius 5.705-14, 751-70.

26See note 19* above.

27Vitruvius 9.2.3-4.

28AlmagestVI.

withheld assent from Heliophotism and Antiphraxis, in accordance with their policy of allow

plural explanations of obscure phenomena, yet even they given pride of place to the two

Presocratic theories.25 Aristotle embraced both Heliophotism and Antiphraxis (pace Atius), and

indeed used the later as a paradigm of successful scientific theory.26 The influence of his

cosmology on posterity was immense. Significantly, the mathematical astronomers also

embraced the new theory and built it into their mathematical models of the heavens. As an

example, we have the account of Aristarchus, who (even in his heliocentic theory) attributes the

moons light to a reflection of solar light.27

After Atius time Claudius Ptolemy would use the

theory as the basis of his study of eclipses.28

Of course what we get from the doxographical tradition is a mere abridgement of

elaborate theories and complex arguments that renders them into their lowest common

denominators. Yet from what we can see, new theories of lunar light and of eclipses were just

not offered after the early fifth century BC. They seem to have become superfluous because the

phenomena they addressed ceased to be problematic. Whereas other astronomical and

cosmological phenomena continued to be hotly debated, lunar light and eclipses ceased to be

mysterious.

What we seem to have, then, in Helophotism and Antiphraxis is two instances of

immediate and long-lasting consensus. That most elusive feature of scientific progress is found


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Greek Astronomy 27

at the heart of Greek cosmology and at the beginnings of Greek astronomy. Anaxagoras

systematized Parmenides insight about lunar light, applied it to the problem of eclipses (and

other phenomena), provided empirical confirmation of his predictions, and convinced peers

(starting with Empedocles) that he had proved his point. Indeed, by Aristotles time his theory

had come to occupy the position of a Kuhnian paradigm, and by Chrysippus time it was perhaps

de rigeurin astronomy. Thus (1) Anaxagoras advanced a novel hypothesis, (2) adduced

evidence for it, and (3) proved it to the satisfaction of his peers and successors. The grand idea

that started the process came from Parmenides, but, as Hippolytus reported and Plutarch

explained, the systematic development and proof of the idea came from Anaxagoras.

More could be said about the reception of this theory, including how it got projected back

to Thales and Pythagoras. But perhaps enough has been said for nowexcept for one last point

that may seem too obvious to make: the consensus that began in the fifth century BC has

continued to the present day. The model of lunar light developed by Anaxagoras is that still

found in astronomy textbooks and manuals, as is his theory of eclipses. To be sure, we

understand now that the earth is spherical rather than flat, the earth orbits the sun rather than vice

versa, and the sizes and distances of the heavenly bodies are different from Anaxagoras. Yet for

all that, he got the source of lunar light and the cause of solar and lunar eclipses just right. He

made a major scientific discovery, one that paved the way for more discoveries, and one that was

so robust that it survived the transition to theories of concentric spheres, deferents and epicycles,

equants and eccentrics, and ultimately the Copernican Revolution itself. In view of Anaxagoras

accomplishment it seems perverse to hold that no Presocratic philosopher ever got beyond airy

speculations or made scientific progress. Scientific astronomy began, if not in the sixth century


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Greek Astronomy 28

as some hold, then surely in the early fifth century BC. If it began with mere speculation, it

quickly transcended its beginnings to offer the first paradigms of successful problem-solving and

models for future astronomical inquiry.

There is one further lesson suggested by this story. In the absence of meaningful dissent

from Heliophotism and Antiphraxis over a period of 2,500 years, it appears that they have ceased

to be problematic theories and morphed into scientifically indubitable descriptions of the world.

One of many competing hypotheses spawned by Presocratic philosophyone of the dream

children of a speculative thinkerhas passed all its tests and settled down into the textbooks

and handbooks of the twenty-first century. A scientific realist could not ask for a happier ending

to the story.


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Documents

Advances in Early Greek Astronomy