The ancient Greek philosophers’ and astronomers’ heliocentric view of the world

We propose the working hypothesis that the actual belief of the early Greek philosophers and astronomers was the heliocentric, rather than the geocentric, view of the Solar System. As an indication of the heliocentric view of the world we take the assumption of a spherical Universe, which is considered as “… the most fundamental assumption of Greek astronomy.” (Evans, 1998: 75, cf. 216-219). It is possible that the idea of a spherical cosmos existed already among the Ionian philosophers, at least from Anaximander onwards (Kahn, 1960: 92-94; Vernant, 1983: 180, 183, 187, 190-211), but perhaps even much earlier, and that it was not expressed clearly, because it ran counter to the conventional religious views and/or because it aided the perception of everyday celestial phenomena.

The earliest evidence about the astronomical knowledge of the ancient Greeks dates from the eighth century B.C. It is found in the epic poems of Homer and Hesiod (Aveni and Ammerman, 2001; Dicks, 1970: 27-38; Evans, 1998: 3-5; Papathanassiou, 2007), while some archaeological correlates to this written evidence have been pointed out recently (Coucouzeli, 2006; Dimitrakoudis et al., 2006). Astronomical knowledge appears to have been used in eighth century B.C. Greece for the purposes of cultivation, navigation, calendar regulation, worship and even politics.

However, astronomical interest in Greece seems to go much further back in time, to the second millennium B.C. An important source of information in this respect is the Orphic texts (Orphica). Although these texts were recorded and translated at the time of Peisistratos (sixth century B.C.) or, mostly, in later times (Kern, 1922; West, 1983), they seem to have existed for many centuries. According to Chassapis (1987), the Orphic Hymns were formulated in the period between 1841 and 1366 B.C. (i.e. during the Minoan and Mycenaean times), since they seem to refer to the vernal equinox and the summer solstice, when these took place in the Taurus and Leo constellations, respectively, up to 1841 B.C., as well as to the phenomenon of the equality of the summer and winter seasons, which occurred around 1366 B.C.

In addition, the Orphics appear to have known about the sphericity of the heavens as well as the two basic postulates of the heliocentric theory, according to which: a) the Earth is spherical and rotates around its own axis; and b) the movement of the Earth around the Sun causes the occurrence of the four seasons. In fact, the Orphics were teaching about the equal duration of the Earth’s rotation and of the apparent motion of the celestial sphere around the same axis (cf. Orpheus saying to his son Musaeus: “…as this (the Earth), which is round, rotates in equal time round its own axis.”—Aristobulus’ fragment ‘Diatheke’ or ‘Testament’ from his Explanation of the Mosaic Law recorded in Eusebius, Praeparatio Evangelica, 13, 12), and they accepted the Sun explicitly as the centre of attraction, around which the Earth describes an ecliptic orbit (dromos, i.e. ‘way’). They also appear to have introduced the notion of the zodiac circle, the names of the constellations, etc. (see also Ovenden, 1966; Papathanassiou, 1991; 2007).

Turning to archaeology, there is now increasing evidence concerning the astronomical interest of the Minoans and Mycenaeans, thanks to numerous archaeoastronomical studies, which were conducted during the last decade in peak sanctuaries, palaces and tombs on Crete (Blomberg and Henriksson, 1996; 2000; 2003; 2005). The study of orientations of buildings has shown that the sunrise and sunset positions at the four solar stands, the full Moon and the heliacal setting of Arcturus, were all taken into account by the ancient inhabitants of Crete since the Early Minoan Age in an effort to establish a physical relationship between themselves and the sky for the sake of keeping a calendar, for navigation, and perhaps also for religious and political purposes (Henriksson and Blomberg, 1996: 113). Apart from orientations, a number of ceramic figurines representing animals, humans or parts of the human body from two peak sanctuaries on Crete have been interpreted, on the basis of ancient written accounts (e.g. Aratos), as having had an astronomical significance related to the recognition of the zodiac (Blomberg, 2000). Finally, a number of Linear B tablets from Pylos and Knossos, dating from later Mycenaean times, record calendar months (Papathanassiou, 2007).

It is therefore possible that Greek astronomy slowly built upon this earlier knowledge going back to the Minoan and Mycenaean civilizations, which were as developed as those of Egypt and Babylon. This knowledge would have been transmitted from generation to generation into the so-called ‘Dark Ages’ (ca. 1100-700 B.C.) and into Archaic and Classical Greek times (e.g. Liritzis and Vassiliou, 2003). Observations may have been carried out by means of various types of sighting aids and measuring devices, such as the gnomon, the klepsydra, the polos or the parapegma, perhaps also including simple forms of armillary spheres (Dimitrakoudis et al., 2006) or even wooden tubes (containing lenses?) in the manner of a primitive dioptra (cf. Evans and Berggren, 2006: 27-42).

To return to our basic assumption of a spherical Universe, it is worth pointing out that the symbolism of the circle was pre-eminent in traditional Greek cosmological thought. The two-dimensional circular shape was considered as the most perfect and sacred, and it must reflect some concept of the wider Universe as a sphere, the most beautiful and divine three-dimensional shape (Edmunds, 2006; cf. also Geminos’ sphairopoieia, i.e. the spherical construction of the cosmos according to nature, in his Introduction to the Phenomena—see Evans and Berggren, 2006: 51-53).

The Greek philosophers’ consideration of the sphere as the shape of the divine substance is attested as early as Xenophanes (sixth century B.C.); it is further elaborated by Plato (Timaeus, 33b) and it is also encountered in Aristotle (On the Heavens, II. 286a10: “But such is the heaven, viz. a divine body, and for that reason it possesses the circular body which by nature always moves in a circle.”; Leggatt, 1995: 227; cf. Vernant, 1983: 183). However, the symbolism of the circle and the sphere may be a lot older. As we have seen, the accounts attributed to the Orphics refer to a spherical Universe with revolving celestial bodies and a solar centre.

As far as the archaeological evidence is concerned, it is worth mentioning that a circle representing the two celestial hemispheres connected with the Dioskouroi, Castor and Pollux, seems to appear on a cryptographic seal dating from ca. 750-700 B.C. (Coucouzeli, 2006), while a series of votive artefacts, dating from ca. 750-480 B.C., may well represent celestial spheres with meridians and sometimes also an equator (Dimitrakoudis, et al., 2006).

The astronomical views and discoveries of the ancient Greek philosophers and astronomers, in particular those regarding the relative positions of the Earth and the Sun, are well-known and they date from the earlier historical era of Thales (ca. 624-547 B.C.) to the later times of Ptolemy (A.D. 87-150).

All of them held the picture of a spherical Universe (see Dicks, 1966: 30) and, as we will show below, most of them seemed to artificially consider the Earth at the centre of the Universe. Indeed, throughout Greek cosmological thought, as a general rule, man’s position in the Universe is considered as a privileged one. Nevertheless, the doctrine that the Earth we inhabit occupies the centre of the Universe was contested. Some philosophers and astronomers even went as far as setting the Sun (or a fiery substance reminiscent of the Sun) in a central position, at the risk of being subjected to public anathema.

Thales (ca. 624-547 B.C.) conceived the Earth as a disc at the centre of the Universe, floating on water (an implication of the celestial equator?). He seems to have predicted the total solar eclipse on 28 May 585 B.C.—a major achievement, which cannot be explained on the basis of the existing evidence about his knowledge—and to have produced a model of the celestial globe (the story about Thales’ prediction of the solar eclipse has been widely discussed, e.g. by O’Grady, 2002). Anaximander (ca. 610-546 B.C.) envisaged the Earth suspended at the centre of a spherical Universe, he distinguished between fixed stars and planets, and he made the first attempt at a ‘mechanical model’ of the Universe, which appeared as a revolving sphere (Lloyd, 1970: 17). Anaximenes (ca. 585-525 B.C.) gave a privileged status to the Sun against the other celestial bodies in the spherical cosmos, arguing that it gave light to the Moon. A central role was also given to the Sun by Heracleitos (ca. 540-480 B.C.), who postulated that the celestial orbit had characteristics related to a constant law of cosmic fire (this is vaguely reminiscent of Newton’s Law of global attraction). Pythagoras (ca. 572-495 B.C.) pictured a spherical Earth kept at the centre of the world by its equilibrium and containing a fiery core, the central ‘hearth’ (‘Hestia’); he also advanced the idea of the revolution of the cosmic sphere on an axis passing through the centre of the Earth and he identified the five zones of the Earth (which were also adopted slightly later by Parmenides, ca. 504-450 B.C.). Oinopides of Chios (ca. 490-420 B.C.) identified the ecliptic as the oblique orbit of the Sun with respect to the celestial equator, which led to the definition of the four solar stands and the four seasons. Anaxagoras maintained that the Sun and all the stars in the spherical Universe are fiery stones, while the Moon is made of earth and receives its light from the Sun, thus providing the clearest explanation of the solar and lunar eclipses. The intriguing theory of Empedocles (ca. 484-424 B.C.), according to which there are two suns, a real or archetypal one (the fire of the Earth in the centre) and an apparent one (the visible Sun), which is a reflection of the archetype on a crystal bowl, a theory that stresses the Sun’s extrapolated projection opposite the Earth, probably implies knowledge of the obliquity of the ecliptic, but also a representation of the Sun revolving around the Earth, which would have served pedagogical purposes.

As for the Pythagoreans, the evidence is somewhat confused, but they essentially denied that the Earth is at the centre of the Solar System. Some inklings of a heliocentric view of the world appear in the theory of the Pythagorean Philolaos (ca. 480-405 B.C.), who posited the existence of a central fire (‘Hestia’ or ‘Tower of Zeus’), around which revolve the celestial bodies, including the Earth (see Gavroglou, et al., 2003; Huffman, 1993).

Plato (ca. 427-347 B.C.) adopted the Pythagorean theory of the circular motion of the Earth (‘winding round’ – eillomenen) up and down on the axis of the Universe. He assimilated the latter with the spindle of Necessity.

In his mystic vision of the Universe, Plato also distinguished between two kinds of motion, the motion along the equator (or ‘circle of the Same’) and the motion along the ecliptic (or ‘circle of the Other’). It is clear that he considers a spherical Earth revolving around itself and around the Sun.

In an interesting passage given to us by Aristotle (On the Heavens, II. 293a17-293b1) it is stated that, besides the Pythagoreans, “… many others …” held the view that fire occupies the centre of the Universe and the reason they gave is that fire, rather than earth, is the most honourable thing and therefore deserves the most honourable place. Up until the time of Aristotle, therefore, there were astronomers who had no qualms about abandoning the traditional view, which gave the Earth the central position.

Aristarchos of Samos (310-230 B.C.) was the first astronomer to put forward a heliocentric astronomical theory in an explicit and unquestionable manner (Heath, 1932; Noack, 1992). Archimedes wrote about him:

His hypotheses are that the fixed stars and the sun remain unmoved, that the earth revolves about the sun in the circumference of a circle, the sun lying in the middle of the orbit … (Psammites, I. 4-7).

Nevertheless, the striking hypothesis advanced by Aristarchus met with great hostility in Athens and—with the sole exception of Seleucos of Seleucia, who espoused it vividly more than a century later—does not seem to have created any solid following.

It seems, therefore, that throughout the history of ancient Greek astronomy theories supporting a geocentric or a heliocentric (or at least a ‘fire-centred’) world coexisted in opposition to each other at any one time. Nevertheless, the geocentric view of the Universe prevailed throughout Hellenistic and Roman times, whereas the heliocentric view was abandoned, only to be rediscovered by Copenicus in the sixteenth century.

The reason for the return, after Aristarchos of Samos, from the heliocentric to the geocentric system —apart from the still existing prejudices and religious beliefs, which set the Earth-Hestia at the centre of the Universe—was the failure of the heliocentric model to ‘save the appearances’. More especially, the heliocentric theory failed to account for a number of physical and astronomical considerations. First, it was inconsistent with ordinary experience of motion: if indeed the Earth was subject to daily axial rotation around the Sun, this would have had a serious effect on the movement of heavy objects (since they naturally travel towards the centre of the Earth) or of objects moving through the air, of winds and clouds (since the Earth would be spinning at incredible rates of speed), whereas no such effects were observed. Second, this theory did not help to explain the apparent absence of stellar parallax (i.e. of any change in the relative positions of the stars as observed from different points of the Earth’s orbit), nor did it account for the inequality of the seasons as defined by the solstices and the equinoxes or for the anomalies in the orbits of the celestial bodies, which became obvious as observations improved. Concerning the objection regarding parallax, it is worth mentioning here in particular the Pythagorean attempt to accommodate the phenomenon of lunar parallax, as reported by Aristotle:

For, since the earth is not the centre, but is distant from it by a whole hemisphere of the earth, nothing … prevents the apparent facts (τα φαινόμενα) occurring in the same way when we do not live at the centre as they would were the earth to be at the centre. For even as it is, nothing makes it obvious that we are at a distance of half a diameter from the centre [i.e. on the Earth’s surface]. (On the Heavens, II. 293b25-30; see also Leggatt, 1995: 255-256).

On the other hand, the model of epicycles and eccentrics, first propounded by Apollonios of Perge and expanded later by Hipparchos and Ptolemy, which assumed a geocentric system, could satisfy with enough accuracy the reconstruction of the celestial phenomena and could compromise with a stationary Earth. Indeed, it was not judged necessary for any mathematical constructions used by astronomical models, such as the model of epicycles and eccentrics or, before it, Eudoxos’ model of concentric spheres, to have a physical basis, but rather to be suitable in predicting the planetary positions, (cf. Plato’s and Ptolemy’s ‘hypotheseis planōmenōn’). Astronomy was a mathematical exercise designed to ‘save the appearances’, to account for the motions of the heavenly bodies by making use of mathematical hypotheses.

The astronomical models aimed at a better estimation of the phenomena connecting the model with the observation. Thus, what counted as phenomena to be saved did not change with time, as Greek astronomy matured. Because of this Ptolemy’s model certainly is not matured astronomy, but rather a culmination of astronomy in terms of complex mathematical models.

It is known that the general frame or model adopted finally was that of the celestial sphere with the sphercal Earth immobile at the centre. This conception of the Universe proved valuable and long lasting. Even today the model of the celestial sphere is used as a necessary basis for the drawing of sky maps, or in planetaria and with orreries, which by their very nature require the observer to occupy a central position. To ordinary observers on Earth, the stars appear to be attached to the inside of a vast hollow globe, which spins round the Earth from east to west once a day.

Although this view is not true—given that the Earth is not at the centre of the Universe, but is only a smallsized planet spinning on its axis in its orbital motion around a brighter than average star in a larger than average galaxy—it could have been useful (and often
still is) for astronomers to pretend that this globe, or celestial sphere in the sky, really does exist. Moreover, the cyclic orbit in the heliocentric model does not explain the planetary positions with any accuracy as the geocentric model does.

Indeed, an understanding of most everyday phenomena is made easier if one constructs an image of the ‘celestial sphere’ having the Earth at the centre, whereby the Sun is projected opposite the Earth’s orbit. Such phenomena include the following:

1) The Earth’s revolution around an axis passing through its centre, and turning from east to west, determining day and night.

2) The appearance of the Earth as suspended in cosmic space.

3) The Earth’s movement around itself and around the Sun.

4) The ecliptic and the celestial equator.

5) The four seasons of the year.

6) Lunar and solar eclipses.

7) The ecliptic circle and the zodiacal band of constellations.

8) The apparent movement of the Sun through the stars.

9) The four solar stands (two equinoxes and two solstices) during a year.

10) The precession of the equinoxes.

11) The obliquity of the ecliptic, i.e. the 23.5o angle between the plane of the ecliptic and the plane of the celestial equator.

12) The determination of the planetary positions at a particular time.

Geocentrism could have also assisted in the fixing of calendric time, as well as in the prediction of weather phenomena by means of the risings and settings of the fixed stars or constellations (Taub, 2003). Moreover, the brilliant idea of considering a celestial sphere having the Earth at its centre, with the Earth’s poles, as well as lines of latitude and longitude projected on it, would have proved extremely useful to the ancient Greek astronomers themselves, as it is often helpful to astronomers nowadays, because it makes it easier to observe far away celestial bodies by placing them on the surface of the ‘celestial sphere’ and by assuming them to be at an infinite distance.

For all the above reasons geocentrism could have apparently won out vis-à-vis heliocentrism. Moreover, the adoption of a geocentric view would have allowed the ancient Greek philosophers and astronomers not only to harmonize their theories with the religious beliefs of their time, but also to facilitate the reception of the surrounding world by the ordinary people so as to ‘save the appearances’, without sacrificing the essence of their ideas. Furthermore, it could have assisted their own observations of the celestial bodies.

Conclusion

The spherical Universe, apparently implying a belief in the heliocentric system, may have been the prevalent view of the ancient Greek philosophers and astronomers.
However, the opposition of such a view to the established religious beliefs, which assumed that the Sun-god circled the Earth-Hearth of the world, and the fear of prosecution for impiety, at least in Athens during the Classical period, may have prevented the promotion of the heliocentric model. Moreover, a model placing the Earth at the centre of the Universe with the Sun revolving around the Earth—much like the modern representation of the celestial sphere—would have ‘saved the appearances’: it would have explained day and night, the four seasons of the year, the solstices and the equinoxes, the apparent movement of the Sun through the stars and constellations, as
well as lunar and solar eclipses, and planetary motions, without exposing the inherent beliefs of the philosophers and astronomers.

(Source: “Ancient Greek heliocentric views hidden from prevailing beliefs?”, by Ioannis Liritzis and Alexandra Coucouzeli)

Research-Selection for NovoScriptorium: Isidoros Aggelos