Early Greek cosmology refers to beliefs about the structure (cosmography) and origins (cosmogony) of the cosmos (Greek "kosmos") primarily from the 8th to 5th centuries BC before it was superseded by ancient Greek astronomy, which was demythologized and involved the systematic study of the world. The main features of early Greek cosmography are shared with those found in ancient near eastern cosmology. The basic elements of the cosmos include (a flat) earth, heaven, the sea, and the netherworld (Tartarus), the first three of which corresponded to the gods Gaia, Ouranos, and Oceanus (or Pontos). 
Some primary sources for early Greek cosmology include the poetry of Homer (the "Iliad" and the "Odyssey"), Hesiod (the "Theogony" and the "Works and Days"), and surviving fragments from Mimnermus.
In the 5th century BC, Greek thinkers began to add new features to this cosmology. One, advocated by figures including Xenophanes, Parmenides, and Empedocles advocated the view that the "kosmos" as a whole (and not the Earth in particular) was spherical. Empedocles and Anaxagoras advocated for a notion called "vortex", which describes a rotation of the cosmos that explains the visible rotation of the stars. When the spherical model of the Earth was proposed, early Greek cosmology as a whole began to be replaced, although this was not immediate. Geographers like Ctesias and Ephorus rejected a spherical Earth in the 4th century BC. Among authors from the early Roman Empire, Strabo, Tacitus, and the Epicureans continued to accept a flat Earth. The last Greek author known to maintain this position was Cosmas Indicopleustes in the 6th century AD.
Overview.
All models of early Greek cosmology shared the following five elements:
Another important element of early Greek cosmology that would distinguish it from the ancient Greek astronomy that would come to dominate in later centuries was the emphasis on the role of the gods in the past and ongoing history of man and the mythological nature of the surrounding world.
Earth, Oceanus, and Hades.
Near the edges of the earth is a region inhabited by fantastical creatures, monsters, and quasi-human beings. Once one reaches the ends of the earth they find it to be surrounded by and delimited by an ocean (Oceanus), as is seen in the Babylonian Map of the World, although there is one main difference between the Babylonian and early Greek view: Oceanus is a river and so has an outer bank, whereas the Babylonian Map does not have an outer rung beyond which the world geography extends. But for Hesiod, beyond Oceanus lies both Hesperides, the daughters of Night (Nyx), and the Gorgons. The only human or hero to traverse past Oceanus is Heracles on his journey where he finds the cattle of Eurytion. The sun does not illuminate the region beyond the river ocean as its circular revolution does not extend over these regions. Instead, according to Mimnermus, it lies during the night in a golden chamber by the banks of the ocean, or perhaps in a barque that had been made by Hephaistos. Oceanus is also overlaid by the rim of a shield that was fashioned by Hephaistos for Achilles. Related to Oceanus, as his sister and wife, is Tethys, who was associated with freshwater, rivers, and springs. Beyond the ocean is the realm of Hades: as such, the ocean separates the domains of the living and the dead. This spatial separation is conceived on a horizontal plain and not a vertical one, and it is reflected by the journey of Odysseus to the afterworld (in this system, the afterworld/Hades is distinct from the underworld/Tartarus). Odysseus crosses the river ocean, takes a long walk across the banks of the ocean, and then arrives at the place where the rivers Cocytus and Phlegethon join to form the Acheron. Hades is also spatially characterized by terms like "erebos" and "zophos" which designate a region of darkness unreached by the sun. Finally, the center of the Earth, the axis mundi, often is said to have a cosmic mountain or cosmic tree, similar to Mount Mashu in Mesopotamian cosmology. In Homer's Iliad, Mount Olympus reaches to heaven.
The island of Circe is a kind of gate that regulates passage into Hades, which one must go through if they wish to enter Hades from the inhabited region or return to the inhabited region from Hades. The island, which is called "the dwelling of early Dawn and her dancing-lawns, and the risings of the sun" at the beginning of Book 12 of the Odyssey, is close to both the places where the sun rises and sun sets. This leads to the description of a counter-intuitive topography where both east and west ultimately collapse into a single point. This leads to Odysseus being disoriented on the island where he says "we do not know where East is, nor where the bright sun goes down under the earth": Odysseus cannot tell apart east from west. Like in Egyptian literature, the exit and entry point of the sun into the inhabited world, circumscribed by Oceanus, lie side by side with each other as a double-gate. Likewise in Hesiod's Theogony (lines 750–756), the paths of the sun and moon are contiguous.
Heaven.
The heaven is a flat and solid firmament supported by pillars. Embedded into the firmament was the sun, moon, and the stars. These astral bodies were personified as, themselves, being gods that could be worshiped or prayed to. According to the Theogony of Hesiod, 116–133:First of all Chaos came into being, and then broad-bosomed Earth (Gaia), a firm seat of all things for ever, and misty Tartaros, deep down in broadpathed earth, and Eros, the most beautiful among the immortal gods, he who loosens our limbs, and subdues the mind and thoughtful counsel of all gods and men. From Chaos, Erebos and black Night came into being, and from Night, again, came Aither and Day, whom she conceived and bore after having mingled in love with Erebos. Now Earth first of all brought forth starry Ouranos, equal to herself, so that it would cover her on all sides, to be a firm seat for the blessed gods forever. She also brought forth large mountains, the beautiful abode of the divine Nymphs who dwell in the woody mountains. She also bore the unharvested sea, seething with its swell, Pontos, without an act of delightful love. Then she slept with Ouranos and bore Okeanos with his deep eddies [...]Heaven is described once as bronze and twice as iron. One passage in the Iliad, where Zeus makes a cosmic threat against any god who dares to intervene in the Trojan War, provides more information on how heaven relates to the rest of the universe:I will seize him and hurl him down to Tartaros wrapped in mist, 
far away, a place where there is a pit deeper than any other, 
where there are iron gates and a bronze threshold, 
as far from Hades as heaven is from earth.Zeus makes a cosmic threat that any such actor will be hurled downwards at an immense distance, whose distance downwards is similar to the distance upwards to the heavens. Later, lines 721–725 reiterate that the region is as far below earth as earth is from heaven: just as it takes ten days for an anvil to fall from heaven to earth, so it takes ten to fall from earth to the underworld. This suggests that the Greeks in this period conceived of the cosmos on a vertical axis, where planes of the cosmos from Tartarus, the earth, and heaven are successively located above each other. Furthermore, the equivalence between the immensity of the directions up and down may also indicate that humans lie on the central plane of this vertical axis.
Underworld (Tartarus).
The underworld in the writings of Hesiod is an immense, dark, and enclosed region called "Tartarus". Tartarus is not a part of the underworld but rather its whole. Three different images of Tartarus can also be painted, depending on the observer. The Titans, sealed into Tartarus by Zeus during the Titanomachy, view it as an inescapable walled enclosure. The entities of Night, Day, Sleep, and Death effectively experience it as a house: it can be entered into and left at will. (These entities all reside in Tartarus, and so Tartarus can be said to house the cyclical phenomena of night, day, sleep, and death.) Finally, from the viewpoint of human topography, it can be understood as a great gorge. These images are also not mutually exclusive: the Titans have an increased difficulty of escaping from Tartarus, such as to the earth, due to the depth of the gorge. Hesiod offers multiple descriptions of features of the underworld, and sometimes they come into tension with each other. According to Johnson, the proper way to read Hesiod so as to avoid encountering such tensions, according to Hesiod's own intentions, is to understand that "Hesiod is not attempting to provide a map of the various structures within the underworld but is giving separate descriptions of the underworld as a whole".
Hesiod refers to a fence enclosing Tartarus, as well as Poseidon's doors and an associated wall; Johnson believes that these terms are referring to the same barrier. Tartarus is a great windy chasm. Some passages locate Atlas in Tartarus but others place it in the far west with the Hesperides, past Oceanus. Other evidence also indicates that Tartarus is both located below the earth but is also to be found at its edges. Thus, Tartarus extends such that some regions of it can be found vertically below the earth whereas others can be found horizontally surrounding it. Hesiod also places in Tartarus a house of Sleep and Death. Finally, the river and goddess Styx, who is the offspring of Oceanus and Tethys, flows into the underworld. The direct source for the water of the Styx river is Oceanus: once Styx parts from Oceanus, Styx flows into much of the underworld in both horizontal and vertical directions. Styx may be visualized as a singular stream starting at the horizon and then parting into multiple individual streams downwards.
Cosmogony and the gods.
Cosmography is related to cosmogony, the original formation of the cosmos, insofar as the latter provides an etiology for the former. Chaos has always existed and is the primordial matter, and out of it creation arises. Cosmic guardians ensure that the creation does not slip back into a status of chaos, for example: "Sun will not overstep his measures; otherwise the Erinyes, guardians of Dike, will fnd him out." Chaos gives rise to Ouranos, Gaia, and Pontos (heaven, earth, sea) who, by association or sexual union, bring forth the rest of the gods. Cosmic struggles between Zeus and the Titans threaten the order of the universe in what is called the Titanomachy: Zeus and the Hundred-Armed eventually drives the Titans, previously Earth-bound, into the netherworld, Tartarus. A wall and fence is also built around Tartarus with giant bronze doors.
One important feature of cosmogony is the intermingling or mating between Oceanus, the salt-water god, with Tethys, the god of all forms of fresh-water (including rivers, springs, and so on). This mimics the earlier Mesopotamian division of the salt and freshwater gods, Tiamat and Abzu. And like in earlier Mesopotamian cosmology, Homer and Hesiod, and later still in Plato ("Timaeus" 40e), the salt and freshwater gods intermingle (or mate) to produce succeeding generations of gods. Ideas regarding the blending of salt and freshwater, personified by these deities, may stem from hydrological observations of these phenomena. The name of the island-country Bahrain also today means "Two Seas", in reference to the meeting and mingling of fresh and salt water seas.
Relationship with other cosmologies.
Early Greek cosmogonies may be related to earlier, Indo-European cosmogony. The god-trio Zeus (king of the gods), Poseidon (god of the sea), and Hades (god of the netherworld) have been described as a "perfect" equivalent to trios of gods in ancient near eastern cosmologies, such as in the god-trio of Ugaritic cosmology, Baal, Yam, and Mot. Hesiod's Theogony is also closely textually related to an earlier Hittite cosmological text, the Song of Kumarbi.
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