This timeline of cosmological theories and discoveries is a chronological record of the development of humanity's understanding of the cosmos over the last two-plus millennia. Modern cosmological ideas follow the development of the scientific discipline of physical cosmology.
For millennia, what today is known to be the Solar System was regarded as the contents of the "whole universe", so advances in the knowledge of both mostly paralleled. Clear distinction was not made until circa mid-17th century. See Timeline of Solar System astronomy for further details on this side.
Antiquity
16th century BCE – Mesopotamian cosmology has a flat, circular Earth enclosed in a cosmic ocean.Horowitz (1998), p. xii 15th–11th century BCE – The Rigveda, a sacred text of Hinduism, contains several hymns concerning the origins of the cosmos. The Nasadiya Sukta (10.129) questions how creation arose and whether its origin can be known.
15th–6th century BCE – During this period, Zoroastrian Cosmology Develops and defines Creation as a manifestation of a cosmic conflict between existence and non-existence, good and evil, and light and darkness. 6th century BCE – The Babylonian Map of the World shows the Earth surrounded by the cosmic ocean, with seven islands arranged around it so as to form a seven-pointed star. Contemporary Biblical cosmology reflects the same view of a flat, circular Earth swimming on water and overarched by the solid vault of the firmament to which are fastened the stars. 6th–4th century BCE – Greek philosophers, as early as Anaximander,This is a matter of debate:
introduce the idea of multiple or even infinite universes. Democritus further detailed that these worlds varied in distance, size; the presence, number and size of their suns and moons; and that they are subject to destructive collisions."there are innumerable worlds of different sizes. In some there is neither sun nor moon, in others they are larger than in ours and others have more than one. These worlds are at irregular distances, more in one direction and less in another, and some are flourishing, others declining. Here they come into being, there they die, and they are destroyed by collision with one another. Some of the worlds have no animal or vegetable life nor any water."
Also during this time period, the Greeks established that the Earth is spherical rather than flat. 6th century BCE – Anaximander conceives a mechanical, non-mythological model of the world: the Earth floats very still in the centre of the infinite, not supported by anything.Aristotle, On the Heavens, ii, 13 Its curious shape is that of a cylinder"A column of stone", Aetius reports in De Fide (III, 7, 1), or "similar to a pillar-shaped stone", pseudo-Plutarch (III, 10). with a height one-third of its diameter. The flat top forms the inhabited world, which is surrounded by a circular oceanic mass. Anaximander considered the Sun as a huge object (larger than the land of Peloponnesus), and consequently, he realized how far from Earth it might be. In his system the celestial bodies turned at different distances. At the origin, after the separation of hot and cold, a ball of flame appeared that surrounded Earth like bark on a tree. This ball broke apart to form the rest of the Universe. It resembled a system of hollow concentric wheels, filled with fire, with the rims pierced by holes like those of a flute. Consequently, the Sun was the fire that one could see through a hole the same size as the Earth on the farthest wheel, and an eclipse corresponded with the occlusion of that hole. The diameter of the solar wheel was twenty-seven times that of the Earth (or twenty-eight, depending on the sources)In Refutation, it is reported that the circle of the Sun is twenty-seven times bigger than the Moon. and the lunar wheel, whose fire was less intense, eighteen (or nineteen) times. Its hole could change shape, thus explaining lunar phases. The stars and the planets, located closer,Aetius, De Fide (II, 15, 6) followed the same model.Most of Anaximander's model of the Universe comes from pseudo-Plutarch (II, 20–28): "[The Sun] is a circle twenty-eight times as big as the Earth, with the outline similar to that of a fire-filled chariot wheel, on which appears a mouth in certain places and through which it exposes its fire, as through the hole on a flute. [...] the Sun is equal to the Earth, but the circle on which it breathes and on which it's borne is twenty-seven times as big as the whole earth. [...] [The eclipse] is when the mouth from which comes the fire heat is closed. [...] [The Moon] is a circle nineteen times as big as the whole earth, all filled with fire, like that of the Sun". 5th century BCE – Parmenides is credited to be the first Greek who declared that the Earth is spherical and is situated in the centre of the universe. 5th century BCE – Pythagoreans such as Philolaus believed the motion of planets is caused by an out-of-sight "fire" at the centre of the universe (not the Sun) that powers them, and Sun and Earth orbit that Central Fire at different distances. The Earth's inhabited side is always opposite to the Central Fire, rendering it invisible to people. They also claimed that the Moon and the planets orbit the Earth. This model depicts a moving Earth, simultaneously self-rotating and orbiting around an external point (but not around the Sun), thus not being geocentrical, contrary to common intuition. Due to philosophical concerns about the number 10 (a "perfect number" for the Pythagorians), they also added a tenth "hidden body" or Counter-Earth (Antichthon), always in the opposite side of the invisible Central Fire and therefore also invisible from Earth. 4th century BCE – Plato claimed in his Timaeus that circles and spheres are the preferred shape of the universe, that the Earth is at the center and is circled by, ordered in-to-outwards: Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, and finally the fixed stars located on the celestial sphere. In Plato's complex cosmogony,"The components from which he made the soul and the way in which he made it were as follows: In between the Being that is indivisible and always changeless, and the one that is divisible and comes to be in the corporeal realm, he mixed a third, intermediate form of being, derived from the other two. Similarly, he made a mixture of the Same, and then one of the Different, in between their indivisible and their corporeal, divisible counterparts. And he took the three mixtures and mixed them together to make a uniform mixture, forcing the Different, which was hard to mix, into conformity with the Same. Now when he had mixed these two with Being, and from the three had made a single mixture, he redivided the whole mixture into as many parts as his task required, each part remaining a mixture of the Same, the Different and Being." (35a-b), translation Donald J. Zeyl the demiurge gave the primacy to the motion of Sameness and left it undivided; but he divided the motion of Difference in six parts, to have seven unequal circles. He prescribed these circles to move in opposite directions, three of them with equal speeds, the others with unequal speeds, but always in proportion. These circles are the orbits of the heavenly bodies: the three moving at equal speeds are the Sun, Venus and Mercury, while the four moving at unequal speeds are the Moon, Mars, Jupiter and Saturn.Plato, Timaeus, 36cPlato, Timaeus, 36d The complicated pattern of these movements is bound to be repeated again after a period called a 'complete' or 'perfect' year.Plato, Timaeus, 39d However, others like Philolaus and Hicetas had rejected geocentrism. 4th century BCE – Eudoxus of Cnidus devised a geometric-mathematical model for the movements of the planets, the first known effort in this sense, based on (conceptual) concentric spheres centered on Earth. To explain the complexity of the movements of the planets along with that of the Sun and the Moon, Eudoxus thought they move as if they were attached to a number of concentrical, invisible spheres, every one of them rotating around its own and different axis and at different paces. His model had twenty-seven homocentric spheres with each sphere explaining a type of observable motion for each celestial object. Eudoxus emphasised that this is a purely mathematical construct of the model in the sense that the spheres of each celestial body do not exist, it just shows the possible positions of the bodies. His model was later refined and expanded by Callippus.
4th century BCE – Aristotle follows the Plato's Earth-centered universe in which the Earth is stationary and the cosmos (or universe) is finite in extent but infinite in time. He argued for a spherical Earth using lunar eclipsesDe caelo, 297b31–298a10 and other observations. Aristotle adopted and expanded even more the previous Eudoxus' and Callippus' model, but by supposing the spheres were material and crystalline. Aristotle also tried to determine whether the Earth moves and concluded that all the celestial bodies fall towards Earth by natural tendency and since Earth is the centre of that tendency, it is stationary. Plato seems to have obscurely argued that the universe did have a beginning, but Aristotle and others interpreted his words differently. 4th century BCE – De Mundo – Five elements, situated in spheres in five regions, the less being in each case surrounded by the greater – namely, earth surrounded by water, water by air, air by fire, and fire by aether – make up the whole Universe. 4th century BCE – Heraclides Ponticus is said to be the first Greek who proposes that the Earth rotates on its axis, from west to east, once every 24 hours, contradicting Aristotle's teachings. Simplicius says that Heraclides proposed that the irregular movements of the planets can be explained if the Earth moves while the Sun stays still, but these statements are disputed. 3rd century BCE – Aristarchus of Samos proposes a Sun-centered universe and Earth's rotation in its own axis. He also provides evidences for his theory from his own observations. 3rd century BCE – Archimedes in his essay The Sand Reckoner, estimates the diameter of the cosmos to be the equivalent in stadia of what would in modern times be called two light years, if Aristarchus' theories were correct. 2nd century BCE – Seleucus of Seleucia advocates Aristarchus' heliocentric universe, presenting it as a fact (whereas Aristarchus presented it as a hypothesis). According to Strabo (1.1.9), Seleucus was the first to state that the tides are due to the attraction of the Moon, and that the height of the tides depends on the Moon's position relative to the Sun.Russell, Bertrand — History of Western Philosophy (2004) – p. 215 2nd century BCE – Apollonius of Perga shows the equivalence of two descriptions of the apparent retrograde planet motions (assuming the geocentric model), one using eccentrics and another deferent and epicycles.Carrol, Bradley and Ostlie, Dale, An Introduction to Modern Astrophysics, Second Edition, Addison-Wesley, San Francisco, 2007. pp. 4 The latter will be a key feature for future models. The epicycle is described as a small orbit within a greater one, called the deferent: as a planet orbits the Earth, it also orbits the original orbit, so its trajectory resembles a curve known as an epitrochoid. This could explain how the planet seems to move as viewed from Earth. 2nd century BCE – Eratosthenes determines that the radius of the Earth is roughly 6,400 km.