Historical models of the Solar System first appeared during prehistoric periods and remain updated to this day. The models of the Solar System throughout history were first represented in the early form of cave markings and drawings, calendars and astronomical symbols. Then books and written records became the main source of information that expressed the way the people of the time thought of the Solar System.
New models of the Solar System are usually built on previous models, thus, the early models are kept track of by intellectuals in astronomy, an extended progress from trying to perfect the geocentric model eventually using the heliocentric model of the Solar System. The use of the Solar System model began as a resource to signify particular periods during the year as well as a navigation tool which was exploited by many leaders from the past.
Past astronomers and prominent thinkers recorded their observations and sought to develop models that accurately interpreted their findings. This approach to modeling the Solar System facilitated advancements towards more precise representations, ultimately enhancing our comprehension of the Solar System.
Early astronomy
The Nebra Sky Disc The Nebra Sky Disc is a bronze dish with symbols that are interpreted generally as the Sun or full moon, a lunar crescent, and stars (including a cluster of seven stars interpreted as the Pleiades). The disc has been attributed to a site in present-day Germany near Nebra, Saxony-Anhalt, and was originally dated by archaeologists to , based on the provenance provided by the looters who found it. Researchers initially suggested the disc is an artefact from the Bronze Age Unetice culture, although a later dating to the Iron Age has also been proposed.
Firmament The ancient Hebrews, like all the ancient peoples of the Near East, believed the sky was a solid dome with the Sun, Moon, planets and stars embedded in it. In biblical cosmology, the firmament is the vast solid dome created by God during his creation of the world to divide the primal sea into upper and lower portions so that the dry land could appear.
Babylonian interpretation Babylonians thought the universe revolved around heaven and Earth. They used methodological observations of the patterns of planets and stars movements to predict future possibilities such as eclipses. Babylonians were able to make use of periodic appearances of the Moon to generate a time source - a calendar. This was developed as the appearance of the full moon was visible every month. The 12 months came about by dividing the ecliptic into 12 equal segments of 30 degrees and were given zodiacal constellation names which were later used by the Greeks.
Chinese theories The Chinese had multiple theories of the structure of the universe. The first theory is the Gaitian (celestial lid) theory, mentioned in an old mathematical text called Zhou bei suan jing in 100 BCE, in which the Earth is within the heaven, where the heaven acts as a dome or a lid. The second theory is the Huntian (Celestial sphere) theory during 100 BCE. This theory claims that the Earth floats on the water that the Heaven contains, which was accepted as the default theory until 200 AD. The Xuanye (Ubiquitous darkness) theory attempts to simplify the structure by implying that the Sun, Moon and the stars are just a highly dense vapour that floats freely in space with no periodic motion.
Greek astronomy Since 600 BCE, Greek thinkers noticed the periodic fashion of the Solar System (then regarded as the "whole universe") but, like their contemporaries, they were puzzled about the forward and retrograde motion of the planets, the "wanderer stars", long taken as heavenly deities. Many theories were announced during this period, mostly purely speculative, but progressively supported by geometry.
Thales of Miletus alleged to have predicted the solar eclipse of 586 BCE. Around 475 BCE, Parmenides claimed that the universe is spherical and moonlight is a reflection of sunlight. Shortly after, circa 450 BCE, Anaxagoras was the first philosopher to consider the Sun as a huge object (larger than the land of Peloponnesus), and consequently, to realize how far from Earth it might be. He also suggested that the Moon is rocky, thus opaque, and closer to the Earth than the Sun, giving a correct explanation of eclipses. To him, comets are formed by collisions of planets and that the motion of planets is controlled by the nous (mind).
Anaximander cosmology
Anaximander, around 560 BCE, was the first to conceive a mechanical model of the world. In his model, 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.
At the origin, after the separation of hot and cold, a ball of flame appeared that surrounded Earth. 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. 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".
Anaximander was the first philosopher to present a system where the celestial bodies turned at different distances.
Pythagorean astronomical system
Around 400 BCE, Pythagoras' students 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. So, the Earth rotates around itself synchronously with a daily orbit around that Central Fire, while the Sun revolves it yearly in a higher orbit. That way, the inhabited side of Earth faces the Sun once every 24 hours. They also claimed that the Moon and the planets orbit the Earth. This model is usually attributed to Philolaus.
This model is the first one that 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.
Platonic geocentrism Around 360 BCE Plato wrote in his Timaeus his idea to account for the motions; he claimed that circles and spheres were the preferred shape of the universe and that the Earth was at the centre and the stars forming the outermost shell, followed by planets, the Sun and the Moon. This is the so-called geocentric model.
In Plato's 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
So, Plato arranged these celestial orbs in the order (outwards from the center): Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, and fixed stars, with the fixed stars located on the celestial sphere. However, this did not suffice to explain the observed planetary motion.
Concentric spheres
Eudoxus of Cnidus, student of Plato in around 380 BCE, introduced a technique to describe the motion of the planets called the method of exhaustion. Eudoxus reasoned that since the distances of the stars, the Moon, the Sun and all known planets do not appear to be changing, they are fixed in a sphere in which the bodies move around the sphere but with a constant radius and the Earth is at the centre of the sphere. To explain the complexity of the movements of the planets, Eudoxus thought they move as if they were attached to a number of concentrical, invisible spheres, every of them rotating around its own and different axis and at different paces.
Eudoxus’ model had twenty-seven homocentric spheres with each sphere explaining a type of observable motion for each celestial object. Eudoxus assigns one sphere for the fixed stars which is supposed to explain their daily movement. He assigns three spheres to both the Sun and the Moon with the first sphere moving in the same manner as the sphere of the fixed stars. The second sphere explains the movement of the Sun and the Moon on the ecliptic plane. The third sphere was supposed to move on a “latitudinally inclined” circle and explain the latitudinal motion of the Sun and the Moon in the cosmos. Four spheres were assigned to Mercury, Venus, Mars, Jupiter, and Saturn, the only known planets at that time. The first and second spheres of the planets moved exactly like the first two spheres of the Sun and the Moon. According to Simplicius, the third and fourth sphere of the planets were supposed to move in a way that created a curve known as a hippopede. The hippopede was a way to try and explain the retrograde motions of planets.