page_title: Historical models of the Solar System
section_title: Greek astronomy
hierarchical_section_title: Historical models of the Solar System / Greek astronomy
caption_attribution_description: Line art drawing of ptolemaic system.
context_page_description: The historical models of the Solar System began during prehistoric periods and is 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 then 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 to eventually using the heliocentric model of the Solar System. The uses of the Solar System model began as a time source to signify particular periods during the year and also a navigation source which is exploited by many leaders from the past.
  Astronomers and great thinkers of the past were able to record observations and attempt to formulate a model that accurately interprets the recordings. This scientific method of deriving a model of the Solar System is what enabled progress towards more accurate models to have a better understanding of the Solar System that we are in.
context_section_description: Since 600 BCE, Greek thinkers noticed the periodic fashion of the Solar System. Many theories were announced during this period. Parmenides claimed that the Solar System is spherical and moonlight is actually a reflection of sunlight. 
   
  Anaxagoras suggested that the Moon is closer to the Earth than the Sun, comets are formed by collisions of planets and that the motion of planets is controlled by the nous (mind). Pythagoras's students thought the motion of planets is caused by a fire at the centre of the universe that powers them and Earth orbits that fire. They also claimed that the Moon, the Sun and planets orbit the Earth.
  Eudoxus, in around 400 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. 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.            However, Aristotle modified Eudoxes' model by supposing the spheres are real. He was able to articulate the spheres for most planets, however, the spheres for Jupiter and Saturn crossed each other. Aristotle solved this complication by introducing an unrolled sphere. 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.
  Around 360 BCE when Plato proposed his idea to account for the motions. Plato 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. However, this did not suffice to explain the observed planetary motion. During the period 127 to 141 AD, Ptolemy deduced that the Earth is spherical based on the fact that not everyone records the solar eclipse at the same time and that observers from the North can not see the Southern stars. Ptolemy attempted to resolve the Planetary motion dilemma in which the observations were not consistent with the perfect circular orbits of the bodies. Ptolemy proposed a complex motion called Epicycles. Epicycles are described as an orbit within an orbit. For example, looking at Venus, Ptolemy claimed that it orbits the Earth, and as it orbits the Earth, it also orbits the original orbit which is demonstrated in the illustration to the right. Ptolemy emphasised that the epicycle motion does not apply to the Sun.  This model were Earth is positioned at the centre of the Solar System, is known as the Geocentric model.
caption: This is the geocentric model of the Solar System with the Earth at the centre. The above image is also a representation of Eudoxus's proposal.
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