Models of the Universeand their Development throughout History

Aristotle (384-322BC)The Greek were the first civilisation to study astronomy as a way of understanding the universe.Before Greek astronomy, study of the stars was performed for religious and astrological purposes only.Aristotle was not a scientist.Even though Aristotles model of the universe was wrong, and not based on scientific fact, Aristotle was such a well respected man, that everyone accepted his model as fact, and it took 2000 years to be cast aside.

The Geocentric ModelAristotles model of the universe was geocentric.This means that the earth was the centre of the universe, and Aristotle believed it was surrounded by the heavens, which were made up of 55 transparent concentric circles which were used to explain the motion of the stars.

Aristarchus (~240BC)Aristarchus proposed the first ever heliocentric model of the universe.His model, with a larger sun at the centre of the universe, and the earth rotating on its axis and orbiting the sun, was actually quite close to the truth.Aristarchus model was not well accepted, because it was not detailed enough to allow for predictions.

The First Heliocentric ModelAristarchus believed that the earth orbited the sun, and the moon orbited the earth.The main problem with Aristarchus view it that the orbits of the earth and planets were circular.

Ptolemy (140AD)Ptolemy was a scientist, who developed Aristotles model of the universe, by applying mathematical evidence to an essentially philosophical model.Ptolemy was interested in creating a mathematical description of the motion of the planets, and so attempted to discard the uniform circular motion presented by Aristotle, and also by the Greek philosopher Plato.

Another Geocentric ModelPtolemys model placed earth slightly away from the centre of the planets cycles, but still believed that it was the centre of the universe. He proposed that the planets moved in epicycles around a deferent.

Nicholas Copernicus (1473-1542)Copernicus proposed once again the theory of a heliocentric model. It was known that Copernicus had studied the work of Aristarchus, and probably derived his model from the Ancient Greek.At the time of Copernicus, the geocentric model was endorsed by the Christian belief, as they thought that it depicted heaven and hell.Copernicus was associated with the church throughout his life, but risked controversy by publishing his views during this time.

Another Heliocentric ModelThe Copernican model was inaccurate, as he imposed uniform circular motion on his model, but the Copernican Hypothesis was correct, sparking the Copernican revolution as other scientists began to adopt his ideas.

Tycho Brahe (1546-1601)Tycho Brahe rejected the Copernican Theory, because he could measure no parallax for the stars, he concluded that the earth must be stationary.Instead, Brahe devised a model of the universe which combined both heliocentric and geocentric ideas.Brahe was also able to devise new and better instruments for studying the universe, and therefore make accurate observations of the positions of the stars, moon, sun and planets.

Combined Geocentric and Heliocentric ModelsDespite obvious flaws in his model of the universe, Brahes observations and calculations of the stars were remarkable due to the fact telescopes hadnt even been invented!

Johannes Kepler (1571-1630)Kepler was a believer in the Copernican theory of the universe. He believed that the spheres containing the six known planets (Mercury, Venus, Earth, Mars, Jupiter and Saturn) were fixed by five regular solids the cube, tetrahedron, dodecahedron, icosahedron and octahedron.Kepler also produced three laws of planetary motion relating to the elliptical orbit of the planets around the sun.

An Improved Heliocentric ModelThis model shows Keplers view of the universe, with the planets being separated by the different regular solids.The sun is at the centre of the universe.

Galileo Galilei (1564-1642)Galileo was a supporter of the Copernican Theory, although he was afraid to support it publicly because of criticisms.Despite popular belief, Galileo did not invent the telescope, but it was instrumental to his model, and was later what encouraged him to publicly defend the heliocentric model.Galileo was interrogated by the Inquisition four times, was condemned and was placed under house arrest for the rest of his life.Many say that this was for heresy, but the case against Galileo stated that he disobeyed orders from the Pope not to promote Copernican theory.

Convincing Proof of the Heliocentric ModelGalileo used his telescope to conduct studies of the moons of Jupiter. These studies supported the Copernican model, because popular belief was that Earth could not move because it would leave the moon behind, and yet Jupiters satellites continued to orbit Jupiter while it moved.

Isaac Newton (1642-1727)Kepler devised three laws of planetary motion, but never understood why they moved around an elliptical orbit.Newton improved Keplers theory by expanding it into a general theory of motion and gravity. Newton never understood exactly what gravity was, and therefore his model was never perfect.

A More Accurate ModelAlthough Newtons model wasnt perfect, his study of the universe led to the discovery of the Law of Universal Gravitation, which was important in later discovery in both physics and astronomy.