Models of the Universe

• Heliocentric Model

  places the sun as the center of the solar system and the planets orbit around it.
• Geocentric Model

  has the earth at the center of the solar system
• Eudoxus of Cnidus

  proposed a model of the universe to explain the apparent opposite motion of some planets. (retrograde motion).
• Eudoxus

  He placed Earth at the center and explained that various spheres are needed to explain the observed movement of objects.
• Eudoxus' model of the universe

  model of the universe

  
• Aristotle


  Just like Eudoxus, he believed that universe is geocentric which places the sun and other planetary objects around Earth.
• Aristotle


  He added that the universe is finite in terms of space and eternal in time
• Aristotle


  In his model, the universe was spherical but infinite. Concentric spheres surrounded Earth, with objects following perfectly circular orbits that were guided by “souls”.
• Aristotle Model of the Universe

  model of the universe

  
• Aristarchus


  was the first person to attempt to calculate the distance of the earth to the sun during the 3rd century
• Aristarchus


  He conducted this by measuring the angle between the moon and the sun and applied the principles of trigonometry.
• Aristarchus


  described Earth as a body that rotated daily around an axis and revolved annually around the sun
• Aristarchus

  The stars were fixed in a large sphere with the sun at the center.
• Aristarchus Model of the Universe

  model of the universe

  
• Ptolemy


  adapted Aristotle’s geocentric theory of the universe.
• Ptolemy


  idea is that the planets do not orbit in perfect circles around the earth
• Ptolemy


  proposed another model where the whole universe moved about a fixed, stationary Earth.
• Epicycle

  He accounted for the apparent motions of the planets around the earth by assuming that each planet moved around a sphere called an
• Deferent

  The center of the epicycle moved on a larger sphere called a
• Ptolemy's Model of the Universe

  model of the universe

  
• Ptolemy

  He noted that the motion of the planet could be described by large loops where the planet moves in a retrograde motion.
• Ptolemy

  his model was able to predict the movements of the planets with certain accuracy but it was not able to explain the observable changes in the phases of Venus.
• Copernicus

  introduced the concept of a heliocentric universe -- the sun is at its center
• Copernicus

  He explained that Earth is only one of the many planets that revolve around the sun.
• Copernican Model of the Universe

  model of the universe

  
• Copernicus and Heliocentric Universe


  Taking the apparent movement of the stars in the sky into account, he explained that Earth spins around its axis.