Geocentric Model
- 3. THE PYTHAGOREAN MODEL • In his model, Earth is at rest at the center of the universe and everything rotates around it.
- 4. Plato’s “Saving the Appearances” • Plato adopted the Pythagorean view of the motion of the heavenly bodies as combinations of circular motion about the earth. • He assumed that all motions in the universe are perfectly circular and all heavenly bodies are ethereal or perfect.
- 5. Plato’s “Saving the Appearances” • Most of the time, planets move from west to east as predicted. • But occasionally, they backtrack for a while; that is, they move westward before resuming their eastward motion or called retrograde motion.
- 7. EUDOXUS’ MODEL • Eudoxus was the first to “save the appearances” that Plato referred to, using a series of 27 concentric spheres on which the sun, the moon and the planets moved in perfect circular motion.
- 8. EUDOXUS’ MODEL • The breakdown of the 27 spheres is as follows: one sphere for fixed stars, three spheres for the sun, three spheres for the moon, and four spheres each of the five known planets at the time – Mercury, Venus, Mars, Saturn, and Jupiter.
- 10. ARISTOTLE’S MODEL • Aristotelian model also used the 27 celestial spheres of Euxodus. • In addition, Aristotle used 27 “buffering” spheres between the celestial spheres of Eudoxus and an outermost sphere that was the domain of what he called the Prime Mover. • Prime Mover rotated this outermost sphere with constant angular speed, causing the other spheres to rotate as well.
- 11. ARISTOTLE’S MODEL • ~Aristotle divided the universe into two realms-the terrestrial and the celestial-with the orbit of the moon as the boundary.
- 12. ARISTOTLE’S MODEL • Earth is a sphere, he based this proposition on several observations. • First, it is only at the surface of a sphere that all objects fall straight down. • Second, the view of the constellations changes as one travels from north to south. • Lastly the shadow of Earth on the Moon during a lunar eclipse was round.
- 13. PTOLEMY’S MODEL
- 14. PTOLEMY’S MODEL • The Greek mathematician Apollonius, known in his time as the “The GreatGeometer,” introduced the idea of an epicycle to explain planetary motion. Apollonius
- 15. PTOLEMY’S MODEL • An epicycle is a circle on which a planet moves. • The center of this small circle in turn moves around Earth along a bigger circular path called the deferent. • To account for the variation in the speed of the sun during its annual motion.
- 16. PTOLEMY’S MODEL • Greek astronomer Hipparchus refined this model by considering that Earth was off- center or eccentric in the deferent where the sun moved.
- 17. PTOLEMY’S MODEL • Around 140 AD, Ptolemy devised a more complex epicyclic model. • He defined a point on the other side of the deferent’s center and called it the equant. • The equant and the center of the Earth are equidistant from the center of the deferent.
- 18. PTOLEMY’S MODEL • Ptolemy’s model of the universe survived for more than 14 centuries. • The Greeks not only knew that the Earth was round. • They also knew the circumference of Earth to be 25 000 miles. • Eratosthenes measured it in 235 BCE using trigonometry and the knowledge of the angle of elevation of the sun at noon in Alexandria and Syene (now Aswan, Egypt).