Eudoxus proposed a geocentric model of the universe with 27 concentric spheres, with Earth at the center. The sun, moon, planets and stars orbited in transparent spheres around Earth. Aristotle later extended this model, proposing the universe was finite but eternal, with all celestial bodies orbiting Earth inside Eudoxus' spheres due to motion from a Prime Mover. Aristarchus first hypothesized a heliocentric model, with the sun at the center and Earth and Moon orbiting it. Ptolemy further developed a geocentric model using epicycles to explain planetary retrograde motion. Copernicus later published his heliocentric model, ending the geocentric era by proposing Earth and planets

2. Eudoxus’ Geocentric Universe Model

3. Eudoxus of Samos
• The universe has 27 concentric spheres with earth as center
• A spherical Earth at the center, while the Sun and the planets were
placed in a giant transparent spheres that orbit the earth.
• The Sun’s spheres orbits the earth every 24 hours.
• Stars are attached to a larger and much farther spheres.

4. Eudoxus’ Five Basic Principles
1. The Earth is at the center of the universe.
2. All celestial motion is circular. All celestial motion is regular.
3. The center of the path of any celestial motion is the center of the
universe.
4. The center of all celestial motion is the center of the universe.
5. The apparent irregular and non-circular motions of celestial bodies
is a result of the combinations of real, circular motions that satisfy
principles.

6. Aristotle and a Finite, Eternal Universe
• He extended Eudoxus’ model.
• The sun, moon, planets and stars are all orbiting the Earth
inside of Eudoxus’ spheres.
• The universe was finite in terms of space but existed
eternally in time.
• Motion of the celestial bodies can be traced to a Prime
Mover that because of its basic movement, all other
crystalline spheres move accordingly.

7. Aristarchus’ Universe

8. Aristarchus of Samos
• The first person to calculate the relative distance of the Earth and Sun
using trigonometry.
• First hypothesized that the Sun was at the center of the universe.
• Visualized that the Moon orbits around a spherical earth which then
revolves around the sun.

9. Ptolemy’s Geocentric Universe Model

11. Ptolemy and Epicycles
The Ptolemaic System
• A planet moves counter-clockwise around the center of the deferent.
• The epicycle’s center also moves counter-clockwise around the center
of the deferent.
• The earth is not exactly at the center of the deferent, or it is eccentric
(off the center)
• The planet moves in a retrograde motion. The planet is brightest at
this point because it is closest to the Earth.

12. Copernicus’
Heliocentric Universe
Model

13. Copernicus and a Heliocentric Universe
• Put an end to the geocentric era by publishing his work “On the
Revolutions of the Heavenly Spheres”.
• The motion of spherical heavenly bodies is uniform, eternal and
circular.
• The earth has 3 different motions, daily rotation on its axis, yearly
motion around the sun and the precession, or change in orientation,
of its axis every 26,000 years.