The document discusses models of the early universe proposed by ancient Greeks. It describes both geocentric models which placed Earth at the center, including those of Pythagoras, Plato, Aristotle, and Ptolemy involving celestial spheres, and heliocentric models like Aristarchus' which placed the Sun at the center. The models attempted to explain observations of planetary motions and positions of celestial objects in the sky through the early Greeks' philosophical and religious perspectives before the invention of advanced astronomical tools.

1. Models of the Universe
Their contributions in order to come up
with the more accurate model of the
universe
Persons that proposed their views of the early universe
Two classification of how the universe might look like according to
the
early Greeks
According to the early Greeks

3.  The early humans relied on the skies as their principal means of telling the time,
of navigation, and knowing when to start planting rice.
 3,000 years ago, the Egyptians established a 365- day calendar based on the track of the stars Sirius.
The track also coincide with the annual flooding of the Nile river.
 The Babylonians and the Assyrian also invented similar calendars to aid them in determining when to sow
and reap crops.
 Humans have come up with several models in an effort to understand the universe. Before the invention
of the telescope, they had to rely on their senses for a picture of the Universe with much philosophical
and religious symbolism.
 Thales of Miletus proposed that the Earth is a disk floating on water.
 Anixamander, also from Miletus, suggested that Earth is a cylinder and that its surface is curved.
Introduction:

4. Classification of the models:
 As civilization flourished, several models were proposed. These models can be grouped under two
categories:
 The geocentric model considers
Earth as the center of the
Universe.
 The heliocentric model assumes the
sun to be the center of the Universe.
Heliocentric
Model
Geocentric Model
 Pythagoras
 Plato
 Eudoxus
 Aristotle
 Apollonius
 Ptolemy
 Aristarchus
 Copernicus
 Galileo
 Kepler
Pyrocentric Model – suggests that a “fire” is the center of the universe
- suggested by Philolaus.
Tychonic Model – a compromise between heliocentric and geocentric model.
- it was proposed by Tyco Brahe.

5. Geocentric Models:
• Pythagorean Model
Pythagoras
 He was acknowledge as the first to assert that the Earth is round, and that the heavenly
bodies moves in circles.
 In his model, Earth is at rest at the center of the Universe, and everything rotates around it.
 He considered the motions of the planets were mathematically related to musical sounds
and numbers.
The music of Spheres
 Anaxagoras, a follower of Pythagoras, was credited with having determined the relative
position of the sun, the moon, and the Earth during solar eclipses and lunar eclipses.

6. A Greek philosopher and a Teacher, He adopted the Pythagorean
view of the motion of the heavenly bodies as combinations of
circular motion about Earth.
• Plato
He assumed that all motions in the universe are perfectly circular and that all
heavenly bodies are ethereal or perfect.
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. “Retrograde
Motion”
Plato’s challenged: “ What circular motions, uniform and perfectly regular, are
to be admitted as hypotheses so that it might possible to save the
appearance presented by the planets.”?
This challenge is known in the history of astronomy as “Plato’s Saving the
Appearances”.

7.  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.
• Eudoxus’ Model
 The breakdown of 27 spheres is as follows:
 One sphere for fixed stars
 Three spheres for the sun
 Three spheres for the moon
 And four spheres for each of the five
known planets at that time.
(Mars, Venus, Mars, Saturn and Jupiter)
 Note that he did not assign any sphere for Earth
because in his model Earth is fixed.
 The last two spheres produced the backward motions of
the planets. He explained the rotation of the 27 spheres
using the notion of “intelligences”.

8. • Aristotle’s Model
The Aristotelian model also used the 27 celestial spheres
of Eudoxus.
He added 27 buffering spheres between celestial spheres of
Eudoxus and an outermost sphere that was the domain of
what he called the “Prime Mover”.
The Prime Mover rotated rotated this outermost sphere
with constant angular speed, causing the other sphere
to rotate as well.

9. According to him the order of the heavenly bodies in
the universe was (from Earth out):
(Earth, moon, Mercury, Venus, sun, Mars,
Jupiter, Saturn, fixed stars, and the
firmament of the Prime Mover)
The Roman Catholic Church adopted this
idea in Medieval times: The Prime Mover
was considered as God, and the sphere
of firmament as heaven.

10. • Aristotle’s Model
Aristotle divided the universe into two realms; terrestrial and celestial.
with the orbit of the moon as the boundary. Below the moon’s orbit was the terrestrial
realm.
Terrestrial objects was composed of four primordial elements in this sequence ;
earth, water, air, and fire.
Celestial objects was consist of the fifth element called aether or ether.
He considered terrestrial matter to be ephemeral and undergoing decay, while
ether was unchanging and perpetual.
This realm was composed of four primordial elements in this sequence ; earth,
water, air, and fire.

11. According to him, the Earth is a sphere.
• Aristotle’s Model
He based this proposition on several observations:
 It is only at the surface of a sphere that all
objects fall straight down.
 The view of the constellations changes as it
travels from north to south.
 The shadow of Earth on the moon during a
lunar eclipse was round.
Aristotle’s mode; was based on
the three types of motion
 Natural
 Violent
 Alteration

12. • Ptolemy’s Model
Apollonius, known in his time as the Great
Geometer, introduced the idea of an epicycle to
explain planetary motion.
An epicycle is a circle on which planet
moves. The center of the small circle in turns
moves around Earth along a bigger circular
path called deferent.
Around 140 A.D., Ptolemy, devised a
more complex epicyclic model.

13. He defined a point on the other side of the
deferent’s center and called it the equant.
The equant and the center of Earth is equidistant
from the center of the deferent
Ptolemy’s model, each planet has its
own epicycle and deferent. His model
survived for more than 14 years
• Ptolemy’s Model

14. Heliocentric Models:
Before heliocentric model came about, a Greek astronomer
Philolaus, proposed a pyrocentric model of the universe.
According to him, neither Earth nor the