The document discusses early models of the universe including the ideas that the Earth was flat and then spherical. It describes models from Greek philosophers like Aristotle who believed the Earth was at the center of the universe and surrounded by spheres carrying the planets and stars. Later models from Copernicus, Brahe and Kepler placed the Sun at the center with Earth and other planets orbiting around it, moving astronomy toward a heliocentric understanding of the solar system.

1. Module 1
Early Models
of the
Universe
Jerome A. Bigael, Leyte Progressive High School
Tacloban City

4. The Spherical Earth
▪During ancient times, Earth was perceived to be flat.
▪Early Babylonian, Chinese, Egyptian, and Hebrew
civilizations believed that Earth had corners.
▪It was then believed that if you set to sail straight in
the ocean, you would get to the edge of Earth and
fall.

6. ▪Flat Earth map drawn by Orlando Ferguson in
1893. The map contains several references to
biblical passages as well as various jabs at
the "Globe Theory".
▪However, many philosophers theorized that
Earth was round and not flat.

7. ▪Navigators such as
Ferdinand Magellan (1480-
1521) and
▪Juan Sebastian Elcano
(1476-1526) were given
credit for circumnavigating
the world and proving that
Earth is not flat.

9. How did the idea of spherical earth
evolve?
Greek philosophers also investigated on the
shape of Earth.
Philosophers like Plato (428 BCE – 348 BCE) and
Aristotle (384 BCE – 322 BCE) described Earth
and the heavens as spherical.

10. In fact, Eudoxus of Cnidus (400 BCE – 350 BCE )
constructed a planetary model based on this.
Then around 6 BCE, these Greek philosophers
speculated that Earth was actually spherical.

11. ▪The timeline shows the ideas of some Greek
philosophers, including Pythagoras (570 BCE –
490 BCE) and Eratosthenes (276 BCE – 194 BCE),
about the sphericity of the Earth.
Pythagoras (started the idea of the spherical Earth)
Plato (educated his students on the sphericity of the
Earth but made no justifications)
Aristotle ( noted that there were stars in Egypt that could
be seen on the other parts of the Earth. This phenomenon
was only possible if Earth had a curved surface.
Eratosthenes (estimated the circumference of the
Earth.

12. Pythagoras (started the idea of the spherical
Earth)
Plato (educated his students on the sphericity of
the Earth but made no justifications)
Aristotle ( noted that there were stars in Egypt
that could be seen on the other parts of the Earth.
This phenomenon was only possible if Earth had a
curved surface.
Eratosthenes (estimated the circumference of
the Earth.

13. Different Models of the Universe
▪Before the telescope was invented,
ancient astronomers only used their
unaided eyes to observe the sky and the
stars.
▪Eventually they created models of the
universe.

14. Eudoxus of Cnidus (400 BCE – 355 BCE)
He was considered as the first astronomer
to explain the retrograde motion of the
planets in the sky.
He found out the differences in the motion
of each planet that should be considered
to give an accurate description of the
universe.

15. What is retrograde motion?
▪Retrograde motion is an APPARENT
change in the movement of the planet
through the sky.
▪Watch this video to fully understand
retrograde motion.

16. ▪According to Eudoxus’s model,
the universe was composed of
Earth, five other planets (that
are visible with the unaided
eye, namely Mercury, Venus,
Mars, Jupiter, and Saturn)

17. Aristotle’s Model
▪Aristotle somehow patterned his model
to the model of Eudoxus.
▪Aristotle’s model showed that the
universe was spherical and finite.
▪Just like Eudoxus, he perceived Earth
was at the center of the universe and
was stationary.

18. ▪He believed so
because Earth is too
big to move; thus it
could not rotate.
▪Other celestial
bodies were built up
symmetrically in
concentric spherical
rings around the
earth

19. ▪Aristotle believed that the Earth was composed of
four elements – Earth, Water, Air, and Fire.
▪Beyond Earth are the planets and the moon which
revolved in spherical rings.
▪He further described that each ring was in physical
contact with one another, which means the motion
of a heavenly body in one sphere will affect the
motion of a nearby heavenly body.

20. Aristarchus of Samos model
310 BCE – 230 BCE
▪He was a Greek astronomer
who made the first attempt to
create a heliocentric model of
universe, which places the sun
and the fixed stars were at
rest, while Earth revolved
around the sun in a circular
path.

22. ▪The only book that survived was entitiled
On the Sizes and Distances of the Sun and
the Moon.
▪In this book, Aristarchus calculated the
sizes of the sun, and the moon and their
distances from Earth by estimating the
relative angles of the moon and the sun
from Earth.

24. ▪Aristarchus had 3 assumptions:
a. Earth was spherical
b. It is far from the sun
c. Moon passes through the Earth’s
shadow when they align.

25. ▪Do you believe that the
astronomical positions of the stars
and planets influence or affect
human lives and events on Earth?
Why or Why not?

26. ▪Why do modern scientists consider
the sun as the center of the
universe? Cite Examples based on
your previous lessons in high
school.

27. Claudius Ptolemaeus 100 AD – 178 AD
▪Ptolemaic universe.
▪In this model, it is assumed that
Earth was at the center of the
universe, while the other celestial
bodies revolved around the Earth
in perfect circles with constant
velocity.

28. ▪Ptolemy’s model
was considered
more refined than
previous geocentric
models because his
model could explain
the motion of the
celestial bodies
accurately.

29. ▪Ptolemy
assumed that
planets revolved
on epicycles
(small spheres)
which moved
around the
deferent (large
sphere).

30. ▪He added that the stars belonged
to the celestial sphere which was
located beyond the planetary
spheres.

31. ▪The center of the
deferent is called the
eccentric.
▪What Ptolemy did next
was to “move” Earth
from its original position
to a position below the
center of the system that
is still inside the
deferent.

32. Nicolaus Copernicus (1473-1543)
▪ During the 16th century, a Polish
astronomer, revived the
heliocentric model of Aristarchus.
▪ He was hesitant to publish his
findings because he was afraid of
condemnation.
▪ But it was published a year before
his death.

33. ▪Copernicus strongly
believed in the
heliocentric model
because there were
loopholes in the
Ptolemaic model in terms
of predicting the
positions of the planets.
▪The Copernicus’s model
of the universe.

34. ▪ In Copernicus’s model shown
above, it could be concluded
that:
1. Heavenly bodies exhibited
constant circular and
perpetual motion along their
epicycles.
2. The sun was at the center of
the universe.
3. The order of planets from
the sun is Mercury, Venus,
Earth, Mars, Jupiter, Saturn
and beyond the planets were
fixed stars.

35. Tycho Brahe (1546-1601)
▪Tycho Brahe was a Danish
astronomer who also made
planetary observations.
▪Unlike Copernicus, Brahe
Believed in a geocentric
universe, but his idea of
the geocentric universe is
slightly different from
Ptolemy’s

36. ▪In Brahe’s model of the
universe, called the
“Tychomic system”, Earth
was at the center and the
sun and the moon revolved
around it, and all the other
planets orbited the sun. Such
a model was a type of
“geoheliocentric system”.

38. After reading the article, write an essay using the
following questions as guide:
▪ 1. Why is Ptolemy’s geocentric astronomy inaccurate?
▪ 2. Why was Copernicus’s idea on the model of the universe described as
revolutionary?
▪ 3. Based on the Prutenic Tables, what can be considered as a weakness of the
Copernican model?
▪ 4. Who is Tycho Brahe? What error(s) did he discover about the astronomical table
created based on Ptolemy’s model of the universe? Based on Copernicus’s model of
the universe?
▪ 5. What did Brahe conclude about these errors?
▪ 6. What did Brahe do to gather more accurate observations?
▪ 7. What scientific attitude did Brahe show during his 20-year observation in Hven?
▪ 8. What can be considered as Brahe’s greatest legacy to the world of astronomy?
▪ 9. Who is Kepler? What is considered as one of his greatest discoveries?
▪ 10. How did Brahe pave the way for Kepler’s discovery?
▪ 11. How would you describe Keplers model of the universe?