PHYSICAL SCIENCE

2. Explain how the
Greeks knew that the
Earth is spherical

3. The Greeks are very much
noted for their major
contributions in different fields.
They were not only great
philosophers. They were great
scientists and mathematicians
as well.

4. • the Greeks used philosophical
arguments to explain the natural
events happening around them
including the movements of the
stars and other heavenly bodies,.
they were observers.

5.  The early Greeks had a
geocentric view of the
universe
-earth is stationary

6. -The Greeks also believed that
stars traveled daily around the
earth. However, they all stayed
in a transparent, hollow sphere
located beyond the planets.
They called this sphere as the
celestial sphere.

7. Celestial sphere

8. Ancient astronomy
• Around 500 B.C., most Greeks
believed that the Earth was round, not
flat.
• Pythagoras and his pupils who were
first to propose a spherical Earth.

9. • In 500 to 430 B.C., Anaxagoras further
supported Pythagoras' proposal through
his observations of the shadows that the
Earth cast on the Moon during a lunar
eclipse. He observed that during a lunar
eclipse, the Earth's shadow was reflected
on the Moon's surface. The shadow
reflected was circular.

10. •Around 340 B.C., Aristotle listed
several arguments for a spherical
Earth which included the positions
of the North Star, the shape of
the Moon and the Sun, and the
disappearance of the ships
when they sail over the horizon.

11. NORTH STAR
•Believed to be at a fixed position in
the sky. However, when the Greeks
traveled to places nearer the
equator, like Egypt, they noticed
that the North Star is closer to the
horizon.

12. The Shape of the Sun and the Moon
•Aristotle argued that if the moon
and the Sun were both spherical,
then perhaps, the Earth was also
spherical

13. Disappearing Ships
•If the Earth was flat, then a ship
traveling away from an
observer should become
smaller and smaller until it
disappeared.

14. •The Size and
Spherical
Earth

15. •Eratosthenes
Astronomer/mathemat
ician in Hellenistic
E
g
y
p
t (c.275-195 BC)
Calculated
circumference of Earth

16. •While he was working at the Library
of Alexandria in Northern Egypt, he
received correspondence from
Syene in Southern Egypt which
stated that a vertical object did not
cast any shadow at noontime during
the summer solstice.

17. •But this was not the case in
Alexandria where, at noon time
during the summer solstice, a
vertical object still casts a shadow.
These observations could only mean
that the Sun, during this time in
Alexandria, was not directly
overhead.

18. Measured altitude of Sun at two
different points on the Earth
(Alexandria & Syene about 800
km apart): found 7°difference
 (Multiplied (360°/7°)x800km
(distance between the 2 sites) to
obtain
circumference~40,000km

19. •To explain the difference, he
hypothesized that the light rays
coming from the sun are parallel, and
the Earth is curved.
•Fro his measurements, he computed
the Earth’s circumference to be
approximately 250,000 stadia about
40,000 km.

22. Important findings of
the following Greek
astronomers:

23. A. Anaxagoras
• Anaxagoras was able to explain what
causes the phases of the moon.
• According to him, the moon shone only by
reflected sunlight. Since it is a sphere,
only half of it illuminated at a time. This
illuminated part that is visible from the
earth changes periodically.

24. B. Eudoxus
• - proposed a system of fixed spheres.
• -believed that the Sun, the moon, the five
known planets and the stars were
attached to these spheres which carried
the heavenly bodies while they revolved
around the stationary Earth.

25. C. Aristotle
-was a student of Plato.
–he earth is spherical in shape since it
always casts a curved shadow when it
eclipses the moon.
–He also believed that the earth was the
center of the universe. The planets and
stars were concentric, crystalline spheres
centered on the earth.

26. •Aristotle lived in ancient Greece
more than three hundred years
before the Common Era (or
Before Christ). In those days,
most people believed that many
gods ruled the universe.

27. • A happy god, for instance, might allow an
abundant harvest while an angry god would
show his fury with storms or earthquakes.
Aristotle decided he could understand the world
through observation and by using logic and
reason. Later scientists called Aristotle the
Father of Natural Science because centuries
after the ancient scholar’s death, his methods
formed the basis of the scientific method.

29. •Aritotle’s Celestial
Physics

30. Four basic (earthly) elements: earth, water, air, fire
Each element tends to move toward its “natural” place:
E.g., rock (earth) falls in air/water, air bubble in water rises
“Natural motions” of earthly objects
straight lines toward center of Earth
bodies in motion naturally tend to come to rest
applied force causes deviation from natural motion
body at rest will remain so unless a force is applied
continual application of force needed to sustain a
n
y
motion other than natural motion

31. Heavens are governed by different laws from Earth
Celestial bodies composed of “aether,” a fifth
element not present on Earth*
*turns out there might a quintessence! (later…)
“Natural motions” of celestial spheres are different from
terrestrial motions:
circular, constant, and eternal
Aristotle needed 55 spheres to explain observed motions of
Sun, Moon, planets, stars
Space is finite, bounded by outer sphere
Edge is unreachable: motions become circular in the ethereal
Time is infinite

32. D. Aristarchus
- Aristarchus is the very first Greek to
profess the heliocentric view. The word
helios means sun; centric means
centered. He learned that the sun was
many time farther than the moon and that
it was much larger than the earth.

33. • He also made an attempt to calculate
the distance of the sun and the moon by
using geometric principles. He based
his calculations on his estimated
diameters of the earth and moon, and
expressed distance in terms of diameter.
However, the measurements he got
were very small and there were a lot of
observational errors.

34. E. Eratosthenes
• The first successful attempt to determine the
size of the earth was made by him. He did this
by applying geometric principles. He observed
the angles of the noonday sun in two Egyptian
cities that were almost opposite each other-
Syene (now Aswan) in the south and
Alexandria in the north. He assumed they
were in the same longitude.

35. F. Hipparchus
• is considered as the greatest of the early Greek
astronomers. He observed and compared the
brightness of 850 stars and arranged them into
order of brightness or magnitude. He developed
a method for predicting the times of lunar
eclipses to within a few hours. Aside from this,
he also measured the length of the year to within
minutes of the modern value.

36. G. Claudius Ptolemy
• is considered as the greatest of the early
Greek astronomers. He observed and
compared the brightness of 850 stars and
arranged them into order of brightness or
magnitude. He developed a method for
predicting the times of lunar eclipses to within
a few hours and measured the length of the
year to within minutes of the modern value.

37. Ptolemic Model
• According to the
Ptolemic Model, the
sun, the moon, and the
other planets move in
circular orbits around
the earth.

38. Ptolemic Model
• However, if observed
night after night, these
planets move slightly
eastward among the
stars. At a certain point,
the planet appears to
stop then moves in the
opposite direction for
some time;

39. Ptolemaic Model
• after which it will resume its eastward motion.
This westward drift of the planets is called
retrograde motion. To justify his earth-
centered model using retrograde motion, he
further explained that the planets orbited on
small circles, called epicycles, revolving
around large circles called deferents.

40. Epic Epicycles
Aristotelian/Ptolemaic view prevailed in
Europe and Islamic empire, through
1400’s
Geocentric model
Creation at finite time in past, for
consistency with Christian theology
Earth known to be round (Columbus
battling against flat Earther is myth!)

41. Copernicus (1473-1543)
Nicholas Copernicus was modern
(re)founder of the heliocentric (Sun
centered) model for the solar system
Rejected Ptolemy’s geocentric model
because it was too complicated
(Occam's razor)
Heliocentric model

42. Mathematics was not simpler than Ptolemy's,
but it required fewer basic assumptions
By Copernicus assuming only
the rotation of the Earth,
revolution about the sun,
tilt of Earth's rotational axis,
he could explain observed “motions” of t
h
e
heavens
Because he retained circular orbits, his
system required the inclusion of epicycles