NAME:__________________________GRADE/SECTION:______________SCORE:_________________
SUBJECT: PHYSICAL SCIENCE_11
TEACHER: ROWENA B. CABRERA_DATE: MAY 21, 2021/week 1
TYPE OF ACTIVTY:
Concept Notes
Individual
Formative
Others
Laboratory
Pair/Group
Summative
Lesson/Topic:
Ancient Astronomy
Learning Target(s): Discuss the thoughts of philosophers about the shape of the Earth.
Describe the size of the Earth.
Realize the importance of the shape of the Earth.
Reference(s):
Self Learning Module 11
Around 500 B.C., most Greeks believed that the Earth was round, not flat. It was Pythagoras and his pupils who
were first to propose a spherical Earth.
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.
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.
North Star
The North Star was 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.
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.
Disappearing Ships
If the Earth was flat, then a ship traveling away from an observer should become smaller and smaller until it
disappeared. However, the Greeks observed that the ship became smaller and then its hull disappeared first before the
sail as if it was being enveloped by the water until it completely disappeared.
The Size of the Spherical Earth
Ancient scholars tried to provide proof of a spherical Earth and its circumference through calculations. It was
Eratosthenes who gave the most accurate size during their time. 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. 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.
Figure 1: Shows how Eratosthenes measured the circumference of the Earth.
Eratosthenes then determined the angle the Sun made with the vertical direction by measuring the shadow that a
vertical stick cast. He found out that in Alexandria, the Sun makes an angle of 7.2° from the vertical while 0° in Syene. To
explain the difference, he hypothesized that the light rays coming from the sun are parallel, and the Earth is curved.
From his measurements, he computed the circumference of the Earth to be approximately 250 000 stadia (a stadium is a
unit of measurement used to describe the size of a typical stadium at the time), about 40 000 kilometers.
Our understanding about the different heavenly bodies can be credited to the important findings of the following Greek
astronomers:
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.
b. Eudoxus
Eudoxus proposed a system of fixed spheres. He 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.
c. Aristotle
Aristotle was a student of Plato. For him, the 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.
d. Aristarchus
Aristarchus is the very first Greek to profess the heliocentric view. The word helios means sun; centric means
centered. This heliocentric view considered the sun as the center of the universe. He learned that the sun was many
time farther than the moon and that it was much larger than the earth. 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.
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 otherSyene (now Aswan) in the south and Alexandria in the north. He assumed they were in the same longitude.
f. Hipparchus
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.
g. Claudius Ptolemy
He believed that the earth was the center of the universe. His Ptolemic Model claimed that the planets moved in a
complicated system of circles. This geocentric model also became known as the Ptolemic System.
The Ptolemic Model
Claudius Ptolemy developed a model that was able to explain the observable motions of the planets.
Figure 2: Ptolemic Model showing geocentrism.
According to the Ptolemic Mode, the sun, the moon, and the other planets move in circular orbits around the earth.
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; after which it will resume its eartward
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.
Aristotle’s Conclusion
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.
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.
Most people in Aristotle’s time believed the earth was flat, but he did not agree. He studied and used scientific methods
to prove that his conclusion was correct. Firstly, Aristotle considered the position of the North Star. The farther north
you journeyed, the closer the North Star seemed to move to the middle of the sky. But if someone were to travel south
of what we now call the equator, the North Star could not be seen at all. He also watched ships sailing into port. He
noticed that at a distance, he could see the tops of their sails before he saw the rest of the ship. Aristotle deduced that
this was because of the curvature of the earth. And lastly he observed the shadow cast during eclipses.
REFLECTION:
If you were given a chance to write something on your journal for your chosen Greek philosopher, who will it be
and what will you tell him about his findings on ancient history?
My Journal
________________________________
(Name of the Greek Philosopher)
__________________________________________________________________________________________________
__________________________________________________________________________________________________
__________________________________________________________________________________________________
______________________________________________________________
Assessment:
Directions: Encircle the letter of the correct answer.
1. Which of the following is the shape of the Earth according to ancient Greeks?
a. cylinder
b. octagon
c. flat disc
d. sphere
2. What is the shape of the Earth as described by modern astronomy?
a. ellipsoid
c. hyperboloid
b. oblate spheroid
d. oblate paraloid
3. Which of the following ancient Greek philosophers computed for the circumference of the Earth?
a. Anaxagoras
b. Pythagoras c. Eratosthenes d. Aristotle
4. According to Erastothenes’ computations, what is the circumference of the Earth?
a. 250,000 stadia
c. 500 stadia
b. 7.2 stadia
d. 40,000 stadia
5. Which of the following is 250 000 stadia equal to?
a. 40,000 kilometers
c. 40,000 meters
b. 40,000 miles
d. 40,000 inches
6. In which of the following events can the circular shadow of the Earth be observed most notably?
a. solar eclipse
c. lunar eclipse
b. summer solstice
d. winter solstice
7. Which of the following describes the position of the North Star if you go nearer the equator?
a. Closer to the horizon
b. Farther away from the horizon.
c.The North Star is fixed wherever you are on the Earth.
d. It disappears completely.
8. Which of the following can be observed of a cruising ship if the Earth is a flat disc?
a. It will shrink then only the sail will be visible until it completely disappears.
b. It will become bigger and bigger.
c. It will not change its size.
d. It will become smaller and smaller until it disappears.
9. During which time did Eratosthenes observe the shadows cast by a vertical stick?
a. noon time in summer solstice
b. noon time in winter solstice
c. during a lunar eclipse
d. during a solar eclipse
10. According to Eratosthenes, which of the following explains why a vertical stick casts a shadow in Alexandria but
not in Syene?
a. The Sun is directly overhead in Syene while in Alexandria, it is only almost directly overhead.
b. The light rays coming from the sun are parallel, and the Earth is curved.
c. The light rays coming from the sun are curved, and the Earth is flat.
d. The Sun is directly overhead in Alexandria while in Syene, it is only almost directly overhead.