Lesson

1 Ancient Astronomy

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.

It was in Greece that the Golden Age of early astronomy was centered. Being philosophers, the
Greeks used philosophical arguments to explain the natural events happening around them
including the movements of the stars and other heavenly bodies. But they were also observers.
They made use of their observational data to explain certain events. They were the ones who
measured the sizes and the distances of the sun and the moon using the basics of geometry and
trigonometry which they also developed.

The early Greeks had a geocentric view of the earth. For them, it was the center of the universe;
hence, a motionless sphere. The sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn orbited
the Earth.

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.

What is It

Key Terms
● Oblate spheroid: the shape of the Earth. It has bulging equator and squeezed poles.
● Solstice: either of the two times in the year, the summer solstice and the winter solstice,
when the sun reaches its highest or lowest point in the sky at noon, marked by the longest
and shortest days.
● Eclipse: an obscuring of the light from one celestial body by the passage of another between
it and the observer or between it and its source of illumination.
● Heliocentrism: the astronomical model in which the Earth and planets revolve around the
Sun.
● Geocentrism: any theory of the structure of the solar system (or the universe) in which
Earth is assumed to be at the center of it all.

Have you ever wondered what the philosophers in ancient astronomy thought about the shape of
the Earth?
Learn About It!
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 times 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 other- Syene (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.

INDIVIDUAL ACTIVITY
Directions: Write the letter of the correct answer. Write your answer on a sheet of paper.
1. Which of the following is the shape of the Earth according to ancient Greeks?
a. cylinder c. flat disc
b. octagon 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 c. Eratosthenes
b. Pythagoras d. Aristotle

2. 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

3. 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

4. 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

5. 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.

6. 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.

7. 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

8. According to Eratosthenes, which of the following explains why a vertical

stick casts a shadow in Alexandria but not in Syene?
 I. The Sun is directly overhead in Syene while in Alexandria, it is only almost directly
overhead.
II. The light rays coming from the sun are parallel, and the Earth is curved.
III. The light rays coming from the sun are curved, and the Earth is flat.
IV. The Sun is directly overhead in Alexandria while in Syene, it is only
almost directly overhead.
a. I only c. III and IV
b. b. I and II d. II and IV
9. Greek philosopher who gave the most accurate size of the spherical earth
during their time?
a. Eratosthenes c. Claudius Ptolemy
b. Anaxagoras d. Hipparchus

10. It is an astronomical model in which the Earth and planets revolve around
the Sun.
a. Geocentrism c. Solstice
b. Heliocentrism d. Eclipse

11. Any theory of the structure of the solar system (or the universe) in which Earth is
assumed to be at the center of it all.
a. Geocentrism c. Solstice
b. Heliocentrism d. Eclipse

12. An obscuring of the light from one celestial body by the passage of another between it
and the observer or between it and its source of illumination.
a. Geocentrism c. Solstice
b. Heliocentrism d. Eclipse

13. Either of the two times in the year, the summer solstice and the winter solstice, when the
sun reaches its highest or lowest point in the sky at noon, marked by the longest and
shortest days.
a. Geocentrism c. Solstice
b. Heliocentrism d. Eclipse
GROUP ACTIVITY

In your most creative way, explain how the Greeks knew

that the Earth is Spherical?

It can be through a song, a role play (experiment), poem, poster, or interpretative dance. A 5-
minute-performance will be evaluated using a common rubric to be given by the teacher.
Criteria 5 4 3 2 1

Content Content is Content is Content is Content is Content is

accurate and accurate but accurate but questionable. inaccurate.
all required some required some required Information is Information is
information is information is information is not presented not presented
presented in a missing missing in a logical in a logical
logical order. and/or not and/or not order, making order, making
presented in a presented in a it difficult to it difficult to
logical order, logical order, follow. follow.
but is still making it
generally easy difficult to
to follow. follow.

Presentation Presentation Presentation Presentation Presentation Presentation

flows well and flows well. flows well. is has no flow.
logically. Tools are used Some tools are unorganized. Insufficient
Presentation correctly. used to show Tools are not information.
reflects Overall acceptable used in a
extensive use presentation understanding relevant
of tools in a is interesting. . manner.
creative way. Lacks
information
about some
and/or
information is
not identified.

Product/ Product is Product is Product is Product is No relevance

appropriate. appropriate. quite inappropriate at all.
Output Details are Details are appropriate. or details are
pleasing to the cluttered. messy.
eye.

Mechanics No spelling Few spelling Some spelling Some spelling Many spelling
errors. No errors. Few errors. Some errors. Some and grammar
grammar grammar grammar grammar errors. Text is
errors. Text is errors. Text is errors. Text is errors. Most copied.
in author’s in author’s in author’s of the text is
own words. own words. own words. in author’s
own words.