1. Lesson 4: Classical Astronomy
Early Ideas of the Heavens
Classical Astronomy
(500 B.C. to A.D. 1400)
Did you ever see something you couldn’t explain? Long ago, people had a lot they couldn’t explain about the night sky. They developed elaborate theories to explain what they observed, but some were more accurate than others.
This is a picture of Ptolemy.

2. Motivation Classical theories (500 B.C. to 150 A.D.)
Some ideas very complicated and wrong
Others ideas simple and correct
People just like you and me
To appreciate what we know today, it’s useful to revisit theories from hundreds or even thousands of years ago.
The people who developed these theories weren’t necessarily scientists as we view scientists today, but their theories seemed believable at the time.
Astronomers of ancient Greece and Egypt were the first to explain the heavens. Men such as Aristotle, Erastothenes, and Ptolemy developed models for the motions of the Sun, Moon, and several planets. They also tried to estimate the sizes of—and distances between—the Earth, Moon, and Sun.
Most of these theories proved incorrect, but they are part of our history. In some cases, people accepted the theories for thousands of years!
They were people just like you and me – with often strange lives. One of your alternate assignments could be to complete a short biography on one of the lives of these ancient astronomers. There are some strange things in some of their lives – like the guy who lost his nose in a swordfight, or those that were condemned by the pope; we’ll discuss some later.

3. Introduction
Astronomers of ancient Greece and Egypt first to explain the heavens
Some ancient Greeks are famous in astronomy: Pythagoras, Aristotle, Aristarchus, Eratosthenes and Ptolemy
Determined important things using naked eye and simple math
Also got some things wrong
Made models to explain motions of Sun, Moon, and planets
Astronomers of ancient Greece and Egypt were the first to explain the heavens. Men such as Aristotle, Erastothenes, and Ptolemy developed models for the motions of the Sun, Moon, and several planets. They also tried to estimate the sizes of—and distances between—the Earth, Moon, and Sun.
Most of these theories proved incorrect, but they are part of our history. In some cases, people accepted the theories for thousands of years!

4. What is the Earth’s shape?
-Today we know the Earth is nearly round—or oblate to be exact.
-- We have proof: pictures taken from the Space Shuttle!
-- We have instruments to measure the slightest changes.
But the ancient astronomers had no cameras and no proof, so some beliefs about the Earth’s shape had little to do with science.
Earth is nearly round
Ancient astronomer’s beliefs about the Earth’s shape not really science

5. Earth’s shape? Pythagoras said Earth was round:
Sphere a perfect shape
Gods made Earth
Pythagoras was right, but he couldn’t prove it.
Pythagoras (famous for his Pythagorean theorem) taught as early as 500 B.C. that the Earth was round because
- The gods considered the sphere a perfect shape.
- The gods would use a perfect form to make something as important as the Earth.
Pythagoras was right, but he couldn’t prove it.

6. Earth’s shape? Aristotle: two things “prove” Earth’s spherical shape
Earth’s shadow on the Moon
A traveler’s view of the sky changes as he moves south
By 300 B.C., Aristotle saw two things that helped “prove” Earth’s spherical shape:
- Earth’s shadow on the Moon during an eclipse is curved, so the whole Earth would be round.
- A traveler moving south sees stars previously hidden by the southern horizon, so Earth must be round. On a flat Earth, these stars would
be visible all the time.

7. How big is the Moon? Aristarchus correctly estimated
Relative size of Earth and Moon
Moon’s diameter related to Earth’s
How big is the Moon?
- Aristarchus used the size of the Earth’s shadow on the Moon during a lunar eclipse to estimate the relative size of the Earth and Moon.
- He estimated the Moon’s diameter was 1/3 of the Earth’s diameter. He was close. It’s actually about ¼.

8. Aristarchus: Measuring the ratio of the size of the Moon to the size of the Earth's shadow at the Moon
The jump Aristarchus made from terrestrial measurements of scale to the celestial is truly remarkable. Without any measures of the sizes of or distances to any celestial objects, he was able to measure both for the Moon. His genius was to use something he knew about the Earth (its size) and a bit of clever geometry to deduce the size of the Earth's shadow at the distance of the Moon. Then by watching the Moon travel through the Earth's shadow (a lunar eclipse), he could determine the ratio between the size of the shadow at the distance to the Moon and the size of the Moon itself.
angular size of Earth's shadow at the distance to the Moon
angular size of the Moon

9. How big is the Earth? Eratosthenes first measured Earth’s size
Used simple math and measurements of shadows
Accurately estimated Earth’s circumference
How big is the Earth?
- Eratosthenes (276−195 B.C.) first measured the Earth’s size.
- He used simple math and measurements of shadows to estimate the Earth’s circumference.
His value for the circumference—25,000 miles—was very close to today’s value.

10. Aristarchus Aristarchus also estimated: Wrong by a lot
Sun about 20 times farther away from the Earth than the Moon
Sun to have seven times Earth’s diameter
Wrong by a lot
How far is it to the Sun and Moon?
- Aristarchus also calculated the Sun to be about 20 times farther away from the Earth than the Moon and to have seven times Earth’s diameter.
- But this time he was wrong by a lot:
-- The Sun is about 400 times farther from the Earth than the Moon.
-- The Sun’s diameter is about 100 times as large as Earth’s diameter.

11. Aristarchus Aristarchus recognized the Sun was larger than Earth
Even though Aristarchus’ estimates of the Sun’s distance and size were wrong, he recognized the Sun was larger than Earth.
And….

12. Aristarchus - sun is center of solar system
He proposed the Sun, not Earth, was the center of our solar system (and the universe)—an unpopular claim at the time.
This is called the Heliocentric (or sun-centered) model
It took 2,000 years for people to see that Aristarchus was right.

13. How could our ancestors believe Earth is the center of the universe?
Geocentric theory
Objects in the sky generally move east to west
How could our ancestors believe Earth is the center of the universe?
- The idea that Earth is the center of the universe is called a geocentric (or earth-centered) theory.
- Because objects in the sky generally move east to west (including the sun and known planets), geocentric theories tended to explain these motions. Because of these motions….

14. Geocentric Model
A geocentric model would look something like this:

15. How could our ancestors believe Earth is the center of the universe?
What Eudoxus believed
What Eudoxus assumed about movement and position of heavenly bodies
Right or wrong?
Eudoxus (400−347 B.C.) proposed a geocentric model that showed each space object mounted on its own revolving transparent sphere.
- He assumed the bodies moving fastest across the sky are those nearest the Earth.
- So the Moon seemed to be the body closest to Earth, which was correct.
- Sounds nice, but this theory proved wrong.

16. Ptolemy’s geocentric theory
What Ptolemy of Alexandria assumed about planetary movement
Theory included epicycles
How well his model predicted motion
Animation: Ptolemy’s Model of Motion of a Planet (
Ensure that you are in slide show so that you get the animation of the epicycles.
- As problems arose that contradicted these theories, scientists would come up with even more complicated designs to explain why the earth was the center of the universe.
- Ptolemy improved on the geocentric theory.
-- About 150 A.D., Ptolemy of Alexandria (Egypt) improved the incorrect geocentric model by assuming each planet moved on a small circle, which inturn had its center move on a much larger circle centered on the Earth.
-- He called the small circles epicycles and used at least 80 of them to explain the motions of the Sun, Moon, and five planets known in his time.
Use the dry-erase board and passage from the text to show what we mean by retrograde motion and how when viewing from earth it would appear as if the planet had reversed directions for a brief time and then reassumed the westward motion (as you look at the planet in the night sky).
- Ptolemy’s model predicted planetary motion quite well -- but alas also incorrectly.

17. Mars’ Retrograde Motion (epicycles) plotted out as seen in skies
This is an example of what ancient observers, like Ptolemy, would have seen in the night sky and why it was reasonable for him to come up with his theory of retrograde motion. What he failed to understand is that this apparent retrograde motion was caused because of the relationship of the earth to Mars as we both revolved around the sun … as shown in this next slide

18. Retrograde motion of Mars
     Retrograde motion of Mars. Under normal circumstances, the planet appears to move to the East (upwards, in the diagram), but when it is being overtaken by the Earth, our faster motion makes Mars appear to be going backwards (downwards, in the diagram). The normal motion is called direct motion, and the backwards motion is called retrograde motion.
A nice picture to explain Ptolemy’s retrograde motion theory

19. Problems with Ptolemy’s geocentric theory
What Ptolemy’s first model couldn’t cover
Ptolemaic models became complex
Belief in his theory declined
But Ptolemy’s geocentric theory had several problems.
Ptolemy’s first model couldn’t cover all the subtle motions of Sun, stars, and planets in the sky.
Very complex Ptolemaic models developed to account for these observations.
Although Ptolemy’s theory survived until the mid-1500s, it became too complicated to believe.

20. Geocentric theory failed the test of “Occam’s Razor”
“If you can choose between a very complicated theory and a simple theory, the simple one probably is better and is more likely to be correct.”
also called the Law of Parsimony
William of Ockham develops a razor-sharp rule that says the simplest explanation with the least assumptions is probably the right one. Ockham's Razor, also called the Law of Parsimony, underpins all of science.
How does Occam’s razor question geocentric models of the universe?
a. Science has a rule called “Occam’s razor,” which the British philosopher William of Ockham (about 1285−1349) often used and
may have created.
b. Occam’s razor said: “If you can choose between a very complicated theory and a simple theory, the simple one probably is better
. . . and is more likely to be correct.”
c. Because of “Occam’s razor” astronomers dropped geocentric models of our solar system and replaced them with a simpler model that we’ll discuss next time (heliocentric, or Suncentered).

21. Summary What is the Earth’s shape? How big is the Earth?
What we know and what Pythagoras believed
What Aristotle showed and how
How big is the Earth?
What we know and how ancients guessed
What Eratosthenes estimated the Earth’s circumference to be and how
What is the Earth’s shape?
- We know it’s spherical, but Pythagoras believed it was spherical because he had faith in the gods.
- Aristotle showed the Earth was round by viewing Earth’s curved shadow on the Moon during eclipses.
How big is the Earth?
- We now know Earth’s size, but in ancient times they had to guess based on limited tools and information.
- Eratosthenes estimated the Earth’s circumference to be 25,000 miles by using math and measurements of shadows. That’s close to today’s measurements.

22. Summary (continued) How far is it to the Sun and Moon?
Aristarchus estimated distances from the Earth to the Moon and Sun—right or wrong?
He estimated the Sun’s and Earth’s relative sizes—right or wrong?
How does Occam’s razor question geocentric models of the universe?
What Ptolemy proposed
What Occam’s razor says about complicated theories
How far is it to the Sun and Moon?
- Aristarchus estimated the distances from the Earth to the Moon and Sun using basic geometry, but his estimate was wrong.
- He estimated the Sun was seven times larger than Earth, which also was wrong.
How does Occam’s razor question geocentric models of the universe?
- Ptolemy proposed a very complicated geocentric model, which was difficult to understand and believe.
- Occam’s razor, a test for scientific theories, says that if you can choose between a very complicated theory and a simple theory, the
simple one is better and probably is correct.