1. Lesson 1-3 Classical Astronomy (500B.C. to 1400 A.D.)

2. Lesson overview Who were the classical astronomers?
What is the Earth’s shape?
How big are the Moon and Earth?
How far is it to the Sun and Moon?
What is the structure of the universe?
What is Occam’s Razor?

3. Who were the classical astronomers?
Astronomers of ancient Greece and Egypt were the first to explain the heavens
Used naked eye observations and geometry
Some constructed idealized models to account for the motion of heavenly bodies
Some famous Greek astronomers:
Pythagoras
Aristotle
Aristarchus
Eratosthenes
Ptolemy

4. What is the Earth’s shape?
Pythagoras ( BC) said Earth was round:
Sphere a perfect shape
Gods made Earth
Pythagoras was right about earth being spherical, but he couldn’t prove it.
Aristotle made observations that the earth was indeed a sphere

5. Aristotle ( B.C.)
Wanted to explain the movement of the sun, moon, 5 visible planets and the stars
Geocentric (Earth-centered), geostatic (earth-fixed) universe
Model used 55 concentric, “solid crystalline spheres” around Earth
Like Pythagorus, he also argued the sphere was the perfect shape the Gods would have used to create Earth

6. Aristotle (384-322 B.C.) Divided universe into two realms:
Sublunar Realm
Everything beneath the Moon’s sphere
Imperfect area containing 4 elements:
Earth and Water (move down)
Air and Fire (move up)
Superlunar Realm
Everything outside the Moon’s sphere
Perfect heavenly area containing “aether” where all elements move in circular motion

7. Aristotle (384-322 B.C.) Outermost sphere was domain of “Prime Mover”
Caused this sphere to move, imparting motion to other spheres
Adjusting velocities of various spheres explained many features of planetary motion
Model became the standard for almost 2000 years
Weaknesses:
Couldn’t explain varying planetary brightness
Couldn’t explain “Retrograde” motion

9. Retrograde Motion

10. Location of Mars among stars after several months of observation
Retrograde Motion
Location of Mars among stars after several months of observation

11. What is the Earth’s shape? (cont’d)
Aristotle—two things “prove” Earth’s spherical shape:
Earth’s shadow on the Moon
What a traveler moving south sees in the sky

12. How big is the Moon? Aristarchus (310-230 BC)
Estimated moon was about 1/3 size of Earth (actually, it is 27%)
Estimated sun’s diameter to be 7 times larger than earth (actually, it is 100 times larger)
First to show the sun was much larger than the earth and proposed it should be at center of universe (heliocentric model)
Stellar Parallax couldn’t be seen

13. Stellar Parallax

14. Eratosthenes (276 – 194 B.C.)
First to accurately calculate the circumference of the earth
Used simple geometry and a protractor
Had to assume the earth was a sphere and that the sun was far enough away that light rays hitting earth at two different locations were parallel

15. Eratosthenes (276 – 194 B.C.)
Heard that in Syene, Egypt, on the summer solstice, the sun left no shadow in a well
Then, on that same day, he was in Alexandria, Egypt, and used a protractor to measure the angle of the sun’s shadow on a tall pole
Then, he used simple geometry to compute the earth’s circumference

16. Distance from Syene to Alexandria = 500 miles
Angle measured with shadow: 7.2 degrees
So, degrees / 7.2 degrees = Earth Circumference / 500
Therefore, Earth’s Circumference = (360 / 7.2) X 500 = 25,000 miles
Actual Earth Circumference at Equator: 24,901 miles

17. You’ve been selected for a team that will travel to Mars
You’ve been selected for a team that will travel to Mars. One of your tasks is to accurately measure its circumference. You land at a spot where the next day at noon your ship will leave no shadow. So the next morning, you get up early, hop on your Mars rover and travel 150 miles to the south. Precisely at noon, you measure the length of your shadow and calculate it’s angle to be 4.1 degrees. What’s the circumference of Mars?

18. Ptolemy (circa 140 A.D.)
Refined Aristotle’s solid-crystalline-sphere theory
Continued three beliefs we now know were completely wrong
All motion in the heavens is circular
Objects in the heavens are perfect and cannot change their intrinsic properties (e.g., brightness)
Earth is the center of the universe

19. Ptolemy (circa 140 A.D.)
Ptolemy used complex combinations of circles (epicycles) to predict orbits for the Sun and Moon
Helped explain retrograde motion
Helped explain varying brightness as his model caused changes in the planets distance from Earth
Results published in Almagest (“The Greatest”)

20. Ptolemy’s geocentric theory
Animation: Ptolemy’s Model of Motion of a Planet

21. Ptolemy (circa 140 A.D.)
To try and explain some of the subtler motions, he included epicycles inside of epicycles
His model survived until the 1500s
It was adopted into Catholic Church doctrine
But its complexity led to its eventual downfall
One of the first to question was William of Occam (1285 – 1349), a British philosopher
Occam’s Razor: Given two equally predictive theories, choose the simpler

22. Arabic Contributions
Translated Ptolemy’s Almagest and wrote extensive commentaries on it
These commentaries made their way to the west and became part of the Renaissance in Europe
Later used by Copernicus who developed theory that put the sun at the center of our Solar System
Arabic numbering (1, 2, 3) replaced Roman numbering (I, II, III)
Gave us algebra and advanced trigonometry
Perfected the astrolabe instrument
Used to chart locations of stars
Used in navigation for travelers

23. Astrolabe Instrument

24. Next: Lesson 1-4 Astronomy in the Renaissance (1400 – 1650 A.D.)