1. Periods of Western Astronomy
Western astronomy divides into 4 periods
Prehistoric (before 500 B.C.)
Motions of Sun, Moon, & stars observed
Keeping time and directions develops
Classical (500 B.C. to A.D. 1400)
Measurements of the heavens
Geometry and models to explain motions

2. Periods of Western Astronomy
Renaissance (1400 to 1650)
Data led to better models
Technology (the telescope) enters picture
Modern (1650 to present)
Physical laws and mathematical techniques
Technological advances accelerate

3. Who? Early Ideas: Pythagoras – 500 BC
Earth was round, based on the belief that the sphere is the perfect shape used by the gods

4. Who? Early Ideas: Aristotle – 300 BC The Earth is round
Shape of Earth’s shadow on the Moon during an eclipse
A traveler moving south will see stars previously hidden by the southern horizon

5. Who? Early Ideas: Aristarchus – 280 BC
Found relative dimensions of the solar system
Earth was much smaller than the distances to the celestial bodies
Heliocentric (Sun-centered) model for the solar system

6. Who? Eratosthenes (276-195 B.C.)
Made the first measurement of the Earth’s size
He obtained a value of 25,000 miles for the circumference, a value very close to today’s value

7. Who?
Eudoxus ( B.C.)
Proposed a geocentric model in where each celestial object was mounted on its own revolving transparent sphere with its own separate tilt

8. Who? Ptolemy of Alexandria (140 AD)
Improved the geocentric model by stating that each planet moved on a small circle, which in turn had its center move on a much larger circle centered on the Earth
The small circles were called epicycles and this tried to explain retrograde motion

9. Who? Nicolas Copernicus (1473-1543)
Re-examined the heliocentric model and used it to explain retrograde motion
Data did not make sense with geocentric model
Heliocentric models explain retrograde motion as a natural consequence of two planets (one being the Earth) passing each other

10. Who? Nicolaus Copernicus (1473-1543)
Found relative distances of the planets from the Sun
However, problems remained:
Could not predict planet positions any more accurately than the model of Ptolemy

11. Who?
Tycho Brahe ( )
Made meticulous measurements & observations of the planets, supernovas, & comets
Proposed a compromised geocentric model
Designed & built instruments of far greater accuracy than any yet devised

12. Who? Johannes Kepler (1571-1630)
Using Brahe’s data, discovered that planets move in ellipses with the Sun located at one of the two foci (not in circles)!
He also created something we will learn about more later called Kepler’s Three Laws!

13. Who?
Galileo ( )
Used the telescope to study the heavens & noticed:
The Moon is made of rock with a surface similar to Earth’s
The Sun has spots. changes its appearance, & rotates
Jupiter has four objects (moons) orbiting it
Milky way is filled with many stars

14. Who? Isaac Newton (1642-1727) - born the year Galileo died – CREEPY?
Major advances in mathematics, physics, and astronomy
“Discovered” gravity & calculus