1. Periods of Western Astronomy
Western astronomy divides into 4 periods
Prehistoric (before 500 B.C.)
Cyclical motions of Sun, Moon and stars observed
Time and directions develop
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)
Accumulation of 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
Evidence: the sphere is the perfect shape used by the gods

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

5. 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

6. Who? Ptolemy of Alexandria (140 AD) Geocentric model (earth- centered)
Assumed: Each planet moved on a small circle, which in turn had its center move on a much larger circle centered on the Earth (epicycles) – explained retrograde motion

7. Who? Nicolaus Copernicus (1473-1543)
Heliocentric (sun-centered) models explain retrograde motion as a natural consequence of two planets (one being the Earth) passing each other
Relative distances of the planets from the Sun
BUT could not predict planet motion

8. Who?
Tycho Brahe ( )
Made observations of a supernova and comets - the heavens were both changeable and more complex than previously believed
Designed and built more precise & accurate instruments
Made measurements of the planets – gathered data

9. Who? Johannes Kepler (1571-1630)
Tycho’s data passed to Kepler, his young assistant
Kepler derived his three laws
One: Using Mars data, Kepler showed the orbit of planets to be an ellipse with the Sun at one foci
Two: Planets will sweep out in equal distances in equal times
Three: The farther the semi-major axis, the longer the orbit (a3=p2)

10. Who? Galileo (1564-1642) – house arrest!
Invent and use the telescope to observe space
Found: The Moon’s surface has craters and is made of rock
The Sun has spots, changes its appearance, and rotates
Jupiter has four moons orbiting it
Milky Ways has many stars

11. Who? Isaac Newton (1642-1727) - born the year Galileo died - CREEPY
Major advances in math (calculus), physics (motion/optics), and astronomy(gravity)
Newton’s laws of motion and gravity