1. History of Astronomy

2. Why did ancient cultures study astronomy?
Several cultures kept very exact records of astronomical events (and could even predict events) such as the Mesopotamians (Babylonians) and especially Chinese.
WHY??????
timekeeping
agriculture (planting crops or hunting and fishing cycles)
astrology/religion
and to keep accurate historical records.
But, there were no real models of why astronomical events happened…that’s what the Greeks did.

6. Greek Astronomy

7. General Information of Historical Models
Models tried to explain motion planets and why they move through the fixed stars.
Models tried to explain motion of moon and sun as well.
Models tried to account for retrograde motion of planets (this was the biggest problem in finding a model that worked accurately).
Astronomy and mathematics advanced side-by-side because it gave cultures problems to solve and calculate
GEOcentric--Earth centered universe
HELIOcentric--Sun centered universe

8. A Timeline of World Events and Famous People (left) and Notable Greek Astronomers (right)

9. The Pythagorean Model of the Solar System

10. Pythagorean Model
Earth moved around a “central fire” but we couldn’t see it because it was blocked by a “Counter Earth”
made music as it spun
Why it’s important:
one of first models
planets are spherical and orbit in circular paths

11. Eudoxus’ Model Tried to account for retrograde motion of planets
opposite motion of spheres produced figure eight motion of planet
Notice: still circular
Problems:
1. Not exact
2. Planets should be same brightness all the time and they aren’t

12. Aristotle’s (Aristotelian) model
~350 B.C.
Geocentric model of the universe
Problem: Not very exact

13. Eratosthenes (~ 200 B.C.): Calculation of the Earth’s radius
Angular distance between Syene and Alexandria: ~ 70
Linear distance between Syene and Alexandria: ~ 5,000 stadia
 Earth Radius ~ 40,000 stadia (probably ~ 14 % too large) – better than any previous radius estimate.

14. Ptolemy’s Model--tried to account for
retrograde motion using epicycles
(still not exact, but better
and was accepted for 1500 yrs !)

15. The Complete Geocentric Model

16. From Ptolemy into the Renaissance
Ptolemy’s model was relatively accurate and stuck around for 1500 years before another replaced it. Note...Ptolemy’s was geocentric.
Aristarchus proposed a heliocentric model around 300 B.C., but no one accepted it because they thought the position of stars should shift if Earth were moving.
Not until 1500’s (only 500 years ago !) did we see that the Earth wasn’t the center of the universe with Copernicus.
Coming up next… Copernicus, Tycho, Kepler and Galileo in the Renaissance.

17. A Timeline of Notable People and Events
A Timeline of Notable People and Events (left) and the Four Most Prominent Renaissance Astronomers (right)

18. Renaissance Astronomy

19. Copernicus
First accepted fully developed heliocentric model (sun at center)
published in 1543
Explains diurnal motion of stars, explains motion of sun
Important: Retrograde motion occurs whenever Earth passes or is passed by another planet
Difference between epicycles (Ptolemy) and Copernican model

20. The Position of the Sun Against the Backdrop of Stars throughout the year

21. The Retrograde Motion of Mars according to the Heliocentric Model
The retrograde motion of mars would occur over a couple of months.
The movement is apparent—mars doesn’t really move like that in space. The position of mars shifts against the backdrop of distant stars.

22. Tycho Brahe
Took very detailed and accurate measurements of stars and planets from his observatory
Also discovered a supernova in 1572 (a brilliant “new star”) that suggests that Aristotle’s starry sphere was not perfect and it changes

23. Tycho and Kepler
Kepler took accurate measurements from Tycho and devised his own mathematical laws.
Kepler’s 1st law: planets move in ellipses and not in circles (the sun is at one of the foci)
Kepler’s 2nd law: planets move faster when close to sun and slower when farther away.
Kepler’s 3rd law: P2  a3 (P=sidereal periods and a=semimajor axis)

24. Kepler’s Second Law

25. Galileo used telescope (but wasn’t the inventor)
found Milky Way is really a lot of stars
showed sunspots on sun—sun isn’t a perfect celestial body
showed moon has mountains and valleys like Earth (suggests Earth is a celestial object…part of the “heavens”)
showed 4 satellites orbiting Jupiter (proof that Earth wasn’t the center of everything)
showed phases of Venus (which crippled Ptolemaic model)…this can’t be explained in the geocentric model

26. Some of Galileo’s Drawings of the Moon

27. Phases of Venus—can’t see this in the geocentric model

28. Galileo’s drawings of Jupiter and the 4 main moons