1. Modern science was born out of an effort over many centuries to understand the motions of celestial bodies.
2. Two competing models were proposed: Geocentric (earth-centered) and Heliocentric (Sun-centered).
3. The final success of the heliocentric model relied on crucial philosophical insights and technological advances.
4. The motions of celestial bodies visible to the naked eye are mostly regular and repeatable.
5. The stars rise in the east and set in the west daily
6. The Sun rises & sets daily, and makes an eastward circuit relative to the stars once a year.
7. The Moon rises & sets daily, and makes an eastward circuit relative to the stars once a month
8. Planetary motions are much more complex, showing occasional retrograde motion.
Planets rise and set daily, and move generally eastward relative to the stars at varying speeds.
But, they occasionally stop, move westward (or retrograde), then stop again and resume moving eastward.
Mars during 2005/6 retrogade.
9. For 2000 years, a geocentric model for the universe was widely assumed.
10. Aristotle (384-322 BC) argued for a geocentric model on physical grounds.
Earth was fixed and unmoving at the center because it is being too big to move, including rotation.
The Sun, Moon, Planets and Stars are affixed to crystalline spheres in uniform circular motion
The combination of perfect motions produces the net retrograde and non-uniform motions observed.
11. The ultimate Geocentric System was formulated Claudius Ptolemy around 150 AD.
12. Ptolemy’s explanation of retrograde motion:
13. Combination of small and large circles creates “loop-the-loop” retrograde motion.
14. Heliocentric Models have the planets, including Earth, in motion around a central Sun.
15. Aristarchus of Samos (310-230BC) proposed the first known Heliocentric model.
16. Nicolaus Copernicus (1473-1543) revived the Heliocentric system in the 16th century after 18 centuries of neglect.
Still clinged to Aristotle’s belief in uniform circular motion.
Had to use epicycles to get the motions correct (fine tuning).
De Revolutionibus Orbium Coelestium (1543)
17. English astronomer Thomas Digges (1546-1595) discarded the idea of a “Celestial Sphere”.
18. Johannes Kepler (1571-1630) took Copernicus’ ideas further and discarded epicycles.
Kepler’s laws of planetary motion state that planets go around the Sun on ellipses rather than circles.
They move with changing speeds rather than at constant speeds.
Kepler’s laws of planetary motion made much more accurate predictions of planetary positions, contributing to the triumph of Heliocentric Models.
19. Galileo Galilei (1564-1642) made crucial discoveries with the newly invented telescope.
20. The observation of craters and mountains on the Moon showed that it had terrain like the Earth.
21. The observation of sunspots showed that the Sun was imperfect and rotating slowly!
22. The Phases of Venus demonstrated decisively that Venus orbits the Sun.
23. COPERNICAN AND PTOLEMAIC PREDICTION