2.1 - Notes Early Ideas in the Heavens: Classical Astronomy

1. 2.1 - NotesEarly Ideas in the Heavens: Classical Astronomy • Ancient Greeks explained the sky by making careful, systematic observations using the naked eye, logic, mathematic, and geometry and they were very successful 

2. Shape of the Earth Ancient Greeks explained earth was round • Pythagoras (560-480BC) – Earth is a sphere because it is a perfect shape, so God would make Earth spherical.

3. Shape of the Earth Aristotle (384-322 BC) – used observations • Earth’s shadow is round during an eclipse • As you travel north or south you see different stars (stars that were once hidden by the horizon)

4. The Size of Earth • Eratosthenes (276-195 BC) was able to calculate the circumference of the Earth as 25,000miles (very close to the actual 24,901) using geometry.

5. Distance and Sizes of the Sun & Moon Aristarchus of Samos (born~310 BC) • Estimated the distance between the Sun, Moon, and Earth by using the angle between the 3 on a quarter moon • Estimated that the Sun was 20X further than the Moon, so it must be 20X bigger since their angular size is the same. • Angular Size: a measure of how large an object looks to you. It is defined as the angle between lines drawn from the observer to opposite sides of an object  moon = .5°

6. Distance and Sizes of the Sun & Moon • Though incorrect, he was the first to say the Earth was not the largest of the three – leading to a heliocentric theory of the solar system

8. Arguments for an Earth Centered Universe • Based on parallax – apparent shift in a stars position relative to its background caused by the movement of the observer

9. 2.2 - NotesThe Planets Take notes on the following topics, be sure to include the people responsible & their ideas: • The Planets • Retrograde Motion • Explaining the Motion of the Planets • Ptolemy • Islamic Astronomy • Asian Astronomy

10. The planets (Greek for “wanderers”) do not follow the same cyclic behavior of the stars The planets move relative to the stars in a very narrow band centered about the ecliptic and called the zodiac Motion and location of the planets in the sky is a combination of all the planets’ orbits being nearly in the same plane and their relative speeds about the Sun Planets and the Zodiac

11. Planets - the Wanderers • Apparent motion of planets is usually from west to east relative to the stars, although on a daily basis, the planets always rise in the east • Even though they followed the ecliptic for the most part they would: • Change speeds • Appear to loop backwards, then forward again • This backward motion is called retrograde motion

12. Retrograde Motion • Occasionally, a planet will move from east to west relative to the stars; this is called retrograde motion • Explaining retrograde motion was one of the main reasons astronomers ultimately rejected the idea of the Earth being located at the center of the solar system

13. Geocentric Model • Earth at the center of the universe and all other bodies move around it. • Employed the perfect form = the circle • Logical – the size of the Sun and Moon remained constant • Remember – the ancient Greeks used logic and reason more than scientific observation and data • Supported by Plato and Aristotle

14. Geocentric Model • Eudoxus (400-347 B.C.) proposed a geocentric model in which each celestial object was mounted on its own revolving transparent sphere with its own separate tilt • The faster an object moved in the sky, the smaller was its corresponding sphere

15. Problems with the Geocentric Model • Mars, Jupiter, and Saturn were brightest in retrograde motion • Planets orbits were perfect circles, so they should always be the same distance from Earth • This simple geocentric model could not explain retrograde motion without appealing to clumsy and unappealing contrivances

16. Ptolemy of Alexandria • Ptolemy of Alexandria (90-168 B.C.)improved the geocentric model by assuming 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 were incorporated so as to explain retrograde motion

17. Epicycles • Each planet moves uniformly around a small circle, whose center moves uniformly around a larger 2nd circle called the deferent • Model included 80 circles overall – very complex

18. Ptolemy of Alexandria • Ptolemy’s model was able to predict planetary motion with fair precision • Discrepancies still remained but the Ptolemaic model lasted until the 1500s and was adopted by the Catholic Church. • Ultimately, all the geocentric models collapsed under the weight of “Occam’s razor” and the heliocentric models prevailed

19. Why was the geocentric model unchallenged for 1400 years? • Had some simple but compelling arguments: • Earth didn’t feel like it was moving • If moving, wouldn’t that create strong winds • No stellar parallax • GOD? Science and religion were not separate at the time and using logic – wouldn’t God put us at the center of the universe???

20. Non-Western Contributions • Islamic Contributions • Relied on celestial phenomena to set its religious calendar • Created a large vocabulary still evident today (e.g., zenith, Betelgeuse) • Developed algebra and Arabic numerals • Asian Contributions • Devised constellations based on Asian mythologies • Kept detailed records of unusual celestial events (e.g., eclipses, comets, supernova, and sunspots) • Eclipse predictions