Chapter 3 The Science of Astronomy

1. Chapter 3The Science of Astronomy

2. Early Astronomy All we know is what we read in the newspapers… and on the rocks , the scrolls, temples, and clay tablets… It is important to recognize parallel developments occurred in many parts of the world including Asia , India, and the Americas, as well as later Europe.

3. How did astronomical observations benefit ancient societies? • Keeping track of time and seasons • for practical purposes, including agriculture • for religious and ceremonial purposes • Aid to navigation

4. What did ancient civilizations achieve in astronomy? • Daily timekeeping • Tracking the seasons and calendar • Monitoring lunar cycles • Monitoring planets and stars • Predicting eclipses • And more…

5. "On the Jisi day, the 7th day of the month, a big new star appeared in the company of the Ho star." "On the Xinwei day the new star dwindled." Bone or tortoise shell inscription from the 14th century BC. China: Earliest known records of supernova explosions (1400 B.C.)

6. North American North American Native Petroglyphs: Anasazi drawing on a ledge in Chaco Canyon, New Mexico is thought to represent the supernova of 1054 AD.

7. Plains Indians Pawnee Indian Sky Map A chart embossed on hide appears to depict constellations of the northern hemisphere. From “ When Stars come down to Earth: Cosmology of the Skidi Pawnee Indians of North America.

8. Babylonian Astronomy • One of the earliest civilizations known to have written records developed along the Tigris and Euphrates rivers in what is now Iraq. • King Hammurabi 1700-BC importance attributed to positions of the planets… astrology governed Babylon life. Detailed observations maintained over several centuries.

9. Babylonian Clock & Calendar Babylonians Knew length of the year to accuracy of about 4 minutes. Divided year into 12 equal months of 30 days each. Babylonian number system based on 60. Angular system based on 360 degrees, 60 minutes to 1 degree, and 60 seconds to 1 minute. They had a 24 hour day they believed in symmetry of 12 hours each. 12 because of 12 lunar cycles per year. Later Chaldeans compiled records into tablets could predict solar, lunar and planetary positions As well as the occurrence of eclipses.

10. Egyptian obelisk: Shadows tell time of day.

11. Ancient people of central Africa (6500 BC) could predict seasons from the orientation of the crescent moon

12. Days of week were named for Sun, Moon, and visible planets

13. England: Stonehenge (completed around 1550 B.C.)

14. England1550B.C.

15. Stonehenge Built in stages from 3000BC to 1800 BC late stone age into bronze age. One of many stone monuments found throughout Europe.

18. Mexico: model of the Templo Mayor The sun rises between the temples on the equinoxes

19. Someone among the ancient Anasazi people carved a spiral known as the Sun Dagger on a vertical cliff face in New Mexico. There the Sun’s rays form a dagger of sunlight that pierces the center of a carved spiral only once each year - at noon on the summer solstice.

20. Our mathematical and scientific heritage originated with the civilizations of the Middle East

21. GREEK DEVELOPMENTS Greek culture started in Crete.. seafaring culture.. 5000 years ago. Source of the legends most of our constellations are named from.

22. Why does modern science trace its roots to the Greeks? • Greeks were the first people known to make models of nature. • They tried to explain patterns in nature without resorting to myth or the supernatural. Greek geocentric model (c. 400 B.C.)

23. Pythagoras (550 BC) • Claimed that natural phenomena could be described by mathematics

24. Pythagoras (c570-500 BC) of Samos established a school teaching the idea that natural phenomenon can be described by numbers. This laid the foundation for modern trigonometry and geometry.   He is thought to have asserted the Earth is round and that all heavenly objects move in perfect circles. Anaxagoras (500-428BC) moon shines by reflected light, explains eclipses

25. Artist’s reconstruction of Library of Alexandria

26. Eratosthenes measures the Earth (c. 240 BC) Measurements: Syene to Alexandria distance ≈ 5000 stadia angle = 7° Calculate circumference of Earth: 7/360  (circum. Earth) = 5000 stadia  circum. Earth = 5000  360/7 stadia ≈ 250,000 stadia Compare to modern value (≈ 40,100 km): Greek stadium ≈ 1/6 km  250,000 stadia ≈ 42,000 km

27. Greek Mainland On mainland democratically ruled city-states arose, Athens, Sparta.5th century BC, Plato ( 428-347BC) teacher of Aristotle., what we see is an imperfect representation of a perfect creation. …we learn more by reason than observation. Dominated western thought for 2000 years.

28. How did the Greeks explain planetary motion? Underpinnings of the Greek geocentric model: • Earth at the center of the universe • Heavens must be “perfect”: Objects moving on perfect spheres or in perfect circles.

29. Aristotle and Plato Aristotle was Plato’s most famous student.…first to adopt physical laws and use the laws to explain what we see.

30. Aristotle becomes Da’ Man Aristotle wrote and taught on philosophy, history, politics, poetry, ethics, drama and science. He did this well. Because of his success his works became the great authority for the next 2000 years. Astronomers (indeed all scholars) cited his work as the authority. If a thought was in conflict with his works it must be wrong… Ideas: The Universe is divided into two parts. The earth corrupt and changeable ..and the heavens perfect and immutable. ( Notice the similarity with some theology.) Earth at the center of the universe. Geocentric. 56 crystalline spheres. Physics: Earth , water, air, fire. Seek natural order. Circular motion expected.

31. But this made it difficult to explain apparent retrograde motion of planets… Review: Over a period of 10 weeks, Mars appears to stop, back up, then go forward again.

32. The most sophisticated geocentric model was that of Ptolemy (A.D. 100-170) — the Ptolemaic model: • Sufficiently accurate to remain in use for 1,500 years. • Arabic translation of Ptolemy’s work named Almagest (“the greatest compilation”) Ptolemy

33. So how does the Ptolemaic model explain retrograde motion? Planets really do go backward in this model..

34. Introduced by Ptolemy (ca. A.D. 140) 0 The Ptolemaic system was considered the “standard model” of the universe until the Copernican Revolution.

35. How was Greek knowledge preserved through history? • Muslim world preserved and enhanced the knowledge they received from the Greeks • Al-Mamun’s House of Wisdom in Baghdad was a great center of learning around A.D. 800 • With the fall of Constantinople (Istanbul) in 1453, Eastern scholars headed west to Europe, carrying knowledge that helped ignite the European Renaissance.

36. Copernicus, Galileo, Tycho, and Kepler challenge the Earth-centered idea Copernicus (1473-1543): • Proposed Sun-centered model (published 1543) • Used model to determine layout of solar system (planetary distances in AU) But . . . • Model was no more accurate than Ptolemaic model in predicting planetary positions, because it still used perfect circles.

37. Galileo (1564-1642) overcame major objections to Copernican view. Three key objections rooted in Aristotelian view were: Earth could not be moving because objects in air would be left behind. Non-circular orbits are not “perfect” as heavens should be. If Earth were really orbiting Sun,we’d detect stellar parallax.

38. Overcoming the first objection (nature of motion): Galileo’s experiments showed that objects in air would stay with a moving Earth. • Aristotle thought that all objects naturally come to rest. • Galileo showed that objects will stay in motion unlessa force acts to slow them down (Newton’s first law of motion).

39. Overcoming the second objection (heavenly perfection): • Tycho’s observations of comet and supernova already challenged this idea. • Using his telescope, Galileo saw: • Sunspots on Sun (“imperfections”) • Mountains and valleys on the Moon (proving it is not a perfect sphere)

40. Overcoming the third objection (parallax): • Tycho thought he had measured stellar distances, so lack of parallax seemed to rule out an orbiting Earth. • Galileo showed stars must be much farther than Tycho thought — in part by using his telescope to see the Milky Way is countless individual stars. • If stars were much farther away, then lack of detectable parallax was no longer so troubling.

41. Galileo also saw four moons orbiting Jupiter, proving that not all objects orbit the Earth

42. Galileo’s observations of phases of Venus proved that it orbits the Sun and not Earth.

43. GG3 In the Ptolemaic model Venus should always be a crescent. .

45. The Catholic Church ordered Galileo to recant his claim that Earth orbits the Sun in 1633 His book on the subject was removed from the Church’s index of banned books in 1824 Galileo was formally vindicated by the Church in 1992 Galileo Galilei

46. Tycho Brahe (1546-1601) • Compiled the most accurate (one arcminute) naked eye measurements ever made of planetary positions. • Still could not detect stellar parallax, and thus still thought Earth must be at center of solar system (but recognized that other planets go around Sun) • Hired Kepler, who used Tycho’s observations to discover the truth about planetary motion.

47. Kepler first tried to match Tycho’s observations with circular orbits • But an 8-arcminute discrepancy led him eventually to ellipses… • “If I had believed that we could ignore these eight minutes [of arc], I would have patched up my hypothesis accordingly. But, since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy.” Johannes Kepler(1571-1630)

48. What are Kepler’s three laws of planetary motion? Kepler’s First Law: The orbit of each planet around the Sun is an ellipse with the Sun at one focus.

49. What is an ellipse? An ellipse looks like an elongated circle

50. Kepler’s Second Law: As a planet moves around its orbit, it sweeps out equal areas in equal times. • means that a planet travels faster when it is nearer to the Sun and slower when it is farther from the Sun.