Lecture 4

1. Lecture 4 100 100 Ancient to Renaissance Astronomy

2. Announcements • Homework 3 – Due Monday, February 12 • Unit 9: Problem 3, Test Yourself 3 • Unit 11: Test Yourself 1 • Unit 12: Problem 1, Test Yourself 1, 3 • Test 1 is one week from today. • Will cover the first five lectures (units 1 – 12, more or less) • Details next time

3. What Is A Cosmology? • View of the universe and our place in it • Always has: • Layout of the universe • Often has: • Associated creation story • Fundamental human question: is there a meaning to our existence? • Method to explain celestial motion (movement of the planets and stars)

4. Earliest Cosmologies (3000+ BC) • Earth is flat • Universe centered on individual • Size of universe limited to local environment • Sun and moon important? • Cave paintings feature disk with rays (sun?) and “spots and crescents” (phases of the moon?) • Hunter-gatherer: value in understanding motions of stars and planets (useful in predicting winter)? • No evidence of concept of “celestial sphere” • Creation myths virtually unknown

5. Stonehenge (2800 BC – 1100 BC) • Horizon astronomy (position of sun during solstices and equinoxes) • Star/planet position not important • Calendar • Associated cosmology? • Probably not: utilitarian tool

6. Stonehenge (2800 BC – 1100 BC) A303 A344

7. Caracol (Maya culture, ~A.D. 1000)

8. Paint Rock, Texas

9. Archeoastronomy • Knowledge of motions of sun and moon allows prediction of seasons. • Important for agriculture • Agriculture important for rise of civilization.

10. Archeoastronomy • Astronomical knowledge important to religious/elite classes. • Perceived power: ability to “control” the objects in the heavens.

11. Babylonian Astronomy • Advanced calendar • Ability to predict planetary motion • Subtle shift in cosmology: • World is flat, but much larger than local environment. • Sky is a “vault” containing the stars. • Sun and moon moves through the sky. Fundamentally different from planets/stars.

12. Babylonian Astronomy • Precise understanding of solar/lunar motion necessary for lunar calendar. • Great interest in understanding planetary motion – associated with predictions of fate (astrology).

13. Babylon and Mathematical Astronomy • Early records are observationally based. • Records found from 1500 BC and before. • Highly accurate mathematics developed to predict future motion. • Math ultimately supplants observation. • No geometrical model – highly abstract.

14. Egyptian Astronomy • Cosmology similar to Babylonian. • Sky is body of the goddess Nut. Sun is divine. • Sky “attached” to land. The Earth is synonymous with the universe. • Afterlife is “elsewhere”

15. Egyptian Astronomy • Little interest in mathematical astronomy, or the sky: • Only five named constellations (records from 1100 BC). • Partial reason: Ancient Egypt lacks the mathematical sophistication of Babylon.

16. Egyptian Astronomy • Limited to the practical: • 36 decans (“star groups”) used to tell time at night. • First appearance of Sirius heralds annual flood of Nile. • Orientation of major monuments based on sun worship and significant constellations (like Orion – Osiris in Egyptian myth)

17. Greek Astronomy • “Classical Revolution” begins c.700 BC. • Astronomy used for timekeeping. Techniques imported from Babylon and Egypt. • Example from Hesiod – Pleiades star cluster used to track time for use in agriculture (Pleiades = harvest time).

18. Greek Astronomy • Pythagoras (c.500 BC): • Earth is a sphere • Morning star and evening star (associated with Venus) are the same object

19. 0 Greek Astronomy • Plato (c. 360 B.C.) • Established a philosophy based on the teachings of Pythagoras that favored mental reasoning power over observational science • Taught that what is seen in the natural world is an imperfect representation of ideal creation • Teachings dominated much of Western philosophy and science for about 2,000 years

20. 0 Greek Astronomy • Models based on the philosophy of Plato were generally wrong because they were based on wrong “first principles”, believed to be “obvious” and not questioned: • Geocentric Universe: Earth at the Center of the Universe and stationary. • “Perfect Heavens”: Motions of all celestial bodies described by motions involving objects of “perfect” shape, i.e., spheres or circles.

21. Greek Astronomy • Eudoxus (c.360 BC) • Complex motions of planets through the sky can be explained through simple circular motion. • Planets located on giant “crystal” spheres with Earth at the center. • Model of 27 nested spheres

22. Greek Astronomy • Greeks are first to devise a “model” that tries to explain (not just document and predict) these motions! • Circles / Spheres important in Greek philosophy (“the most perfect shapes”) – so model is based on spheres.

23. Aristotle (384 – 322 BC) • Eudoxus’ ideas refined by Aristotle (c.350 BC): • Earth is spherical at center of universe. • “Edge” of universe is a literal celestial sphere holding the fixed stars. Sphere rotates around earth. • Sun moon and planets on concentric spheres that rotate around the earth.

24. 0 Parallax • Aristotle could not measure stellar parallax. • He concluded the earth must not be moving. • Distances to stars are too great to measure parallax.

25. Aristotle’s Universe Sun, Moon and five known planets attached to 7 spherical “heavens” Would dominate astronomical thought for the next 2,000 years!

26. Aside – Days of the Week These seven heavenly bodies were associated with the days of the week. In fact, we still use their names:

27. Aristarchus (310 BC – 230 BC) • Measured relative distances/sizes of sun and moon: • Moon is about 1/3rd the size of Earth. • Sun 20 times farther than moon and 7 times larger than Earth. • Since sun is larger, reasons it must be at the center of universe. • First known heliocentric cosmology.

28. Some Incredible Insights • Aristarchus Develops Heliocentric Idea: • Moon shines because it reflects light from the sun. • Stars show no parallax: • Must be extremely far away • If viewed close up, must be as large and bright as the sun • Must be “distant suns” • Universe must therefore be MUCH bigger than indicated by Aristotle’s geocentric model.

29. Aristotle Hijacks Science • Aristarchus’ hypothesis fails in the face of Aristotle’s philosophy. • Why no perception of motion: • Earth rotates? Why don’t dropped objects fly off to the west? • Earth circles the sun? Why don’t we fly off its surface? • Ironically: Why no parallax? • Geocentric model wins the day!

30. 0 Calculation of the Earth’s radius • Eratosthenes (~ 200 B.C.) • Angular distance between Syene and Alexandria: ~ 7° • Linear distance between Syene and Alexandria: ~ 5,000 stadia •  Earth Radius ~ 40,000 stadia (probably ~ 14 % too large) – better than any previous radius estimate.

31. 0 Hipparchus (c. 180 B.C.) • Used naked-eye sighting instruments to aid observations • Introduced the use of celestial coordinates, made star maps (850 stars) • Invented the magnitude scale

32. 0 Hipparchus • Estimated and compared stellar brightness's • Used (possibly invented) trigonometry in astronomy • Discovered the precession of the Earth's axis • Maintained the idea of the Earth-centered (geocentric) universe, because he could not detect any hint of stellar parallax.

33. 0 Later Refinements Epicycles Introduced to explain retrograde (westward) motion of planets Deferent (off-center rotation) used to help fix deviations from the observed positions

34. 0 Ptolemy (c. 85 - 165 A.D.) • Summarized all previous Greek astronomy in his book the Almagest. • Further refined the deferent/epicycle concept using the equant. • The point of uniform motion of the planet is off-center.

35. 0 Ptolemy’s Geocentric Model Of The Cosmos • His model was the most accurate predictor of planetary positions in the ancient world and was used as the model of the Cosmos up to the time of the Renaissance.

36. Other Greek Contributions • Names and mythological identities for nearly all of the northern hemisphere and many southern hemisphere constellations. • Names of the major planets (carried over by Romans, using Roman names).

37. The Dark Ages • Fall of Rome, Rise of Christianity • Scriptural Basis for Geocentric Cosmology • Slight philosophical alterations to Ptolemy’s model • Earth is center of universe – “crown jewel” of God’s creation.

38. The Dark Ages • Sin is centered on Earth: “Flawed” surface • Taint reaches to moon – mostly perfect, except for some dark splotches. • Other celestial bodies are perfect, luminous orbs (perfection of Heaven).

39. The Dark Ages • Problems creeping up with geocentric cosmology. • The number of “tweaks” needed getting out of hand. • Support is waning among some of the educated in Europe.

40. The Renaissance • Nicholas Copernicus (1473 – 1543) • Influenced by ideas of Aristarchus. • Sun near center of universe. Planets orbit sun, including Earth. • Naturally explains retrograde motion.

41. 0 Copernicus’ new explanation for retrograde motion of the planets Retrograde (westward) motion of a planet occurs when the Earth passes the planet.

42. 0 Heliocentric Model • Copernicus did not have strong observational evidence to support his theory (stellar parallax), nor did he propose new laws of motion to explain the motion of the Earth. • Copernicus was rather conservative and harkened back to the "pure" ideas of Plato and Aristotle such as "perfect" circular motion in the heavens and the platonic idea of the Sun/God deity. • He greatly disapproved of the more recent ideas of the eccentricand equant used to modify the geocentric model.

43. Problems With Copernicus • Assumes circular motion: • Needs epicycles to match observed motion as well as geocentric model. • Makes predictions that cannot be verified with technology of the time (e.g. phases of Venus). • Why no stellar parallax (Brahe)? • “Philosophical” argument for next 50 years.

44. Tycho Brahe (1546-1601) • Was the last and greatest of the naked eye observational astronomers • Built an observatory with large sighting devices to make accurate records of planet and star positions for over 20 years • Observed and reported on the supernova of 1572, and he observed and estimated the distance to a comet in 1577 • Both observations helped to discredit the Aristotelian view of the Cosmos.

45. 0 Parallax Shift • Did not accept the Copernican (heliocentric) model, because he could not measure an annual parallax shift in the positions of the stars.

46. 0 Tycho’s Model • Proposed his own modified geocentric model of the Cosmos that was mathematically nearly identical to the Copernican model.

47. For Next Time • Read Unit 12. • Homework 2 due at the beginning of next class period! • Lab Report 1 due now (by beginning of lab) • Please be next door ready for lab in about 5 minutes.