ETSU Honors Program 2005-06

1. ETSU Honors Program 2005-06 Great Ideas in Science Topic 2: Cosmology Dr Martin Hendry HENDRY@etsu.edu University of Glasgow, UK Room 377 Brown Hall

2. Cosmology – the study of the Universe as a whole: • Origin • Evolution • Eventual Fate

3. Cosmological theories depend on the available data

11. Retrograde motion of Mars

12. Ptolemy proposed a model which could explain planetary motions – including retrograde loops Ptolemy: 90 – 168 AD

13. Early Greek Astronomy The Greeks inherited ideas from Babylonia and Egypt, but approached astronomy in a scientific way Thales (624 – 547 BC) taught that the Universe was rational

14. Early Greek Astronomy The Greeks inherited ideas from Babylonia and Egypt, but approached astronomy in a scientific way Plato (428 – 347 BC): reality a distorted shadow of a Perfect Form. Circle = most perfect form in nature All celestial motions are combinations of circular motions u

15. Early Greek Astronomy The Greeks inherited ideas from Babylonia and Egypt, but approached astronomy in a scientific way Pythagoras: “The Earth is a sphere, and the Sun, Moon and planets are divine, perfect spheres moving in perfect circles”

16. Early Greek Astronomy The Greeks inherited ideas from Babylonia and Egypt, but approached astronomy in a scientific way Eudoxus (409 – 356 BC) constructed a system of 27 nested spheres, rotating at different rates

17. Early Greek Astronomy The Greeks inherited ideas from Babylonia and Egypt, but approached astronomy in a scientific way Aristotle (384 – 322 BC): Universe divided into two parts: Corrupt, changeable Earth Perfect, immutable heavens

18. The Observations of Galileo Galileo Galilei: (1564 – 1642)

19. v • Aristotle’s Theory: • Objects move only as long as we apply a force to them • Falling bodies fall at a constant rate • Heavy bodies fall faster than light ones • Galileo’s Experiment: • Objects keep moving after we stop applying a force (if no friction) • Falling bodies accelerate as they fall • Heavy bodies fall at the same rate as light ones

20. v • Aristotle’s Theory: • Objects move only as long as we apply a force to them • Falling bodies fall at a constant rate • Heavy bodies fall faster than light ones • Galileo’s Experiment: • Objects keep moving after we stop applying a force (if no friction) • Falling bodies accelerate as they fall • Heavy bodies fall at the same rate as light ones

21. a r v • Aristotle’s Theory: • Objects move only as long as we apply a force to them • Falling bodies fall at a constant rate • Heavy bodies fall faster than light ones • Galileo’s Experiment: • Objects keep moving after we stop applying a force (if no friction) • Falling bodies accelerate as they fall • Heavy bodies fall at the same rate as light ones

22. Early Greek Astronomy The Greeks inherited ideas from Babylonia and Egypt, but approached astronomy in a scientific way Pythagoras (570 – 500 BC): nature governed by mathematical relations Motion of celestial bodies follows the same rules as musical notes – the Harmony of the Spheres

23. Galileo’s empirical approach codified in the Scientific Method: • Characterisation • Hypothesis (hypothetical explanation) • Prediction (logical deduction from hypothesis) • Experiment (test of all of the above) • Evaluation and iteration (peer review / reproducibility)

24. Karl Popper (1902-1994) Galileo’s empirical approach codified in the Scientific Method: • Characterisation • Hypothesis (hypothetical explanation) • Prediction (logical deduction from hypothesis) • Experiment (test of all of the above) • Evaluation and iteration (peer review / reproducibility)

25. Karl Popper (1902-1994) Galileo’s empirical approach codified in the Scientific Method: • Characterisation • Hypothesis (hypothetical explanation) • Prediction (logical deduction from hypothesis) • Experiment (test of all of the above) • Evaluation and iteration (peer review / reproducibility) Theories should be Falsifiable

26. Ockham’s Razor “Frustra fit per plura, quod fieri potest per pauciora.” “It is vain to do with more what can be done with less.” William of Ockham (1288 – 1348 AD)

27. Ockham’s Razor “Frustra fit per plura, quod fieri potest per pauciora.” “It is vain to do with more what can be done with less.” Everything else being equal, we favour models which are simple. William of Ockham (1288 – 1348 AD)

28. Ptolemy proposed a model which could explain planetary motions – including retrograde loops Ptolemy: 90 – 168 AD

29. The Ptolemaic Universe

31. How does Ptolemy’s world model measure up? • Characterisation Retrograde loops of Mars • Hypothesis Epicycles on Mars’ orbits (hypothetical explanation) • Prediction Future position of Mars (logical deduction from hypothesis) • Experiment Observe where Mars actually is (test of all of the above) • Evaluation and iteration Error – add more epicycles?... (peer review / reproducibility)

33. Ptolemy’s Universe Mercury and Venus are kept ‘close’ to the Sun by introducing a ‘fudge factor’

34. Errors in Ptolemy’s model grew with time – required updates (e.g. extra epicycles) but only temporary ‘fixes’. By 1500s, Ptolemy’s model a poor descriptor of planetary positions, but remained accepted model among astronomers

36. The Copernican Revolution Nicolaus Copernicus (1473 – 1543) “In the true centre of everything resides the Sun” De Revolutionibus Orbis (1543)

37. The Copernican Revolution Simpler, more elegant explanation of retrograde motion Earth ‘overtakes’ e.g. Mars

40. The Observations of Galileo Galileo Galilei: (1564 – 1642)

41. The Observations of Galileo The Moon is an imperfect world with mountains and valleys, just like the Earth

42. The Observations of Galileo Moons of Jupiter: supported idea of Earth moving through space, contradicted Aristotelian view of all motions around Earth

43. The Observations of Galileo Earlier observed phases of Venus

44. The Observations of Galileo Earlier observed phases of Venus Geocentric model Sun

45. The Observations of Galileo Earlier observed phases of Venus Geocentric model Heliocentric model Sun Sun

46. The Observations of Galileo Phases of Venus impossible to explain in geocentric model Clear evidence that the Earth went round the Sun, and not the other way round

47. Early Greek Astronomy Aristarchus (310 – 230 BC) did argue for a sun-centred model: from eclipse geometry, Sun much larger than the Earth, so shouldn’t the Earth orbit the Sun?... Idea not widely accepted – no stellar parallax shift

48. Parallax Shift

49. Parallax Shift Nearby stars do show an annual parallax shift, but it is tiny! First detected only in the mid 19th Century

50. Parallax Shift Even the nearest star shows a parallax shift of only 1/2000th the width of the full Moon