Chapter 3

1. Scales and Motion in the Universe Investigating Astronomy, Slater & Freedman Chapter 3

2. In this chapter you will discover… • What makes a theory scientific • The scientific revolution that changed the idea of an unmoving Earth and allowed the Earth to move • Copernicus’s argument that the planets orbit the Sun • Why the direction of motion of the planets on the celestial sphere sometimes appears to change • That Kepler’s determination of the shapes of planetary orbits depended on the careful observations of his mentor Tycho Brahe • How Isaac Newton formulated an equation to describe the force of gravity and how he thereby explained why the planets and moons remain in orbit

3. The Ancient Mystery of the Planets What was once so mysterious about the movement of planets in our sky? Why did the ancient Greeks reject the real explanation for planetary motion? Our goals for learning:

4. Planets Known in Ancient Times Mercury difficult to see; always close to Sun in sky Venus very bright when visible — morning or evening “star” Mars noticeably red Jupiter very bright Saturn moderately bright

5. Eratosthenes and Aristarchus (310-~230 BC) Using simple tools and basic geometry to measure: 1. the size of the Earth, Moon, and Sun 220? – 143? BC 2. the distances to the Moon and Sun 1st heliocentric theory Sun 18 to 20 x Moon distance Size of sun ~ 7x earth

6. What was once so mysterious about planetary motion in our sky? Planets usually move eastwardfrom night to night relative to the stars. You cannot see this motion on a single night; rather, planets rise in the east and set in the west. • But sometimes they go westward for a few weeks or months: retrograde motion

7. Early models of the universe attempted to explain the motion of the five visible planets against the background of “fixed” stars. The main problem was that the planets do not move uniformly against the background of stars, but at times appear to stop, move backward, then move forward again. This backward motion is referred to as retrograde motion. Time-lapse images of Mars during retrograde Jupiter retrograde motion

8. The retrograde motion of Mars as shown in a series of images taken on the same photographic plate.

9. Explaining Apparent Retrograde Motion Easy for us to explain: occurs when we “lap” another planet (or when Mercury or Venus lap us) But very difficult to explain if you think the solar system is geocentric and the Earth is unmoving In fact, ancients considered but rejected the correct explanation…

10. We see apparent retrograde motion when we pass by a planet in its orbit.

11. Why did the ancient Greeks reject the real explanation for planetary motion? • Their inability to observe stellar parallax was a major factor.

12. The Greeks knew that the lack of observable parallax could mean one of two things: Stars are so far away that stellar parallax is too small to notice with the naked eye Earth does not orbit Sun; it is at the bottom of the universe With rare exceptions such as Aristarchus, the Greeks rejected the correct explanation (1) because they did not think the stars could be that far away… Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems.

13. 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”) • Greeks also tended to Believe that planets were living beings influencing man’s life.. • Ptolemy’s book, Tetrabiblios, is the bible of astrology. Ptolemy

14. The Ancient Greek Model An Earth-centered, or geocentric, model of the universe

15. Ptolemy’s model used a geocentric (Earth-centered) model of the solar system in which the planets orbited the Earth indirectly by moving on epicycles which in turn orbited the earth.

16. The Ptolemaic system was an ingenious and complicated system of circular orbits centered on other circular orbits called epicycles. It remained the best model for over 1500 years (with many modifications). Celestial sphere Earth Centered

17. The The assumptions for this model were commonly accepted: 1. the earth did not move 2. the earth was the center of the system 3. the stars were located at a fixed distance on a transparent celestial sphere that rotated from E to W 4. the celestial realm was unchanging, and celestial motion was perfect, i.e. circular!

19. The Marriage of Aristotle and Christianity • In the 13th century St. Thomas Aquinas blended the natural philosophy of Aristotle, which included the Ptolemaic model, with Christian beliefs. • A central, unmoving Earth fit perfectly with prevalent Christian thinking, and various scriptures where found, whose literal interpretation, seemed to agree with this model. • 1 Chronicles 16:30: “He has fixed the earth firm, immovable.” • Psalm 96:10: “He has fixed the earth firm, immovable ...” • Psalm 104:5: “Thou didst fix the earth on its foundation so that it never can be shaken.” • Isaiah 45:18: “...who made the earth and fashioned it, and himself fixed it fast...”

20. Timeline of Ancient Astronomy

21. Ptolemy’s system worked well in general detail. It was used to create tables predicting the occurrence of astronomical events.... eclipses conjunctions etc. Over several hundred years, small errors in the tables accumulated to produce large error in the timing of events - as much as a month by 1200 AD

22. A major revision was done in 1250 by a group of scholars under King Alfonso of Spain. Ptolemaic system was modified to include deferents (off center circles). They produced the Alfonsine Tables By 1500 even these tables were in error by several hours and even days in some cases

23. epicycle Epicycle with deferent center of epicycle

24. Copernicus, a contemporary of Columbus, worked 40 years on a heliocentric—sun-centered—model for two reasons: Ptolemy’s predicted positions for celestial objects had become less accurate over time. (2) The Ptolemaic model was not aesthetically pleasing enough. He wanted to restore perfect” or circular motion and get rid of off-center circles!

25. Because both models (Ptolemaic & Copernican) were based on the assumption that the planets move at constant speed, Copernicus was forced to add small epicycles of his own to improve accuracy. Copernicus would not abandon the circle as the preferred planetary orbit because he thought circles are the best representation of the perfect motions of the heavens.

26. Advantages of Copernican System: Simpler! [not more accurate!] simple explanation of retrograde motion explained the phases of Venus explained why Mercury & Venus always close to the sun Using trigonometry was able to calculate the relative distances to all visible planets

27. Timeline of Renaissance Astronomy

28. But What about the Scriptural Evidence for the Geocentric Model? • As more and more evidence began to build which indicated the correctness of Copernicus’ model, faithful Christians had to ask some fundamental questions about their interpretation of scripture. • By the end of the 17th century, most Christians had come to accept the heliocentric model. • These Christians had to make adjustments to their interpretation of certain scriptures: the Earth being “fixed” must be interpreted differently.

29. The basic scriptural re-interpretation typically involved asking the question, “What is the scripture talking about in the verses interpreted previously as a fixed Earth”? The re-examination of scripture continues even today as we seek the message of scripture that God intended to deliver – which we have discovered is almost never scientific information.

30. Comparing The Two Models There were strong argument against Copernican idea of a moving earth: Inertia-if earth is moving, why don’t objects thrown upward fall behind as the earth rotates under the object? Parallax-if earth moves, one should see stellar parallaxes (stars seem to move as viewed from different locations)

31. Celestial Sphere

32. Stellar parallax is quite small- 0.75 arcseconds for largest shift detected — because the stars are so far away from us. • Stellar parallax,the apparent annual shifting of nearby stars with respect to background stars, was not observed until 1838.

33. The Copernican Model had good Predictive Power • A good model (or theory) will make verifiable predictions that might allow the the theory to be disproved. • Using theAstronomical Unit (AU)—the average distance between Earth and Sun— Copernicus predicted with amazing accuracy the Sun-to-planet distances for the 5 planets visible from Earth in the 1500s.

34. Planetary Distances in AU Copernicus Value Actual Value Mercury 0.38 0.39 Venus 0.72 0.72 Earth 1.00 1.00 Mars 1.52 1.52 Jupiter 5.22 5.20 Saturn 9.17 9.54

35. The Copernican model was more aesthetic since it could explain the motions of Mercury and Venus without resorting to special rules needed by the Ptolemaic model. • Copernicus offered a simpler explanation for retrograde motion that required no use of epicycles. 16

37. Copernicus, who died in 1543 just as his book De Revolutionibus was published, started such an upheaval in people’s thinking that the word “revolution” took on a second meaning that is so familiar to us today. • Tycho Brahe • Tycho was born 3 years after Copernicus died. 17

38. Tycho Brahe • Tycho built the largest and most accurate naked-eye instruments yet constructed. • He could measure angles to within 0.1º, close to the limit the human eye can observe. 18

39. He not only made careful measurements, but he recorded the accuracy of each measurement. 1563 close conjunction of Jupiter & Saturn. Alfonsine tables were off by a month, while Copernican tables were off by several days. Tycho wanted to correct the tables 19

40. Woodcut of Tycho Silver nosepiece is visible in the picture Tycho lost the tip of his nose in a duel at age 20 over a question in math.

41. 11/11/1572 Nova appeared in the sky and was closely observed by Brahe: he observed.... a. no apparent parallax, therefore the nova was not inside celestial sphere b. thus an obvious change in the unchanging celestial sphere Tycho writings about the Nova gained the attention/approval of Frederick, king of Denmark who built Tycho the world’s best observatory (Uraniborg) on island of Ven

42. Tycho’s supernova today Woodcut of Tycho’s Stella Noveau

43. Woodcut of the inside of Brahe’s observatory Blaeu ´s Atlas , 1663 Stjerneborg, 1584, partly underground

44. Aerial view of site of Uraniborg on the Island Ven

45. Sweden Denmark Poland

46. After Frederick’s death, Tycho fell out of favor and thus disassembled his observatory and moved it to Prague under HRE Rudolph II Castle Benatky His commission was to revise Alfonsine tables Hired several mathematicians to handle the drudgery of the computations, one of whom was Johannes Kepler

47. Tycho’s model

48. Tycho Brahe died 24th October 1601 of a urinary bladder infection that he may have tried to cure himself, with a medicine containing mercury Teyn Church in Prague where Tycho was buried Body exhumed in 1901 to determine cause of death 1996 Particle Induced X-ray Emission (PIXE) showed recent high levels of Mercury in Brahe’s hair samples implying mercury poisoning.

49. 1610 painting by unknown artist Born 1571 Died 1630 (58) Johannes Kepler

50. Kepler was a sickly child of a protestant family living in predominantly catholic area. Got scholarship to become Lutheran minister, but liked math better. Had influential teacher who was a Copernican. Became Math teacher at Graz (not very good, only had 1 student last year) 1595 wrote almanac with astronomical & astrological weather predictions. They were correct and got reputation as astrologer