1. Slide 1 Western Scientific Astronomy begins with the ancient Greek civilization about 600 BCE

2. Slide 2 Early Astronomers Unfortunately, there are no written documents about the significance of stone and bronze age monuments. First preserved written documents about ancient astronomy are from ancient Greek philosophy. Greeks tried to understand the motions of the sky and describe them in terms of mathematical models.

3. Slide 3 Ancient Greek Astronomers Models were generally wrong because they were based on “first principles”, believed to be “obvious” and not to be questioned: 1.Geocentric Universe: Earth at the Center of the Universe. 2.“Perfect Heavens”: Motions of all celestial bodies described by motions involving objects of “perfect” shape, i.e., spheres or circles (Plato).

4. Slide 4 Ancient Greek Astronomers Eudoxus (409 – 356 B.C.): Model of 27 nested spheres Aristotle (384 – 322 B.C.), major authority of philosophy until the late middle ages: Universe can be divided in 2 parts: 1. Imperfect, changeable Earth, He expanded Eudoxus’ Model to use 55 spheres. 2. Perfect Heavens (described by spheres)

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7. Slide 7 Using the distance between the Earth and the Moon as a baseline

8. Slide 8 Eratosthenes of Cyrene Born: 276 BC in Cyrene, North Africa (now Shahhat, Libya) Died: 194 BC in Alexandria, Egypt Measured circumference of the Earth He got R = 6739 km Actual radius 6378 km

9. Slide 9 Eratosthenes (~ 200 B.C.): Calculation of the Earth’s radius Angular distance between Syene and Alexandria: ~ 7 0 Linear distance between Syene and Alexandria: ~ 5,000 stadia  Earth Radius ~ 40,000 stadia (probably ~ 14 % too large) – better than any previous radius estimate.

10. Slide 10 The problem of retrograde motion

11. Slide 11 Retrograde motion Backward travel of planets through the moving night sky.

12. Slide 12 Later refinements (2nd century B.C.) Hipparchus: Placing the Earth away from the centers of the “perfect spheres” Ptolemy: Further refinements, including epicycles

13. Slide 13 Claudius Ptolemy 85-165 AD Mathematical Syntaxis (Almagest)

14. Slide 14 Epicycles The Ptolemaic system was considered the “standard model” of the Universe until the Copernican Revolution. Introduced to explain retrograde (westward) motion of planets

15. Slide 15 Epicycles

16. Slide 16 The Copernican Revolution Nicolai Copernicus (1473 – 1543): Heliocentric Universe (Sun in the Center)

17. Slide 17 1. There is no one centre in the universe. 2. The Earth's centre is not the centre of the universe. 3. The centre of the universe is near the sun. 4. The distance from the Earth to the sun is imperceptible compared with the distance to the stars. 5. The rotation of the Earth accounts for the apparent daily rotation of the stars. 6. The apparent annual cycle of movements of the sun is caused by the Earth revolving round it. 7. The apparent retrograde motion of the planets is caused by the motion of the Earth from which one observes. Church cleric, but rejected a 2000-yr old paradigm Seven axioms written in a pamphlet “Little Commentary” (1514) Born: 19 Feb 1473 Died: 24 May 1543

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19. Slide 19 Copernicus’ new (and correct) explanation for retrograde motion of the planets This made Ptolemy’s epicycles unnecessary. Retrograde (westward) motion of a planet occurs when the Earth passes the planet.

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22. Slide 22 Tycho Brahe (1546-1601) Contributions to Astronomy: Tycho was the first to suggest a non-circular orbit for a celestial body (a comet). Used calibrated and bigger instruments, new techniques to measure angles (similar to a sextant). Built an observatory (remember - no telescopes yet) and made accurate and continuous measurements for 20 years. His measurements helped to prove that planets orbited the sun. Accurate map of the stars with 777 stars. Observed “a new star” – supernova 1572 (Tycho’s Supernova). Measured length of the year to within 1 second. Was still unable to choose between the geocentric and heliocentric model. He had his own model with the Earth at the center, orbited by the sun and the moon, with planets orbiting the sun. Never worked out the mathematical details, and his model was never accepted. Using Tycho's data, a German astronomer (Kepler) was able to refute the geocentric model

23. Slide 23 Use of Parallax

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26. Slide 26 Johannes Kepler (1571 – 1630)

27. Slide 27 Kepler hypothesized that a physical force moved the planets, and that the force diminished with distance. Planets closer to the sun feel a stronger force and move faster. Elliptical orbits – key to the problem of the planetary motion

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29. Slide 29 Kepler’s Laws of Planetary Motion 1.The orbits of the planets are ellipses with the sun at one focus. Eccentricity e = c/a c

30. Slide 30 e = R a - R p R a + R p Vocab: perihelion, aphelion, semimajor axis, eccentricity Elliptical orbits

31. Slide 31 Eccentricities of Ellipses e = 0.02 e = 0.1e = 0.2 e = 0.4e = 0.6 1)2)3) 4) 5)

32. Slide 32 Eccentricities of Planetary Orbits Orbits of planets are virtually indistinguishable from circles: Earth: e = 0.0167 Most extreme example: Pluto: e = 0.248

33. Slide 33 LAW 2: A line joining a planet/comet and the Sun sweeps out equal areas in equal intervals of time The closer to the sun, the larger the orbital velocity

34. Slide 34 Planetary Orbits (2) A planet’s orbital period (P) squared is proportional to its average distance from the sun (a) cubed: P y 2 = a AU 3 A line from a planet to the sun sweeps over equal areas in equal intervals of time. (P y = period in years; a AU = distance in AU)

35. Slide 35 LAW 3: The squares of the periods of the planets are proportional to the cubes of their semimajor axes: For the Earth P 2 = 1 yr, a 2 = 1 AU Note units!!

36. Slide 36 Johannes Kepler (1571 – 1630) Used the precise observational tables of Tycho Brahe (1546 – 1601) to study planetary motion mathematically. 1.Circular motion and Planets move around the sun on elliptical paths, with non-uniform velocities. Found a consistent description by abandoning both 2.Uniform motion.

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38. Slide 38 Galileo Galilei (1564-1642) Was the first to report using the telescope to view the heavens. Telescope invented in 1604 by Hans Lippershay. Galileo used the telescope in 1609. Built his own. Two lenses in a metal tube about 4 feet long, diameter = 4 cm (1.6 inches). Magnification 3X to 33X. His observations between 1609 and 1612 changed our ideas about the universe. What did he see? New stars (Milky Way made up of stars) Mountains and valleys on the moon Four moons orbiting Jupiter (now called Galilean moons) Phases of Venus Sunspots (rotating around the sun about once a month) The rings of Saturn (sketches. was puzzling; not identified as rings until about 50 years later.) Planets are disks, not pinpoints of light like the stars

39. Slide 39 32X power

40. Slide 40 Major Discoveries of Galileo Moons of Jupiter (4 Galilean moons) Rings of Saturn (What he really saw)

41. Slide 41 Major Discoveries of Galileo (2) Surface structures on the moon; first estimates of the height of mountains on the moon

42. Slide 42 Major Discoveries of Galileo (3) Sun spots (proving that the sun is not perfect!)

43. Slide 43 Major Discoveries of Galileo (4) Phases of Venus (including “full Venus”), proving that Venus orbits the sun, not the Earth!

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45. Slide 45 The significance of what he saw: Cast doubt on the view of the "perfection of the heavens" (of Aristotle and Plato) Showed deficiencies of the geocentric (Ptolemaic) model Rotation of sunspots around sun suggested that if the sun could rotate, perhaps the Earth could too. Phases of Venus would be a natural consequence of the heliocentric model. Jupiter's moons showed that centers of motion other than Earth existed. Galileo published in Italian, not Latin. Widely read. Language of the people, rather than language of the scholars. Arguments against the geocentric model were so forceful that he came under fire from the Catholic Church and was forced to give a public denial of the heliocentric/Copernican system, and was placed under house arrest for the last 10 years of his life. Was not pardoned by the Church until 1992. Science in Italy was dealt a severe blow. The center of scientific investigation shifted to northern Europe. Many scholars refused to believe his ideas and a few even refused to look through the telescope. Many clung to old ideas.

46. Slide 46 Galileo Galilei (1594 – 1642) Invented the modern view of science: Transition from a faith-based “science” to an observation-based science. Rejected the “old” view (still alive now!) that the only path to true understanding is through religious faith Observations are correct even if they contradict the Scripture “The Bible tells us how to go to heaven, not how the heavens go.”

47. Slide 47 Isaac Newton English scientist Sir Isaac Newton (1642—1727) explained gravity as the force that holds planets in orbit around the Sun.

48. Slide 48 The Birth of Modern Astronomy Early Astronomy  Sir Isaac Newton Although others had theorized the existence of gravitational force, Newton was the first to formulate and test the law of universal gravitation. The universal law of gravitation, helped explain the motions of planets in the solar system.  Universal Gravitation Gravitational force decreases with distance. The greater the mass of an object, the greater is its gravitational force.

49. Slide 49 Gravity’s Influence on Orbits

50. Slide 50 Newton’s Laws of Motion 1 st Law unbalanced force –A body at rest, or in uniform motion, will remain so unless acted upon by an unbalanced force. 2 nd Law acceleration –The change in motion ( acceleration ) is proportional to the unbalanced force 3 rd Law action reaction –For every action there is an equal and opposite reaction

51. Slide 51 Gravity Gravity is the force that –holds us to the Earth –causes a rock to fall towards the ground –causes the Earth to go around the Sun –causes the Sun to be pulled towards the center of the Milky Way galaxy Gravity acts between any two objects even if they are far apart. action at a distance –“ action at a distance ”

52. Slide 52 Historical Overview