History of Astronomy

1. History of Astronomy • How did we get to where we are today? • We stand on the shoulders of giants. • Greek, Egyptian, Asian, European, Islamic, Indian scientists all observed the heavens and tried to explain what they saw • What did the ancients know? • And how did they know it? • 4 distinct periods of astronomical advances

2. Lecture Outline • 4 periods of astronomy • Prehistoric: motions of sun, moon, and stars used to benefit mankind. • Classical: astronomers use simple geometry to try to explain what they see. • Renaissance: the boom of observational astronomy. • Modern: search for underlying physical laws that describe the heavens.

3. Prehistoric Astronomy • Before 500 B.C. • Cyclic motion of sun, moon, and stars • Time-keeping • Direction determination • Fortelling the future? • Prehistoric astronomers identified recurring patterns in the night sky

4. Great Pyramid of Giza To Orion To Thuban Astronomical alignments built in to the pyramid

5. Prehistoric Astronomy • Celestial Sphere • Easy way to visualize the observable universe • Stars embedded in spherical shell that rotates around the Earth • Describes movement of stars, but totally unrealistic • No sense of distance from earth • North Star (Polaris) is the top of the C.S. • All stars appear to rotate around Polaris over the course of a night.

6. Prehistoric Astronomy • Constellations • Star-patterns on the C.S. • Used for navigation • North star • Southern cross • keeping track of seasons • winter, summer, planting season, harvesting season

7. Prehistoric Astronomy • Motion of Sun & Stars • Daily/Diurnal: like Sun, stars rise in east, set in west • Ancients believed this due to rotation of C.S. • from Earth rotating underneath C.S. • To them, the Earth does not appear to move, so easier to believe Earth is stationary • Annual: new patterns emerge and old ones dissapear • Earth’s orbit around sun changes visible constellations • Earth itself blocks ½ of C.S. from view

8. Prehistoric Astronomy • Motion of the Planets • Ancients observed bright “stars” that seemed to move through the sky faster than others • These are the planets of the solar system • Observed to move through same narrow band of sky  the “Zodiac” (“little animal”) • Because planets lie in roughly same orbital plane (but ancients did not know this) 17° swath

9. Prehistoric Astronomy • Sometimes, retrograde motion of these Zodiac “stars” was observed! • What is going on?

10. Prehistoric Astronomy • Observations of the planets led to the downfall of many early ideas about the nature of the universe and sky • Thought to be calm and unchanging • Rigid C.S. • …but planets move differently from everything else, so C.S. cannot be static and unchanging • Also, observations of comets (which were first thought to be phenomena in the Earth’s atmosphere)

11. Prehistoric Astronomy • The Moon & Eclipses • Solar eclipse: sky becomes dark as night for a short while • Lunar eclipse: moon turns red and/or disappears for a while • Occur infrequently…need near perfect alignment of sun, moon, and earth • Prehistoric astronomers did not know • seen as omens (good or bad, depending on the circumstances)

12. Prehistoric Astronomy

13. Classical Astronomy • 500 B.C. - 1400 A.D. • The “Big Names” of classical astronomy • Pythagoras: Greek philosopher, geometry whiz (because, well, he invented it as we use it today) • Aristotle: Greek philosopher, student of Plato • Eratosthenes: geographer & astronomer • Pioneers in determining the size and shape of the earth

14. Classical Astronomy There is geometry in the humming of the strings... there is music in the spacing of the spheres. • Pythagoras • His ideas would influence generations of scientists and philosophers for centuries • The solid body we call the “sphere” is perfect. • What is meant by this? • So naturally, the Gods would model the earth as a sphere during the Creation… • True, to some extent… • Equatorial bulge…Earth is “pear-shaped” • Small-scale deviations from sphericity…mountains, canyons, deep-sea trenches, etc.

15. Classical Astronomy • Aristotle • More quantifiable evidence than Pythagoras • Observations of curved shadow on the moon during lunar eclipses • It’s earth’s shadow, so earth must be curved like a sphere! • Observations of night sky • Travelers moving south begin to see “new” (unfamiliar) stars. • Only possible if a horizon exists, therefore, earth must be curved! (There is no horizon on a flat earth)

16. Classical Astronomy • Eratosthenes • Brilliant experiment to determine the size of the earth • A large obelisk in Alexandria casts a ~7°shadow when the sun is at it’s highest point in the sky… • 5000 stadia (~500 miles) away in Egyptian city of Syene, the sun casts no shadow at it’s highest point in the sky… • There must be ~7°between the lines joining Alexandria and Syene to the center of the (spherical) earth

18. Eratosthenes (cont.) • Therefore, the distance from Alexandria to Syene must be ~ 1/50th of the distance around the earth (circumference) • 360°/7° ≈ 50 • Cearth = 50 x 5000 stadia = 250,000 stadia ≈ 25,000 miles • Accepted value today: 24,044.5 miles • amazingly accurate for that time…although the precise value of the stadia is disputed • some claim E’s estimate off by ~ 16%

19. Classical Astronomy • Motion of the planets • Geocentric universe = earth-centered • Sun, moon, and each planet occupied its own transparent, revolving sphere, nested between the stationary earth and the celestial sphere • Ptolemy • Attempt to describe retrograde planet motion by epicycles: frisbee on a bike wheel • circles moving in circles moving in circles… • Too complicated, required many nested epicycles for even smallest amount of accuracy in predictions…

20. How did classical astronomers explain this strange backwards motion?

21. Ptolemy Ptolemy attempts to explain retrograde motion by hypothesizing that the planets move in circles within circles… …his theories remained in use for another 1500 years, until it became too complex to be physically meaningful

23. Classical Astronomy • Aristarchus • Measured size of moon • Estimated distance to Sun (but wrong by factor of ~ 20) • Proposed Sun was center of the heavens • Heliocentric model first appears • Problem: should see stars shift positions (stellar parallax)

24. Distances to even nearest stars are so huge, stellar parallax is a VERY small quantity, and so could not be measured until the middle of the 19th century! Evidence for heliocentric model existed, but was just outside our grasp to observe…  “Absence of evidence is not evidence of absence.”

25. The Renaissance • 1400 A.D. – 1650 A.D. • Revolutions in astronomy • The Big Names: • Nicolas Copernicus • Tycho Brahe • Johannes Kepler • Galileo Galilei • Isaac Newton

26. The Renaissance • Copernicus • Polish doctor & lawyer • Revisited ideas of Aristarchus’ 2000 year old “heliocentric” model • Now able to explain retrograde motion: planets on different, sun-centered orbits pass each other… • But still some discrepancies… • Model had perfectly circular orbits • Still no parallax observed • Published his ideas shortly before his death • First person to describe the details of the heliocentric model

27. At last, a simple & accurate model for predicting retrograde motion

28. The Renaissance • Tycho Brahe • Danish Nobility • utilized his wealth to study the sky • Built most accurate pointing & measuring instruments of his time • Still could not observe parallax  one of the last to hold on to the geocentric model • Actually, built at least 4 of each, operated simultaneously, directed by Brahe himself • One of the first to establish importance of repeatability in scientific measurement • More data = more accurate results

29. Tycho Brahe (cont.) • Large, fine-toothed “gear” system • Brahe and/or his assistant would sight stars along an “aiming” direction • Assistant would record the angular positions based on the scale of the gear-teeth • Incredibly accurate star & planet positions • Observations in favor of heliocentric model • Revealed true shape of planetary orbits, but not to Brahe himself…

30. The Renaissance • Kepler • One of Brahe’s assistants • inherited his work when he died • Superior data (by volume and precision) showed that Mars’ orbit is not circular, but elliptical • Kepler noticed the sun was not at the center of the calculated orbit, but off to the side, at a “focus” of the ellipse • Proceeding with this model, Kepler calculated the orbits of the other planets, and found excellent agreement with the actual observations!!

31. Ellipses

32. Kepler (cont.) • Kepler’s 3 Laws • Planets move in elliptical orbits, with the sun at one focus of the ellipse • Planets do not move with constant speed. They move faster when nearer the sun, and slower when they are farther away • The amount of time it takes a planet to orbit the sun exactly once is related to the size of the orbit (semimajor axis): P2 years = a3 AU

33. Kepler (cont.) 1st Law 2nd Law 3rd Law equal areas in equal time elliptical orbits, Sun at one focus period-major axis relation

34. Kepler’s Laws are empirical, which means they are based on actual data & measurements, not just theory…

35. The Renaissance • Galileo • Italian scientist, fascinated by motion • He used telescope to: • Observe and draw surface features of the moon • Concluded moon was ball of rock • Observe changing sunspots (sun NOT a constant orb…) • Observe 4 large moons of Jupiter, proving that some bodies in the solar system did NOT orbit the earth • Observe evidence of Saturn’s rings • Observe phases of Venus as proof of the heliocentric model • Establish large size of Milky Way galaxy

37. The phases of Venus, As observed from Earth

38. No matter what configuration, epicycles cannot explain the phases of Venus…

39. Galileo (cont.) • Galileo’s experiments with motion • Falling bodies, pendulum, etc… • Coupled with his observations of 4 large moons of Jupiter • Called “Galilean Moons” in his honor • Raised questions about nature of planetary motion and what held planets in orbit • Condemned for his findings… • Enter British Scientist Sir Isaac Newton…

40. The Renaissance • Newton • Born same year Galileo died • Attempts to understand motion of the moon • Leads him to deduce the law of gravity (as we still use it today!) • Requires him to invent new mathematics • Leads him to deduce the general laws of motion More on Newton and his Laws later…

41. these Renaissance astronomers are pictured on currency & postage alongside kings & queens

42. Modern Astronomy • 1650 A.D. to the present • New discoveries! • More planets & more moons • Binary stars, galaxies, exotic phenomena • New technologies! • Better optics • bigger, better telescopes • Photographic film • CCDs

43. Modern Astronomy • Nature of matter & heat • Up to 100 years ago, no idea atoms existed • Theorized to exist by greeks during 5th century B.C. • “atom” = “indivisible” or “uncuttable” • Rutherford & the gold foil experiment • Probe structure of the atom • Link between heat/temperature and molecular motion discovered • Lord Kelvin, working on improving steam engines

44. Modern Astronomy • The Kelvin Temperature Scale • Temperature directly proportional to speed of molecular motion… • H2O freezing point ≈ 273 K • Room temperature ≈ 300 K • H2O boiling point ≈ 373 K • Surface of sun ≈ 6000 K • Absolute Zero = 0 K (NO molecular motion)