Classical Greek Astronomy Mr. Catt Astronomy

1. Classical Greek Astronomy Mr. Catt Astronomy

2. Quick History • The first traces of settlement date to around 3,000 B.C, • Several major civilizations rose and fell over the centuries • Minoans: 2700-1450 B.C., mainly on Crete • Mycenaeans: 1550-1050 B.C, mainland Greece • Classical Greece: 776-323 B.C. • Macedonians: 323-88 B.C. • Romans/Byzantines: 88 B.C.-1453 A.D.

3. Problems • The biggest problem of studying Greek astronomy is the lack of the original works • This is due to both the destruction of libraries, such as at Alexandria, and the early Christian Church's attitudes toward Greco-Roman culture • Most writings have been completely lost and are known only through writings of later authors • Since the original works no longer exist, only the general ideas, not the specific details are known

4. The Beginning • The earliest astronomical references in Greece were found in the works of Hesiod and Homer, from nearly 3,000 years ago. • These pieces give practical advice for navigation and farming • “When Orion and Sirius are come to the middle of the sky and the rosy fingered dawn confronts Arcturus, cut off all your grapes and bring them home with you”-Hesiod, 700 B.C.

5. The First Scientists • The Greeks were notable in that they were the first people that attempted to explain the world in terms of natural phenomenon, leaving out gods and myths • Obviously, many of the following theories are wrong and may seem funny today, but they were closer to explaining the world than the stories of gods used by all previous cultures • The key innovation is the Greeks' attempt to explain the world rationally

6. The Astronomers of Miletus

7. Thales • Lived 624-547 B.C. • One of the legendary Seven Sages, the first known Greek to explain the world naturally • Originally a merchant who gathered knowledge on his business travels, especially to Egypt • Learned to calculate distances/heights by using geometry, which would later factor into astronomy • Was said to have predicted a solar eclipse

8. Anaxamander • Lived 611-547 B.C. • Said to have been a student of Thales • Often called “The Father of Cosmology” • Gave the first physical explanations of the celestial realm • The earth was a cylinder floating freely in space • The sun, moon, and stars were fire filed wheels with holes that allowed their light to escape

9. Anaximenes • Lived 585-526 B.C. • Also gave mechanical explanations of the universe • Believed that the stars were like nails, fixed to the interior of a crystalline vault surrounding the earth. • This idea would eventually grow into the celestial sphere, which would dominate astronomical thought for almost 2000 years

10. Pythagoras

11. Pythagoras • Lived 582-500 B.C. • Best known for his theorem on right triangles and his 5 perfect solids • Left no writings of his own, so it's hard to determine what ideas were his and what ideas were those of his followers,

12. Astronomical Innovations • Believed that the Celestial bodies revolved in circles around a hidden “central fire,” which was rendered invisible by a “counter Earth” • Believed that the Earth and all other celestial bodies were spheres • These two ideas were quickly accepted • Also believed that all of the celestial motion makes a lot of noise, but that we can't recognize it because we have no silence for comparison

13. The Pythagoras Model

14. Eudoxus

15. Universe Model Updated • Lived 408-355 B.C. • Expanded greatly on the idea of the crystalline sphere by using many spheres to explain the motions of celestial objects • The stars' motion was explained by the rotation of a single sphere • The sun needed two spheres, one rotates Westward once a day to account for rising/setting and the other rotates Eastward to account for motion through the Zodiac

16. Universe Model Updated • Planets were even more complicated in this model, each needing 4 spheres • Spheres 3 and 4 rotate in the planet's synodic period in opposite directions and slightly tilted to account for the cyclical figure eight motion • Sphere 2 rotates Eastward to explain motion through the Zodiac. This, combined with spheres 3/4, accounts for retrograde motion • Sphere 1 rotates Westward once a day • What this model fails to account for is the changing brightness of planets

17. The Eudoxus Model

18. Aristotle

19. Proofs of Earlier Ideas • Lived 384-322 B.C. • Offered proofs of earlier ideas • Earth is a sphere: • All falling objects move to the center of the Earth, which is straight down. If the Earth were a cube, this wouldn't always be true. • The Earth casts a circular shadow on the moon during an eclipse • Some stars are visible in Greece that aren't in Egypt • Other celestial objects are spheres: • The shapes of the shadow on the moon in an eclipse can only be true with a spherical moon

20. Long Lasting Ideas • Aristotle had the best reasoned argument for a geocentric solar system • Taking the earlier idea of Empedocles that the universe is made of earth, air, fire, and water, Aristotle reasoned that since all of the heavy materials are in the Earth, it is heavy and therefore must be stationary • This reasoning helped cement the idea of an Earth-centered solar system, which would dominate for almost 2000 years

21. Argument for Spherical Earth

22. Spherical Earth

23. Flat Earth

24. Aristarchus

25. Astronomy and Geometry • Lived 310-230 B.C. • Used geometry to calculate the distance to the moon and the ratio of the distances between the earth and sun and earth and moon • Used sound geometry, but wrong data for the distances, which resulted in estimates that weren't even close • Once the size of the Earth was determined (later) new calculations were made that were remarkably accurate

26. Heliocentricism • Aristarchus was one of the few astronomers to support the idea of a sun centered solar system • Aristarchus calculated the sun to be 7 times bigger than the earth • To Aristarchus, it made no sense to place the much smaller Earth at the center • To account for the motion of the sky, Aristarchus reasoned that the motion was only apparent, caused by the Earth spinning on its axis once a day

27. Objections • The heliocentric model was largely rejected, the only other astronomer known to support it was Seleucus of Selucia • One objection was that, if the Earth moved, there should be a wind caused by its motion • A second was that anything dropped should land to the West of where it was dropped from as Earth rushed by during the fall • A third was that if the Earth orbited the sun, two stars should appear different distances relative to each other in different seasons (Parallax)

28. Objections (Parallax) • Aristotle argued that the absence of parallax for the stars in the sky implied that the Earth must be at the center of the solar system. • This is a valid scientific argument. • Parallax is the apparent shifting of nearby objects with respect to distant ones as the position of the observer changes.

29. Objection (Parallax)

30. Eratosthenes

31. Earth Measured • Lived 276-195 B.C. • Used geometry to calculate the circumference of the Earth • The drive to do this came from reports that the Sun cast no shadows at noon in Syene, located in Southern Egypt, on the first day of summer • At the same time on the same day in Alexandria, located in Northern Egypt, the Sun cast shadows

32. The Method • Knowing the sun was directly 90 degrees overhead in Syene, Eratosthenes measured the sun's height in Alexandria, which was 82.8 degrees, meaning that the latitudes were 7.2 degrees, 1/50th of a circle in difference • He then had the distance between the two cities measured, which was done by men specially trained in pacing out the distance. • The end result came within a few percent of the Earth's actual circumference

33. Measuring the Earth • Therefore the Earth’s circumference was about 360/7  50 times the distance between the two cities. • Knowing this distance he was able to find the Earth’s diameter. • His calculation was very close to the correct value. Linear distance between Syene and Alexandria: ~ 574 miles Earth Radius ~ 4,597 miles (~ 14 % too large) – better than any previous radius estimate. (Actual radius is 3,963 miles)

34. Hipparchus

35. Antiquity's Greatest Astronomer • Lived 190-120 B.C. • Is responsible for recording the first variable star, which drove him to make a catalog of the sky, noting stars' position and brightness. • We can thank Hipparchus for magnitudes • When charting positions, he noticed that all stars were about 2 degrees off from older reference materials he was using • With this observation, Hipparchus discovered the Precession of the Equinoxes

36. Precession Pictured

37. Claudius Ptolemy

38. Last Great Classical Astronomer • 90-168 A.D. • Worked in Alexandria, Egypt • His greatest accomplishment was the perfection of the geocentric system • The earlier model of Eudoxus failed to account for the changing brightness of the planets, the fact that different planets retrograde differently, and was very complex. • Ptolemy's system was far simpler and worked even better for explaining planets' motion

39. Ptolemy's Universe • Geocentric with the Earth at the center, planets and sun orbit. • Each of the planets, in addition to orbiting the Earth, moves in circles, called epicycles, on their own orbits, which explains both retrograde motion and planets' changing brightness • Since the system wasn't totally accurate for predicting planetary motion, Ptolemy modified the model, moving the Earth slightly out of center • This system goes unchallenged for 1500 years.

40. Ptolemy's Model • An epicycle is the circular orbit of a planet, the center of which revolves around the Earth in another circle.

41. In Motion

42. The Marriage of Aristotle and Christianity • In the 13th century St. Thomas Aquinas blended the natural philosophy of Aristotle and Ptolemy’s work with Christian beliefs. • A central, unmoving Earth fit perfectly with Christian thinking and a literal interpretation of the Bible. • People during the Middle Ages placed a great reliance on authority, especially authorities of the past.