History of Astronomy

1. History of Astronomy The knowledge of the Ancients

2. Passage of astronomical knowledge Ancient Babylon Ancient Egypt Ancient Greece Ancient Roman Empire

3. Mesopotamia/Babylon Modern political boundaries Ancient Babylon

4. Mesopotamia/Babylon

5. Ancient Egypt Nile River Valley Great Pyramid of Kufu

6. Ancient Greece

7. What did ancient civilizations achieve in astronomy? • Daily timekeeping • Tracking the seasons and calendar • Monitoring lunar cycles • Monitoring planets and stars • Predicting eclipses • And more…

8. Ancient people of central Africa (6500 B.C.) could predict seasons from the orientation of the crescent moon.

9. Egyptian obelisk: Shadows tell time of day.

10. England: Stonehenge (completed around 1550 B.C.)

11. Scotland: 4,000-year-old stone circle; Moon rises as shown here every 18.6 years.

12. Mesopotamian Astronomy • MESOPOTAMIANS built observatories starting ~6000 years ago: • the ziggurats had seven levels, one for each wandering object in the sky: • Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn • Thus 7 days to the week • They tracked stars --- groups rising before sun at different times of year implied seasonal beginnings for planting and harvesting (zodiac). • Divided circles in 360 degrees, each degree into 60 minutes and each minute into 60 seconds -- we still use! • Left written records in cuneform so we understand them better

13. Other Ancient Accomplishments • Mesopotamians could predict planetary positions -- synodic periods, e.g., Mars returns to same location roughly every 780 days: 22 synodic periods = 47 years, so • records of old planetary positions could give good locations. • Knew about the SAROS cycle 2700 years ago: lunar eclipses definitely occurred every 18.6 years. • Chinese, Indians and Mayans also knew these patterns • Egyptians used astronomical events to forecast Nile floods and harvest times.

14. Mesopotamian Astronomy and Influence • By 2000 BC Ur and other Sumerian and Babylonia cities had large temples, or ziggurats, usually aligned N-S, like most Egyptian pyramids • Egyptian and Mesopotamian knowledge spread to Europe, Asia and Africa

15. Cuneiform-first system of writing

16. Cuneiform tablet with envelope

17. Greek Astronomy

18. Greek Astronomy • About 624 BC to 547 BC • One of the earliest Greek philosophers. • His areas of interest included geometry and astronomy. • There are no books or writings that have survived from Thales. Thales

19. Greek Astronomy • About 569 BC to 475 BC • One of the greatest mathematicians of all time. • Founded secret society based on his mathematical discoveries and their religious implications. • Since the society was extremely secretive, very little is known about his life or personal works. Pythagoras

20. Greek Astronomy Plato • 427 BC to 347 BC • Well known for political and social philosophy but he also made contributions to astronomy. • He was most noted for his belief in the perfect and unchanging nature of the heavens. • Plato was the head of the School of Athens & was Aristotle’s teacher.

21. Greek Astronomy Aristotle • 384 BC to 322 BC • Founded his own school called the Lyceum in Athens • Made contributions to all areas of philosophy but math was his weakness • He did not believe that empirical evidence was necessary to prove ideas.

22. Greek Astronomy Aristotle • Aristotle is one the first to attempt to create a scientific model of the universe. • This model has now become known as the “Geocentric Model” which places the “imperfect” Earth at the center and all of the “perfect” celestial objects go around us in perfect circular motion

23. Greek Astronomy Euclid • About 325 BC to 265 BC • Great mathematician but so little is known about him that people question whether he was an actual person or a group of mathematicians.

24. Greek Astronomy • The book “Elements” is possible the most famous and long lasting math text books in history. Euclid

25. Greek Astronomy Aristarchus • About 310 BC to 230 BC • We have no writings that have survived but Aristarchus is referred to in the writings of other philosophers

26. Greek Astronomy Aristarchus • Most noted for proposing the idea of a heliocentric universe with the earth as one of the planets moving around the Sun. • Aristarchus is the first to attempt to measure the relative distance between the Earth-Moon and the Earth-Sun without the aid of trigonometry. • Actual angle = 89° 50” not 87°

27. Greek Astronomy Eratosthenes • 276 BC to 194 BC • Librarian at the Great Library of Alexandria in Egypt. • Developed a calendar with a leap year. • Measured the circumference of the Earth in 325 BC !! • Achieved an accuracy of about 90% of the actual number.

28. Eratosthenes measures the Earth (c. 240 B.C.) • Measurements: • Syene to Alexandria • distance ≈ 5,000 stadia • angle = 7°

29. Eratosthenes measures the Earth (c. 240 B.C.) Calculate circumference of Earth: 7/360  (circum. Earth) = 5,000 stadia  circum. Earth = 5,000  360/7 stadia ≈ 250,000 stadia Compare to modern value (≈ 40,100 km): Greek stadium ≈ 1/6 km  250,000 stadia ≈ 42,000 km

30. Greek Astronomy • 190 BC to 120 BC • Hipparchus is consider the first great astronomer and scientist • Hipparchus is one of the first of the ancient philosophers to realize that ideas must be proven with empirical evidence. • He realized that more data meant more certainty in the idea or model Hipparchus

31. Greek Astronomy Hipparchus • Created highly accurate star atlases in an attempt to measure the length of the year more accurately. • He measured the length of the year to within 6.5 minutes of the actual time. • He discovered precession by examining ancient star position data (mostly Mesopotamian) and comparing them to his own measurements.

32. Greek Astronomy Hipparchus • Based on measurements during an eclipse, he is able to place a range on the distance to the moon. • He estimated it to be between 59 and 67 Earth diameters. The actual number is 60. • He marks the first truly scientific astronomical mind and his influence is still felt today.

33. Greek Astronomy Ptolemy • About 85 AD to 165 AD • Ptolemy is the most influential astronomer in his day and his models of the universe will prevail for the next 1400 years • He is a great admirer of Hipparchus and his rigorous method of verifying ideas with empirical data. • Ptolemy is also accused of stealing ideas without crediting his source.

34. Greek Astronomy • Ptolemy is a great proponent of the geocentric model. • He saw that there were problems with Aristotle’s simplistic idea of a geocentric model. • Ptolemy employed an old idea of epicycles to explain help explain the discrepancies in the evidence for geocentrism. • Although he was able to make more accurate predictions than previous astronomers, his model still failed to be completely accurate. Ptolemy

35. Why does modern science trace its roots to the Greeks? • Greeks were the first people known to make models of nature. • They tried to explain patterns in nature without resorting to myth or the supernatural. Greek geocentric model (c. 400 B.C.)

36. Two Different Early Models… • GEOCENTRIC • Earth is the center of everything • Earth doesn’t spin or move

37. Two Different Early Models… • HELIOCENTRIC • The Sun is the center of the solar system • Earth spins (rotates) to create day/night • Earth orbits (revolves)to create the year

38. Geocentric Models made “sense” • GEOCENTRIC: Earth doesn’t move • If we did, we’d feel it! • If we did, we’d lose the moon! • If we did, the stars around us would shift! • THEREFORE: • Sky (& Stars!) rotated around us • Sun & Moon & Planets actually move among key constellations of the Zodiac by design/choice

39. Two Different Early Models… • HELIOCENTRIC: Earth moves about the Sun • So do all of the planets • The Moon goes around us, too • Earth spins to create night and day • THEREFORE: • Sky (& Stars!) just SEEM to rotate around us • Sun & Moon & Planets moved among key constellations of the Zodiac because of OUR motion

40. Parallax

41. Parallax results from shift in viewing position • IfCLOSE to Earth, a star would be seen in different locations (at different angles)

42. Parallax results from shift in viewing position • If FAR from Earth, a star would NOT be seen in different locations (at different angles)

43. KEY IDEA: Retrograde motion of the planets • We make the observation that planets – and only the planets – “dance” in front of the stars. How is this observation explained in each model?

44. The Motion of the Planets in the sky over time

45. The Motion of Mars In 2009-2010

46. Retrograde Motion Explanations • Ancient (and geocentric) • The planets move on their own around us • God(s) control their motions • Heaven’s realm – doesn’t concern us! • It just is…

47. Ptolemaic Retrograde Motion

48. So how does the Ptolemaic model explain retrograde motion?Planets really do go backward in this model.

49. Retrograde Motion Explanations • Modern (and heliocentric) • Earth and other planets orbit the sun at different rates • Earth “laps” slower-moving outer planets – and they appear to loop