Chapter 3 The Science of Astronomy. 1/28/09 Astro Lecture 6 1

Transcription

1 Chapter 3 The Science of Astronomy Astro Lecture 6 1

2 3.1 The Ancient Roots of Science Our goals for learning: In what ways do all humans employ scientific thinking? How did astronomical observations benefit ancient societies? What did ancient civilizations achieve in astronomy? Astro Lecture 6 2

3 In what ways do all humans employ scientific thinking? Scientific thinking is based on everyday ideas of observation and trial-and-error experiments. Astro Lecture 6 3

4 How did astronomical observations benefit ancient societies? In keeping track of time and seasons for practical purposes, including agriculture for religious and ceremonial purposes In aiding navigation Astro Lecture 6 4

5 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 Astro Lecture 6 5

6 Modern measures of time come directly from ancient observations of motion in the sky Length of day = time it takes Sun to make one full circle in sky Length of month = Moon s cylce of phases Length of year = Cycle of seasons 7 days in a week = named after 7objects that could be seen with the naked eye = Sun, Moon and 5 planets Astro Lecture 6 6

7 Ancient people of central Africa (6500 B.C.) could predict seasons from the orientation of the crescent moon. They noticed that the orientation of the horns of the crescent relative to the horizon is closely related to the local rainfall patterns. Astro Lecture 6 7

8 Days of the week were named for Sun, Moon, and 5 visible planets. Astro Lecture 6 8

9 Time of Day Use of sticks to cast shadows and tell time: Egyptian obelisk Can trace origins of modern clock to ancient Egypt about 4000 yrs ago. Divided daylight into 12 equal parts Astro Lecture 6 9

10 Marking Seasons Many ancient cultures built structures to help them mark the seasons England: Stonehenge (2750 BC, completed around 1550 B.C.) Astro Lecture 6 10

11 Mexico: model of the Templo Mayor - Sun rose directly through the notches between temples on the equinoxes (March and September) Astro Lecture 6 11

12 New Mexico: Anasazi kiva aligned north south Astro Lecture 6 12

13 SW United States: Amazi people carved a 19-turn spriral Sun Dagger on a vertical cliff in Chaco Canyon: Sun s rays form a dagger of sunlight that pierces the center of the carved spiral only once a year mark summer solstice (around June 21 when receive most direct sunlight) Astro Lecture 6 13

14 Macchu Pichu, Peru: structures aligned with solstices Astro Lecture 6 14

15 The Temple at Abu Simbel, Aswan, Egypt (13th century BC) The massive facade of the main temple is dominated by the four seated colossal statues of Ramesses. These familiar representations are of Ramesses II himself. Each statue, 67 feet high, is seated on a throne and wears the double crown of Upper and Lower Egypt. Astro Lecture 6 15

16 The temple at Abu Simbel, Aswan, Egypt The axis of the temple is arranged so that on two days of the year, in February and October (around equinoxes), the rising sun shoots its rays through the entrance and through the halls until it finally illuminates the sanctuary statues. Astro Lecture 6 16

17 Peru: lines and patterns, some aligned with stars Astro Lecture 6 17

18 South Pacific: Polynesians were very skilled in the art of celestial navigation. Astro Lecture 6 18

19 "On the Jisi day, the 7th day of the month, a big new star appeared in the company of the Ho star." "On the Xinwei day the new star dwindled." Bone or tortoiseshell inscription from the 14th century B.C. China: earliest known records of supernova explosions (1400 B.C.) Astro Lecture 6 19

20 Lunar calendar Some ancient civilizations used lunar phases for basis as a calendar (29 or 30 days) <29.5 days> A 12 month lunar calendar has days (11 days < solar calendar), e.g. Muslim calendar shifts 11 days every year Lunar calendars can be synchronize with solar calendars through interesting coincidence: 19 yrs solar = 235 months lunar Lunar phases repeat on same dates about every 19 years (Metonic cycle) Astro Lecture 6 20

21 Scotland: 4,000-year-old stone circle Moon rises (as shown here) every 18.6 years. Astro Lecture 6 21

22 France: Cave paintings from 18,000 B.C. may suggest knowledge of lunar phases (29 dots). Astro Lecture 6 22

23 What have we learned? In what ways do all humans employ scientific thinking? Scientific thinking involves the same type of trial-and-error thinking that we use in our everyday lives, but in a carefully organized way. How did astronomical observations benefit ancient societies? Keeping track of time and seasons; navigation Astro Lecture 6 23

24 What have we learned? What did ancient civilizations achieve in astronomy? To tell the time of day and year, to track cycles of the Moon, to observe planets and stars. (Many ancient structures aided in astronomical observations.) Astro Lecture 6 24

25 Astronomy of Babylonians What were the purposes of Astronomy in ancient Babylonia? What are the scientific breakthroughs of the Babylonians? How did the western astronomy benefit from these breakthroughs? Astro Lecture 6 25

26 Our mathematical and scientific heritage originated with the civilizations of the Middle East. Astro Lecture 6 26

27 Babylonian context Mesopotamia is the large area associated with the Tigris and Euphrates rivers, in what is now Iraq. Civilization had developed in this fertile valley by 4000 BCE, with irrigation for growing crops, domesticated animals, houses, temples, an army, and all the other features of civilization. Mesopotamia is generally considered by historians the cradle of western civilization, and it is from here we begin to trace the development of western astronomy. From ca to 500 BCE the city of Babylon was the capital of the Babylonian empire. Astro Lecture 6 27

28 The Ancient Near East Euphrates R. Tigris R. Astro Lecture 6 28

29 Babylonian context There was continual fighting for supremacy among various tribes in the region over the centuries (Sumerians, Akkadians, Amorites, Hittites, Assyrians, Chaldeans, Persians). In ca BCE the Amorites, a tribe from the Syrian desert to the West reached ascendancy. The sixth Amorite king was the famous Hammurabi (ca BCE). Hammurabi introduced the famous code of laws: an eye for an eye, a tooth for a tooth. He also introduced a common calendar for the empire, and this was based on astronomical measurements. His capital city was Babylon, on the Euphrates River about 55 miles south of present-day Baghdad. The large empire was administered from this central capital. Astro Lecture 6 29

30 Cuneiform writing The Babylonians also invented a system of writing called cuneiform: wedge-shaped marks made on clay tablets, which were then baked in the Sun. Our knowledge of Babylonian astronomy is derived from such cuneiform tablets, many of which give positions of celestial objects much like a modern ephemeris. Astro Lecture 6 30

31 Cuneiform tablets Astro Lecture 6 31

32 Hammurabi This column shows Hammurabi receiving his code of laws from the god Shamash (the god of justice, seated). Below the two figures is the cuneiform text of Hammurabi s code. Astro Lecture 6 32

33 Ishtar Gate The city of Babylon was famous throughout the ancient world. This is one of 8 gates leading into the city of Babylon, and the most magnificent. The walls of the city were 85 ft thick. This gate stood next to the palace of Nebuchadnezzar. Just south of the palace was a ziggurat (pyramidal temple) 300 ft high, probably the Tower of Babel mentioned in the Bible. Astro Lecture 6 33

34 Babylonian astronomy The Babylonians had two main interests in observing the heavens: to establish and maintain a reliable calendar. This was essential to administer their large, land-based empire. to make astrological predictions. Their astronomical observing program: They acquired long and continuous records of observations of the stars and planets over many 100s of years They looked for cycles or patterns in these. They also looked for patterns in deviations from cycles or patterns. They used these patterns to extrapolate to the future. Astro Lecture 6 34

35 Purposes of Astronomy in ancient Babylonia As in most ancient cultures, astronomy was actually practiced as astrology. Astronomical events, whether they were every-day occurrences or rare incidents, had a deep religious meaning for the people. It was believed that all things happened for a reason! Lives were lived according to the advice of these astronomers/astrologists! Orientation of the constellations was used to mark seasons for harvesting or sowing crops. Certain constellations were noted for their yearly rising or setting times, and provided an accurate clock by which time could be measured. Astro Lecture 6 35

36 What are the scientific breakthroughs of the Babylonians? Babylonians not only recognized Venus as the same object whether it appeared in the morning or evening, but they actually developed a method for calculating the length of the Venus cycle! Babylonians were able to predict solar and lunar eclipses. They applied a simple method, which made future predictions based on past observations! They almost certainly knew about the saros cycle; the general pattern of eclipses repeats every 18 years (and 11.3 days). Babylonians created the zodiac the circle of little animals which marked the twelve constellations that the Sun, Moon, and planets travel between during their movements through the sky. Astro Lecture 6 36

37 What are the scientific breakthroughs of the Babylonians? (Cont.) Constellations that we still use today, such as Leo, Gemini, Capricorn, etc. were invented by the Sumerians between 2,000-3,000 BC. A great deal of astronomical mythology was handed down from the Sumerians. Babylonian system of mathematics was sexagesimal, or a base 60 numeral system. From this we derive the modern day usage of 60 seconds in a minute, and 60 minutes in an hour. Babylonians invented the degree system to distinguish positions in the sky (360 degrees, 60 in 1 degree, etc.). Greeks adopted the degree system and also many of the Babylonian constellations, which they renamed in Greek! Astro Lecture 6 37

38 How did western astronomy benefit from these breakthroughs? The Babylonian underpinnings of western astronomy are extensive! It was from the Babylonians that the Greeks gained their knowledge of the five visible planets and the constellations of the zodiac, and centuries of recorded astronomical observations. Greeks also adopted the idea that the motions of the planets could be predicted with accuracy! Astro Lecture 6 38

39 Artist s reconstruction of the Library of Alexandria Astro Lecture 6 39

40 3.2 Ancient Greek Science Our goals for learning: Why does modern science trace its roots to the Greeks? How did the Greeks explain planetary motion? How did Islamic scientists preserve and extend Greek science? Astro Lecture 6 40

41 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.) Astro Lecture 6 41

42 Special Topic: Eratosthenes measures the Earth (c. 240 B.C.) Measurements: Syene to Alexandria distance 5,000 stadia angle = 7 Calculate circumference of Earth: 7/360 (circum. Earth) = 5,000 stadia circum. Earth = 5, /7 stadia 250,000 stadia Compare to modern value ( 40,100 km): Greek stadium 1/6 km 250,000 stadia 42,000 km Astro Lecture 6 42

43 How did the Greeks explain planetary motion? Plato Underpinnings of the Greek geocentric model: Earth at the center of the universe Heavens must be perfect objects move on perfect spheres or in perfect circles. Aristotle Astro Lecture 6 43

44 But this made it difficult to explain the apparent retrograde motion of planets Review: Over a period of 10 weeks, Mars appears to stop, back up, then go forward again. Mars Retrograde Motion Astro Lecture 6 44

45 The most sophisticated geocentric model was that of Ptolemy (A.D ) the Ptolemaic model: Sufficiently accurate to remain in use for 1,500 years Arabic translation of Ptolemy s work named Almagest ( the greatest compilation ) Ptolemy Astro Lecture 6 45

46 So how does the Ptolemaic model explain retrograde motion? Planets really do go backward in this model. Ptolemaic Model Astro Lecture 6 46