And folks who tried to figure what

Transcription

1 Chapter 03 Part 1 Ancient Astronomy In the beginning, there were stars. And folks who tried to figure what they really were.

2 In the Beginning It Was About Time Astronomy is the oldest science because almost every ancient culture, understanding the need to predict the coming of the seasons, became expert at tracking and predicting the motions of the sun, moon, and planets. At right is the sun dagger, a cleft of rock in NM used to mark the summer solstice. Interesting list of ancient observatories (click here)

3 New Mexico: Anasazi kiva aligned north south

4 Monuments Were Observatories The Egyptian Pyramids, Stonehenge, and the Native American, Mayan, and Aztec temples all are complex astronomical observatories designed for direction-finding or the prediction of the planets and seasons. The Great Pyramid at Giza in Egypt (at left) is precisely oriented to north-south lines from that era. The tunnels to the tombs are also designed to shine light on specific days.

5 Determining the Time of Day How can you figure out time by the sun? Can you say sundials? Day length varies over the course of the year, so knowing the exact time that the Sun rises or sets can be easy, if you use a Sun Dagger.

6 Egyptian gyp obelisk: Shadows tell time of day.

7 Determining the Time of Year Ancient people also needed d to mark the year for rituals and crops. Many cultures used sky signs Sirius in Canis Major rises in August dog days African peoples marked rainy seasons by the Moon s orientation. Stonehenge marked out the rise and set of different objects with massive stones.

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

9 Using Lunar Cycles Marking the phases of the moon was the most important time-keeping observation since it helped to mark smaller parts of the year. 12 lunar months brings us to only 355 days a year, so it is off from the solar year. Some cultures still worked with this: Jewish calendar is lunar (that s why holidays move) Christian i Easter is lunar (that s t why it moves) Muslim Ramadan moves.

10 Scotland: 4000-year-old stone circle; Moon rises as shown here every 18.6 years.

11 Peru: Lines and patterns, some aligned with stars

12 Macchu Pichu, Peru: Structures aligned with solstices

13 South Pacific: Polynesians were very skilled in the art of celestial navigation.

14 Greek Astronomy The first mathematical study of the heavens was by the Greek philosopher Aristotle in the second century B.C. It was given final form by Ptolemy in the second century A.D. After the fall of Rome and throughout the Dark Ages, this work became the geocentric ( earthcentered ) view of the heavens: Earth was the center about which h all orbited. All orbits were circular about the Earth.

15 Islamic Astronomy Following the rise of Islam in 700 AD, Muslim scientists began detailed studies of astronomy. At first, this was to mark the passage of the moon exactly (important for holidays). Later, they discovered the lost Greek astronomy and began naming the heavens (many stars have Arabic names). By 1200 AD, astronomers like al-sufi, al-biruni, al-tusi and Omar Khayyam had left a lasting mark on astronomy.

16 The Rise of Astronomy The great philosopher-scientists scientists of Greece, Rome, and other Mid-East cultures brought astronomy together as a science just around the time of Jesus (400 BC to 100 AD). This was based on observation and experiment and lead to the first scientific model of the universe: the Geocentric Theory.

17 The Geocentric (Ptolemaic) Theory By 100 AD, there were competing theories about the movements of the heavens, but the main one was a geocentric ( earth-centered ) theory. Since circles are perfect, all orbits and objects should be exactly circular. The main problem was explaining the movement of planets they seemed to wander, not move in smooth circular paths. To explain this, Ptolemy included epicycles that were essentially orbits within orbits.

18 But perfect circles made it difficult to explain the apparent retrograde motion of planets

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

20 The Power of Geocentrism Geocentrism was popular p because it explained the universe well. Observational data didn t show problems until the use of the telescope ~1600 AD. Epicycles look weird, but this is before gravity, so we didn t really understand what kept planets in orbits.

21 Geocentric Views and the Church By 1400 AD, much of the Islamic astronomical data and discoveries had been taken by the Catholic Church as spoils of the Crusades. The Church h upheld the geocentric view of the universe from Ptolemy because it supported the Church s view of the perceived importance of humans in the universe from Genesis 1. Since the Catholic Church funded most astronomical research in Europe, you had to support the Church s view to get money.

22 Why Was Geocentrism Popular? At the time, humans believed strongly they were fallen and suffering from God s wrath: Black Plague kills 1/3 of Europe in 50 years. Misery was the norm. It was a bright spot to think we were the center of Creation. Also, Aristotle and Ptolemy s math were based on observations that were fairly crude and nobody could reliably do better (you could make more precise observations with a telescope from Wal-mart today).