1 Gravity and Motion The Early History of Astronomy

2 Topics l Introduction l Geocentric Astronomy l Summary

3 Introduction

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5 Geocentric Astronomy 4000 BC – 700 AD

6 Origins of Astronomy l 4000 – 500 BC Chaldean, Babylonian, Egyptian, Chinese, Indian,... l Grouped stars into constellations. l Babylonians divided circumference into 360 degrees; 1 degree into 60 minutes; 1 minute into 60 seconds. l Believed stars controlled human destiny.

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8 Ancient Greece

9 Greek Astronomy l BC Thales l Founded Ionian School of Philosophers. l Declared universe is understandable in terms of simple rules. l Rejected superstition. l Earth floats in a vast ocean. l BC Anaximander l Earth isolated, and unsupported, in space.

10 Greek Astronomy – II l 560 – 480 BC Pythagoras of Samos rational numbersmusical intervals l Discovered relationship between rational numbers and musical intervals. Philosophos l The goal of the Philosophos, the Lover of Wisdom, was to discover rules of Nature through deep reflection. l Decided upon spiritual grounds that the Earth was spherical – the perfect shape.

11 Greek Astronomy – III l 428 – 348 BC Plato l Space is infinite and contains a finite, spherical, universe at the center of which lies the Earth. l The visible world is but a distorted copy of the real world of Ideas. Pythagorean program abandoned. sphere circular l The shape of the Earth must be a sphere and all heavenly motion must be circular.

12 Greek Astronomy – IV l BC Aristotle l Joined Plato's Academy in 367 BC. Alexander the Great l Tutored young prince who would become Alexander the Great, the founder of the city of Alexandria in 332 BC. l Like Pythagoras, Aristotle believed that mathematics was the key to a true understanding of Nature. circular motion geocentrism. l Promoted idea of circular motion into a dogma of astronomy along with geocentrism.

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14 Greek Astronomy – V l 310 BC Aristarchus of Samos l Last of the great Pythagoreans. heliocentric model l Developed a heliocentric model. l Alas his ideas all but died with him. l This model re-discovered by Copernicus 17 centuries later.

15 Greek Astronomy – VI l BC Eratosthenes of Alexandria l Determined circumference of the Earth degrees l At the Summer Solstice the Sun is vertical at Syene but at Alexandria it is 7.25 degrees south of the vertical. Alexandria Syene NS

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17 The Circumference of the Earth A A Sunlight Alexandria Syene A = 7.5 o D = 500 miles C = Circumference D C / D= 360 / A C = D * 360 / A = 500*360 / 7.5 = 24,000 miles Center of the Earth Shadow

18 End of Greek Astronomy l 190 – 120 BC Hipparchus l The greatest astronomer of antiquity. l From his precise observations he became convinced that Aristotle was wrong about circular motion. epicycles geocentric l Created a more accurate model of planetary motion in which he introduced epicycles into the geocentric model years l Became the accepted model of the universe for the next 1600 years!

19 The Empire of Rome l 47 BC – 30 AD Roman Empire at its height l Era of Cleopatra, Julius Caesar and Mark Anthony. l Era of Jesus of Nazareth. l Roman conquest of the Middle East. l AD Claudius Ptolemeus (Ptolemy) The Almagest l Wrote great textbook: The Almagest summarizing Greek astronomy and his own astronomical theories.

20 Planets: The Vagabond Stars Prograde Motion Retrograde Motion

21 The Ptolemaic System

22 Descent into Darkness l 642 AD Fall of Alexandria l After a 14-month siege by Arabian troops, Alexandria was all but destroyed. l The great library was razed to the ground. l Dark Ages (700 AD – 1400 AD) l The West enters a period of stagnation. l Greek astronomy passed on to the Arabs, via India. This knowledge is preserved and extended by Islamic scholars in Baghdad.

23 Period of Western history characterized by terror and despair, in populations oppressed, famished, and wretched to a degree almost unimaginable today. To the miseries of constant war, political and social disintegration, there was added the dreadful affliction of inescapable, mysterious, and deadly disease. Mankind stood helpless as though trapped in a world of terror and peril against which there was no defense. The Sleepwalkers by Arthur Koestler The Dark Ages

24 The Great Re-Awakening l 760 AD l Islamic leaders in Baghdad sponsored translations of old Greek texts. l 1000 AD l The Islamic empire spreads to Spain. l Greek-Arabian science enters the West. l 1400 AD l Gradually, the West awakens from its long dark sleep.

25 Summary l 4000 BC ~ 500 BC astronomy astrology l The earliest ideas about astronomy grew from astrology, the superstitious belief in the influence of stars on human affairs. l 600 BC ~ 100 BC l Abandonment of superstition and embrace of reason. Golden age of ancient Greece l 100 BC ~ 200 AD l Consolidation of ancient Greek astronomy