1 Gravity and Motion The Early History of Astronomy

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Gravity and MotionGravity and Motion

The Early History of Astronomy

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TopicsTopics

Introduction Geocentric Astronomy Summary

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IntroductionIntroduction

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Geocentric AstronomyGeocentric Astronomy

4000 BC – 700 AD

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Origins of AstronomyOrigins of Astronomy

4000 – 500 BC Chaldean, Babylonian, Egyptian, Chinese, Indian, ... Grouped stars into constellations. Babylonians divided circumference into 360

degrees; 1 degree into 60 minutes; 1 minute into 60 seconds.

Believed stars controlled human destiny.

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http://plato-dialogues.org/tools/gk_wrld.htm

Ancient GreeceAncient Greece

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Greek AstronomyGreek Astronomy

624 - 548 BC Thales Founded Ionian School of Philosophers. Declared universe is understandable in terms

of simple rules. Rejected superstition. Earth floats in a vast ocean.  

610 - 547 BC Anaximander Earth isolated, and unsupported, in space.

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Greek Astronomy – IIGreek Astronomy – II

560 – 480 BC Pythagoras of Samos Discovered relationship between rational rational

numbersnumbers and musical intervalsmusical intervals.  The goal of the PhilosophosPhilosophos, the Lover of

Wisdom, was to discover rules of Nature through deep reflection.  

Decided upon spiritual grounds that the Earth was spherical – the perfect shape.

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Greek Astronomy – IIIGreek Astronomy – III

428 – 348 BC Plato Space is infinite and contains a finite,

spherical, universe at the center of which lies the Earth.

The visible world is but a distorted copy of the real world of Ideas. Pythagorean program abandoned.

The shape of the Earth must be a spheresphere and all heavenly motion must be circularcircular.

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Greek Astronomy – IVGreek Astronomy – IV

384 - 322 BC Aristotle Joined Plato's Academy in 367 BC. Tutored young prince who would become

Alexander the GreatAlexander the Great, the founder of the city of Alexandria in 332 BC.

Like Pythagoras, Aristotle believed that mathematics was the key to a true understanding of Nature.

Promoted idea of circular motioncircular motion into a dogma of astronomy along with geocentrismgeocentrism..

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Greek Astronomy – VGreek Astronomy – V

310 BC Aristarchus of Samos Last of the great Pythagoreans. Developed a heliocentric modelheliocentric model. Alas his ideas all but died with him. This model re-discovered by Copernicus 17

centuries later.

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Greek Astronomy – VIGreek Astronomy – VI

276 - 194 BC Eratosthenes of Alexandria Determined circumference of the Earth.  At the Summer Solstice the Sun is vertical at

Syene but at Alexandria it is 7.25 degrees7.25 degrees south of the vertical. 

Alexandria Syene

N S

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http://www.geocities.com/Athens/Forum/8740/Alexander.htm

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The Circumference of the EarthThe Circumference of the Earth

A

ASunlightSunlight

AlexandriaAlexandria

SyeneSyene

A = 7.5o

D = 500 milesC = Circumference

DC / D = 360 / A

C = D * 360 / A= 500*360 / 7.5= 24,000 miles

Center of the Earth

Shadow

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End of Greek AstronomyEnd of Greek Astronomy

190 – 120 BC Hipparchus The greatest astronomer of antiquity. From his precise observations he became

convinced that Aristotle was wrong about circular motion.

Created a more accurate model of planetary motion in which he introduced epicyclesepicycles into the geocentricgeocentric model.

Became the accepted model of the universe for the next 1600 years1600 years!

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The Empire of RomeThe Empire of Rome

47 BC – 30 AD Roman Empire at its height Era of Cleopatra, Julius Caesar and Mark

Anthony. Era of Jesus of Nazareth. Roman conquest of the Middle East.

100 - 170 AD Claudius Ptolemeus (Ptolemy) Wrote great textbook: The AlmagestThe Almagest

summarizing Greek astronomy and his own astronomical theories.

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Planets: The Vagabond StarsPlanets: The Vagabond Stars

Prograde Motion

Retrograde Motion

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The Ptolemaic SystemThe Ptolemaic System

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Descent into DarknessDescent into Darkness

642 AD Fall of Alexandria After a 14-month siege by Arabian troops,

Alexandria was all but destroyed. The great library was razed to the ground. 

Dark Ages (700 AD – 1400 AD) The West enters a period of stagnation. Greek astronomy passed on to the Arabs, via

India. This knowledge is preserved and extended by Islamic scholars in Baghdad. 

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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 AgesThe Dark Ages

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The Great Re-AwakeningThe Great Re-Awakening

760 AD Islamic leaders in Baghdad sponsored

translations of old Greek texts. 

1000 AD The Islamic empire spreads to Spain. Greek-Arabian science enters the West.

1400 AD Gradually, the West awakens from its long

dark sleep. 

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SummarySummary

4000 BC ~ 500 BC The earliest ideas about astronomyastronomy grew from

astrologyastrology, the superstitious belief in the influence of stars on human affairs.

600 BC ~ 100 BC Abandonment of superstition and embrace of

reason. Golden age of ancient Greece

100 BC ~ 200 AD Consolidation of ancient Greek astronomy