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The Egyptians through the Greeks This logo denotes A102 appropriate The Practical Sky Probably before there was civilization there was a need to keep track of days Migration of prey animals Rutting season for domesticated animals Agriculture: planting and harvesting (8000 years ago) The phases of the Moon, the rising position of the Sun, and prominent constellations provided clues for the time of year Timeline Dates in some cases are approximate; some civilizations achieved various milestones at later times. The dates cited are generally the earliest verifiable dates. Prehistoric Civilizations Clockwise from top: Varna 4400BC, Mesopotamia 3100 BC, Tell Hamoukar 4000BC, Tell Qaramel 9600 BC, Minos 2700 BC Dangers in Interpreting Ancient Structures We must take care not to superimpose our understanding of the sky onto ancient builders One way to avoid this error is to look at many supposedly astronomically inspired structures Why do you think the preferred orientation is slightly south of east? Orientation of Late Stone Age communal tombs on the Iberian peninsula. M. Hoskins Egyptian Astronomers • Constructed by nomadic cattle herders • ~ 7000 years old • Covers an area of 2.9 km by 1.2 km • 10 slabs about 2.7 meters high, bovine burial mounds, and a calendar circle Goseck Circle Oldest Solar Observatory Discovered in Germany 2002 Bronze Age (7000 years ago) Reconstructions on upper, middle right Nebra disk (bottom) found nearby but not nearly so old Sumerian Planosphere Stonehenge Solstice Sunrise at Stonehenge Carhenge (not so ancient) Babylon Ziggurat (Ur, Iraq; 2000 BCE) Celestial Observatory and temple Cuneiform citing lunar eclipse Centuries of recorded planetary observations Thirteen Towers of Chankillo Peru, 2300 years old Towers stand 6 to 13 feet, extending 1,00 feet away, marking positions of the Sun over a year Oldest Solar Observatory in the Americas From central position the arc of the rising Sun is subtended Ceremonial buildings in the rings Observers stood here Mayan Observatories circa 900 AD El Caracol Chichen Itza The Antikythera An instrument, not an observatory 1st Century BCE Named for the Greek island near the shipwreck from which this was discovered, circa 1900 A mechanical computer for celestial positions Similar to Byzantine sundial calendar, 500 AD Perhaps made by Archimedes of Syracuse, Sicily Known for great inventions Between 29 and 70 gears wheels for more complicated calculations But this is much simpler (some gears missing) Tower of the Winds in Athens Sebius water clock inside Egyptian inventor Antikythera mechanism A computer for horoscopes Perhaps every town had a tower, a water clock, and a mechanism The Greeks: First Millennium BCE Greek Reason • Thales ~700BCE – Lodestone – Amber • Anaximander ~ 650 BCE – Attempted to describe the mechanics of celestial bodies • Socrates ~400 BCE – “The Cave” – We see reality as shadows on the cave wall • Plato: student of Socrates Plato’s Republic: The Myth of Er • Er was a character in Republic • Whorl (spindle) Of Necessity • Celestial Spheres Eudoxus of Cnidus ( 408-355 BCE) Colleague of Plato Originator of scientific astronomy No works survive Envisioned a system of 20 concentric spheres Spheres because of their symmetry and unity Earth centermost Rotational axis fixed to next largest sphere Introduction of epicycles First Principles To account for: stability in the world. find what is fundamental, constant and unchanging.. diversity in the world. find what is different, what changes, and how it changes; this will reveal the rules or agencies that control the change process. use these rules to organize the fundamentals into a "neat array". pattern in the world.. which the Greeks called: arché physis cosmos Timeaus: • 2 Circles Split • Circle of the Same – Celestial Equator • Circle of the Different – Ecliptic Quadrivium: Greek Learning Arithmetic: Geometry: Music: Astronomy: number number in space number in time number in motion Five Perfect Solids (Things to come) Terrestrial Elements Qualities Earth Cold and Dry Water Cold and Wet Air Warm and Wet Fire Warm and Dry Celestial Element Quintessence, or aether quintessence is perfect and immutable Aristotle 384-322 BCE Student at Plato’s Academy / Teacher for Alexander Each terrestrial element (earth, water, air, fire) has a natural place or state; some amount of each element is present in every body Gravity/Levity If a body (animate or inanimate) is removed from the natural place or state of its predominant element (violent change), it will naturally strive to return where it belongs (natural change) physis Metaphysics For Aristotle, his “First Philosophy”, coming after physis For us, the study and doctrine of internal, active principles in things Intrinsic properties or qualities Examples The intrinsic nature of rock is ‘heavy’, and it seeks the center of the Earth The intrinsic nature of fire is ‘not heavy’ and it reaches upward NOT that Earth’s mass pulls it down NOT that convection currents steer the flames Not until the 17th century will these ideas be challenged Influence felt for almost 2000 years Adopts (and adapts) Eudoxus’ spherical model Geocentrism Not a mathematician Simplifies model Calls the outermost sphere the “Prime Mover” Earth at the center because it’s heavy And spherical (!) contrary to myths of a flat world Remember, each axis fixed to the next outer sphere Adopted later by the Roman Church as God Earth where man lives The Underworld of fire and brimstone The Hellenistic Era The time right after the death of Alexander 323 - 31 BCE, when Rome emerges Commerce flourishes Greek culture had been spread over a wide area Museum at Alexandria: 400,000 scrolls Center of learning for hundreds of years Burned by zealots in 415 AD Librarian and Philosopher Hypatia killed for heresy The Hellenistic World Aristarchos 310-230 BCE • Had a different cosmological view than Aristotle: Heliocentrism • Found the ratio of the Earth to Moon diameter Erastosthenes 276 - 194 BCE • Determined the size of the Earth, the distance to the Moon, and the size of the Moon, all very accurately. Apollonius of Perga 190 – 120 BCE Mathematician Conic sections Math 80 Introduced eccentric orbits Hipparchus of Rhodes 190-120 BCE Precession of the equinoxes Devised the Magnitude Scale for his catalog Clumsy but still in use (modified heavily) today Then, brightest star (Sirius) = 1, dimmest = 6 Now Vega is labeled zero, brighter objects are negative, very dim objects are 20-30 Took centuries of Babylonian observations and tried to reconcile the data with a combination of epicycles and eccentric orbits In sexigesimal! He made it work for the moon… Retrograde Motion • Apparent backwards motion of planētēs in the sky • Wanderers; not the Sun or Moon • Difficult to reconcile with Aristotle - geocentrism *Ptolemy (Claudius) 90-168AD • Mathematician, Astronomer, and Astrologer • Star catalog of 1000 stars in 48 constellations • Maybe Hipparchus’ catalog • Tetrabiblios: Astrology text • Scornful of previous 3 centuries of astronomy/astrology *Ptolemy I was one of Alexander’s generals. He was given governorship of Egypt by Alexander and the Ptolemys ruled the country for centuries. Note the cross-staff The Mathematike Syntaxis “The Mathematical Compilation” …that the heavens are spherical and move spherically; …that the earth, in figure, is sensibly spherical also when taken as a whole …[that the earth] in position, lies right in the middle of the heavens, like a geometrical center; …[that the earth] in magnitude and distance, has the ratio of a point with respect to the sphere of the fixed stars, having itself no local motion at all. Incorporates Hipparchus’ equinox precession for accurate for predicting planetary positions for many centuries Epicycles, Eccentrics, and now the Equant deferent Equant point Planetary Hypothesis Ptolemy reasoned that longer period planets are closer to the stars, furthest from Earth Saturn, Jupiter, Mars far, Moon closest Sun, Mercury, Venus a problem Since Mercury and Venus are solar companions, he put the Sun nearer than Mars, then Venus and Mercury with little evidence Distances: Moon: 64 Rearth ~256,000 miles; Stars: 19,865 Rearth ~ 75 million miles