The Egyptians through the Greeks

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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

Oldest Solar Observatory in the Americas Towers stand 6 to 13 feet, extending 1,00 feet away,

marking positions of the Sun over a year From central position the arc of the rising Sun is subtended

Ceremonial buildings in the rings

Observers stood here

Mayan Observatories circa 900 AD

Chichen ItzaEl Caracol

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 But this is much simpler

Perhaps made by Archimedes of Syracuse, Sicily Known for great inventions

Between 29 and 70 gears wheels for more complicated calculations (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 sphereIntroduction of epicycles

First Principles

To account for: which the Greeks called:

stability in the world.

find what is fundamental, constant

and unchanging.. arché

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.

physis

pattern in the world..

use these rules to organize the fundamentals into a "neat array".

cosmos

Timeaus:

• 2 Circles Split

• Circle of the Same– Celestial Equator

• Circle of the Different– Ecliptic

Quadrivium: Greek Learning

Arithmetic: number Geometry: number in space Music: number in time Astronomy: 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 NOT that Earth’s mass pulls it down

The intrinsic nature of fire is ‘not heavy’ and it reaches upward

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

Earth at the center because it’s heavy And spherical (!) contrary to myths of a flat world

Not a mathematician Simplifies model

Calls the outermost sphere the “Prime Mover” 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