Unit 3: Ancient Greek Astronomy

Introduction to Ancient Greek Astronomy

• time period: ~3000 BCE ↔ ~300 CE

  • <3000 BCE: Greeks arrive at Aegean Peninsula

  • ~750 BCE: start of recorded history (founded Rome & wrote alphabet)

    e.g. 776 BCE Olympic games

  • ~750 BCE ↔ 480 BCE: Archaic Period (city states loosely united and expanded to new territories

    e.g. Turkey, Spain, France, Italy, Russia, Africa)

  • 480 BCE ↔ 323 BCE: Classical Period (Macedonia dominated and Alexander the Great expanded to Egypt & Babylon)

  • 323 BCE ↔ 156 BCE: Hellenistic Period (culture expanded to Middle East & Egypt)

  • 156 BCE ↔ 30 BCE: Greco-Roman Period (empire joined Roman Republic and last Egyptian Greek ruler Cleopatra VII died)

1st documented group taking astronomy beyond tracking time & astrology & religion → science culture

• reason: fragmented geography (city states) & decentralized rule

established first higher education

academy word origin: first school location (just outside of Athens)

• - Plato

• 600 BCE: Ionian

  e.g. Thales, Anaximander

• 500 BCE ↔ 200 CE: Pythagorean

The Ionian Philosophers - Thales and Anaximander

Thales = Anaximander’s teacher

• birthplace: Miletus, Ionia

  • large port city → multicultural

Thales

money & means & acquaintances → travel e.g. Middle East and Egypt

• learned about science achievements

• recognized Greeks and Egyptians explained phenomena with different god & mood

  e.g. earthquake: Greeks → angry Poseidon v.s. Egyptians → laughing Geb

• sought explanation beyond god

  e.g. earthquake: Earth floated on water → large wave moved ground

• gained popularity by predicting event

585 BCE Eclipse

• 600 BCE: Ionia was in war

learned from Babylon

went to both army generals: ‘solar eclipse’s going to occur’

solar eclipse: Zeus randomly blocked sun

• result: Medes and Lydians ended war, gods created knowable laws

Herodotus observed event

Anaximander

• birthdate: 610 BCE

1st cosmologist

cosmology → universe study

• goal: explain universe origin & method

  • create model organizing known universe and motion patterns

    • placed known universe on wheels around cylinder Earth

    • result: day / moon / eclipse / season … cycle

thought process revolution: idea that model could be proposed & tested

set Earth in space

• finite ground

  • reason: N/A

The Pythagoreans

Anaximander = Pythagoras’ teacher

• date: ~ 550 BCE

• birthdate: 570 BCE

• birthplace: Samos

  • statue

travel → Egypt math & Babylon astronomy

Pythagoras gathered followers

• cult → math & universe = god

  rules e.g. communal, vegetarian, specific on science, men = women

  • cosmos: universe follows equation laws

    cosmos word origin: Greek, order

  • result: N/A

Pythagoras was only one hearing celestial music

Pythagoras and Blacksmith

walked past blacksmith forge

some sounds were un/harmonious

method: measured dimensions

result: bar length determined sound, musical pitch followed natural law = universe followed natural law

• if 2 bar lengths were simple ratio, harmonious

  v.v.s.

• if 2 bar lengths were not simple ratio, unharmonious

Perfection

celestial bodies move in perfect circles

earth = sphere

• reason: simplest and purest geometric shape

Evidence

horizon disappearance

flat: decrease size then fall off of edge / fade to single point

v.s.

round: disappear bottom-first

• on curved surface, top remains visible as object goes over horizon

lunar eclipse

Earth’s shadow on Moon = round

star shift

flat: specific rate based on walk speed

v.s.

round: different rate based on walk speed

• constellation shift = travel distance

The Heliocentrists - Philolaus, Herakleides, and Aristarchus

• 500 BCE ↔ 400 BCE: Pythagorean persecution

  e.g. arson and exile

  • result: division

went on in Anaximander

• developed and hypothesized and tested and observed sky models

• result: ~ 300 years → close to uncovering solar system

Philolaus

• birthdate: 470 BCE

one of Pythagoras’ star pupils

1st to suggest Earth moves through space

• once a day around Central Fire

  • invisible

  Greece faced away from Central Fire

• Counter-Earth between Central Fire and Earth

  • invisible

  • purpose: keep earth from getting burnt by central fire

  • Aristotle: ‘added to bring celestial body number to 10’

• result: rid Anaximander’s complexities

Counter-/Earth spins around Central Fire once a day

v.s.

Sun spins around Central Fire once a year

v.s.

fixed stars

Herakleides

• birthdate: 390 BCE

set Earth on daily spin

• reason: no central fire evidence

rid central fire and Counter-Earth

mummified remains location: Los Angeles museum

• moved Mercury and Venus orbits around Sun

  • 5 and 6

  • reason: makes nearer loops to rising/setting sun

  • result: + motion and brightness

Aristarchus

• birthdate: 310 BCE

  • ~ Herakleides’ deathdate

• birthplace: Samos

last great heliocentric

famous texts

1. “On the sizes and distances of the Sun and Moon”

• gained him fame

developed sun/moon distance measure method and size

• correct method

  • lunar eclipse

    • method: monitored Earth’s shadow size on Moon

    • size result: moon < Earth

  • Sun’s distance compared to Moon

    • method: waited for quarter moon phase → Sun’s in lit direction

      solved right triangle, measured ∠A (Sun ↔ Moon angle) by quadrant: ~87° ≅ 89.9°

    • result: Sun’s farther than Moon

      • despite total solar eclipse

conclusion: size → if Sun > Moon and Moon < Earth, Sun > Earth

• incorrect measurement

  • size estimate: moon 2 = Earth * 10 = Sun

  • size answer: moon 4 = Earth * 100 = Sun

  • values: moon distance → N/A and sun distance → N/A

  • distance equation estimate:

    \cos a=\frac{B}{C}=\frac{moondistance}{sundistance}\cong\frac{1}{19}

  • distance equation answer:

    \cos a=\frac{B}{C}=\frac{moondistance}{sundistance}\cong\frac{1}{382}

2. heliocentric universe

• lost but spoken and written for centuries

‘Is Earth at centre if Sun’s largest celestial body?’

• conclusion: no, only Moon orbits Earth because it’s the only celestial body that doesn’t have the weird pattern

Apparent Planetary Motion

• ~100 BCE: heliocentric track ended

  • reason: Aristarchus wasn’t interested in followers

features:

model must explain that motion don’t maintain…

• direction

• speed

• brightness

Venus and Mercury

clockwise loop

e.g. Venus

temporary retrograde motion v.s. normal direct motion

• September → December: down middle → up left

• December → March: up left → down right

changing apparent brightness

• helical rise: bright

Mars, Jupiter, and Saturn

west → east

e.g. Mars

temporary retrograde motion v.s. normal direct motion

• May → September 8: slight downwards, west → east

• September 8 → October 15: slight upwards, east → west

• October 15 → January: slight downwards, west → east

changing apparent speed

• May 1 → June 1: 15° per month

• September 8 → October 8: 4° per month

changing apparent brightness

• retrograde motion: bright and big

  v.s.

• direct motion: dim and small

Plato and Eudoxus