Greek Astronomy: The First “Scientific Revolution”

Athens, ca. 400 BC

Puzzlah #20 Puzzlah #20 Why might the Greeks have been more dependent on astronomy than a tropical culture like the Maya? (A) More extended empire to keep coordinated (A) More extended empire to keep coordinated (B) Navigation needed for seafaring (B) Navigation needed for seafaring (C) Clear weather presented more astronomical phenomena (C) Clear weather presented more astronomical phenomena that demanded explanations that demanded explanations (D) More extremes in weather require better calendars (D) More extremes in weather require better calendars

Puzzlah #20 Puzzlah #20 Why might the Greeks have been more dependent on astronomy than a tropical culture like the Maya? (A) More extended empire to keep coordinated (A) More extended empire to keep coordinated (B) Navigation needed for seafaring (B) Navigation needed for seafaring (C) Clear weather presented more astronomical phenomena (C) Clear weather presented more astronomical phenomena that demanded explanations that demanded explanations (D) More extremes in weather require better calendars (D) More extremes in weather require better calendars

Temple of Poseidon, Sounion

Gateway, Citadel of Mycenae

Parthenon, Athens

Dolphins & Lions of Delos of Delos

The School of Athens (Raphael)

Pythagoras Ptolemy Plato & Aristotle

800 Years of Greek Astronomy: 650 BC – 150 AD 650 BC – 150 AD

Greek Astronomical Timeline to 250 BC

- Rejects mythological/supernatural explanations of nature

Greek Astronomical Timeline to 250 BC Atomic theory. Plurality of worlds. Infinite universe.

Greek Astronomical Timeline to 250 BC Physics, biology, astronomy.

Greek Astronomical Timeline to 250 BC Physics, biology, astronomy. Mostly erroneous!

Greek Astronomical Timeline to 250 BC

Greek Astronomical Timeline 250 BC – 150 AD

First proved for all right triangles by Pythagoras, ca. 530 BC

“Irrational numbers” – a remarkable, even disturbing, discovery of the Pythagoreans E.g. Square root (2) = This is not a "rational" number* It cannot be expressed as the ratio of ANY two whole numbers. There is an infinity of rational numbers, but the square root of 2 is NOT among them. * “Rational” numbers: 2, 5, 2/3, 3/2, 13/17, 129/97, /747253,

Impact of Mathematical Success on Greek Thinking Applied mathematical logic to thinking in other areas leading to "rational thinking" leading to "rational thinking" Preference for making deductions about nature from axioms or abstract principles from axioms or abstract principles

Impact of Mathematical Success on Greek Thinking Applied mathematical logic to thinking in other areas leading to "rational thinking" leading to "rational thinking" Preference for making deductions about nature from axioms or abstract principles from axioms or abstract principles Their science based largely on non-empirical premises Disdained making experiments Great progress in some areas, but overall success circumscribed by these biases success circumscribed by these biases x

Puzzlah # 21 You have two objects, A and B, both of which are the same shape. B weighs twice as much as A. You drop both simultaneously from a height of 3 feet. What happens? (A) A (the lighter object) hits the ground first. (A) A (the lighter object) hits the ground first. (B) B (the heavier object) hits the ground first. (B) B (the heavier object) hits the ground first. (C) They hit at the same time. (C) They hit at the same time.

Puzzlah # 21 You have two objects, A and B, both of which are the same shape. B weighs twice as much as A. You drop both simultaneously from a height of 3 feet. What happens? (A) A (the lighter object) hits the ground first. (A) A (the lighter object) hits the ground first. (B) B (the heavier object) hits the ground first. (B) B (the heavier object) hits the ground first. (C) They hit at the same time. (C) They hit at the same time.

You have just done a simple experiment that invalidates assumptions in Aristotle’s physics which were accepted for over 1300 years.

Greek Astronomy: Great! Rational, mathematical, logical, mostly empirical Knew shape and size of Earth and MoonKnew shape and size of Earth and Moon Understood origin of lunar phasesUnderstood origin of lunar phases Understood origin of eclipsesUnderstood origin of eclipses Detected (Hipparchos) polar precessionDetected (Hipparchos) polar precession Realized (Aristarchus) that Sun is much more distant (& therefore larger) than MoonRealized (Aristarchus) that Sun is much more distant (& therefore larger) than Moon Constructed first cosmological models that reproduced the dataConstructed first cosmological models that reproduced the data

Greek Astronomy: Spherical Shape of Earth Curvature of ocean horizon Curvature of ocean horizon Different different latitudes Different different latitudes Different length of “ “ Different length of “ “ Circular Earth shadow during lunar eclipses Circular Earth shadow during lunar eclipses Shadow lengths differ at different latitudes and can be used to measure the diameter of Earth (Eratosthenes) Shadow lengths differ at different latitudes and can be used to measure the diameter of Earth (Eratosthenes)

Lunar eclipse

If know size of the Earth, can use shadow to estimate size of Moon.

Eratosthenes and the shape and size of Earth Syene Alexandria Sun at noon, June 21: overhead at Syene, but not at Alexandria

Eratosthenes Method (200 BC) Apply plane geometry: Measure d, H, S. The two (approximate) triangles are congruent. This means that S/H = d/R so R = dH/S Eratosthenes answer: R = 4025 miles True value: R = 3950 miles Syene Alexandria S H

Hipparchos, ca 150 BC Star catalogs Magnitude system Precession Planetary data

Aristarchus’ Heliocentric Cosmology (ca. 250 BC)

Aristarchus’ Heliocentric Cosmology (ca. 250 BC) Although Aristarchus was right, the geocentric cosmology favored by Aristotle prevailed.

Ultimate Greek Cosmology Cosmic bodies are inanimate, physical objects, not supernatural beingsCosmic bodies are inanimate, physical objects, not supernatural beings Model must explain their known motionsModel must explain their known motions Spherical Earth at center of universe (“geocentric”)Spherical Earth at center of universe (“geocentric”) “Superlunary” region pure, eternal, unchanging“Superlunary” region pure, eternal, unchanging –In contrast to corrupted, changeable Earth & sublunar region Only purely circular (“perfect”) motions allowedOnly purely circular (“perfect”) motions allowed Earth is stationary; universe revolves once per day around the EarthEarth is stationary; universe revolves once per day around the Earth

Summary of easily visible motions of celestial objects: an acceptable model must reproduce ALL

= 'Retrograde motion" Summary of easily visible motions of celestial objects: Acceptable model must reproduce ALL

“Retrograde Motion” of Mars

Greek Universe of Nested Spheres

Early Greek Model : geocentric, nested, rotating spheres

Early Greek Model : geocentric, nested, rotating spheres Does this seem archaic, backward, naive, dumb?

A Traditional “Projective” Cosmology (Indian) Well, try this:

Ptolemy (ca. 130 AD): constructs ultimate Greek cosmological model

Ptolemy’s Geocentric Universe Center of Universe "Epicycle"

Epicycles are needed in a geocentric model to produce retrograde motion

Ptolemy: how epicycles produce retrograde motion

Puzzlah #22 Ptolemy's geocentric model for the solar system may have been wrong, but it was scientific. It made definite predictions, for example about phases shown by Venus in a telescope. In the model, when should Venus show a "gibbous" phase? A) Once per Earth year A) Once per Earth year B) About half the time B) About half the time C) Once per epicyclic rotation C) Once per epicyclic rotation D) Never D) Never n.b.: "gibbous" means more than half-full but less than full

Puzzlah #22 Ptolemy's geocentric model for the solar system may have been wrong, but it was scientific. It made definite predictions, for example about phases shown by Venus in a telescope. In the model, when should Venus show a "gibbous" phase? A) Once per Earth year A) Once per Earth year B) About half the time B) About half the time C) Once per epicyclic rotation C) Once per epicyclic rotation D) Never D) Never n.b.: "gibbous" means more than half-full but less than full

The Dark Ages

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