Greek Astronomy:
The First “Scientific Revolution”
Athens, ca. 400 BC
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
(B) Navigation needed for seafaring
(C) Clear weather presented more astronomical phenomena
that demanded explanations
(D) More extremes in weather require better calendars
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
(B) Navigation needed for seafaring
(C) Clear weather presented more astronomical phenomena
that demanded explanations
(D) More extremes in weather require better calendars
Temple of Poseidon, Sounion
Gateway, Citadel of Mycenae
Parthenon, Athens
Dolphins & Lions
of Delos
The School of Athens (Raphael)
Plato & Aristotle
Ptolemy
Pythagoras
The School of Athens (Raphael)
800 Years of Greek Astronomy:
650 BC – 150 AD
Greek Astronomical Timeline to 250 BC
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) = 1.41421356...
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, 1489001/747253, .......
Impact of Mathematical Success
on Greek Thinking
Applied mathematical logic to thinking in other areas
leading to "rational thinking"
Preference for making deductions about nature
from axioms or abstract principles
Impact of Mathematical Success
on Greek Thinking
Applied mathematical logic to thinking in other areas
leading to "rational thinking"
x
Preference for making deductions about nature
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
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.
(B) B (the heavier object) hits the ground first.
(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.
(B) B (the heavier object) hits the ground first.
(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.
(C) They hit at the same time.
Greek Astronomy: Great!
Rational, mathematical, logical, mostly empirical
• Knew shape and size of Earth and Moon
• Understood origin of lunar phases
• Understood origin of eclipses
• Detected (Hipparchos) polar precession
• Realized (Aristarchus) that Sun is much more
distant (& therefore larger) than Moon
• Constructed first cosmological models that
reproduced the data
Greek Astronomy:
Spherical Shape of Earth
• Curvature of ocean horizon
•
•
•
•
Different stars @ different latitudes
Different length of day @ “ “
Circular Earth shadow during lunar eclipses
Shadow lengths differ at different latitudes and can
be used to measure the diameter of Earth
(Eratosthenes)
Lunar eclipse
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
S
H
Alexandria
so R = dH/S
Eratosthenes answer:
R = 4025 miles
True value:
R = 3950 miles
Syene
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 beings
• Model must explain their known motions
• Spherical Earth at center of universe (“geocentric”)
• “Superlunary” region pure, eternal, unchanging
– In contrast to corrupted, changeable Earth & sublunar region
• Only purely circular (“perfect”) motions allowed
• Earth is stationary; universe revolves once per day
around the Earth
Summary of easily visible motions of celestial objects:
an acceptable model must reproduce ALL
Summary of easily visible motions of celestial objects:
Acceptable model must reproduce ALL
= 'Retrograde motion"
“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?
Well, try this:
A Traditional “Projective” Cosmology (Indian)
Ptolemy (ca. 130 AD): constructs ultimate
Greek cosmological model
Ptolemy’s Geocentric Universe
"Epicycle"
Center of
Universe
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)
B)
C)
D)
Once per Earth year
About half the time
Once per epicyclic rotation
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)
B)
C)
D)
Once per Earth year
About half the time
Once per epicyclic rotation
Never
n.b.: "gibbous" means more than half-full but
less than full
The Dark Ages
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