Transcript Greek Astronomy - Physics & Astronomy

Greek Astronomy
Ancient View of the Cosmos
 Universe is 2-D
 All celestial objects attached to a sphere.
 Celestial Sphere is close
 Climb a high mountain and touch the sky
 Celestial objects are self-luminous
 Earth is the center of the universe
 Objects move on perfect circles
Pre-disposition for success
 Not Fatalists like Babylonians
 Greeks had a curiosity about nature
 Model builders
Lunar Phases and Eclipses
Two important changes in thought
needed
 Universe is three dimensional
 Some celestial objects are dark
Lunar Phases
Phases
1st Quarter
Full Moon
New Moon
Moon (3476 km)
Earth (12,756 km)
Lunar Eclipses
Sun
penumbra
umbra
Earth
Moon
Lunar Eclipses
b Eclipse shadow
is always a
section of a circle
b Earth must be a
sphere
b Color of the
eclipsed moon
Solar Eclipses
Total Solar Eclipse
Eratosthenes
North Pole
Equator
Alexandria
7°
Sun’s Rays
Syene
Video
Eratosthenes
a 7° is about 1/50th of a circle
a Alexandria and Syene are separated
by 800 km
 That 800 km must be 1/50th the
circumference of the Earth
 Earth Circumference = 800*50 = 40,000 km
Accepted value is 40,074 km
Relative sizes and distances
 By 350 BC Greek Natural
Philosophers knew the relative
diameters and distances of the
Sun, Moon, and Earth.
 The Sun was very large and very far away
and the Earth was bigger than the Moon.
Cosmologies
 Heliocentric
Model
 Aristarchus
of Samos
Cosmologies
 Geocentric Model
 Aristotle
350 BC
Stationary Earth
 Earth is heavy
 Easier to imagine the sky can move
 It looks like the sky is moving
 We have no sensation of our motion
 Rotating Earth would make
objects fly off of the surface
 Stellar Parallax
Stellar Parallax
 Hipparchus 150 BC
June
Sun
January
Why did Hipparchus fail to
observe Stellar Parallax?
0%
0%
0%
0%
1.
2.
3.
4.
He only looked at bright stars.
He was a poor observer.
The stars are too far away.
The Earth does not orbit the Sun.
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Aristotelian Universe
Terrestrial Realm
 Composition predicts motion
natural tendencies
Fire and Air tend to rise
Earth and Water tend to sink
 Overall tendency to seek rest
 Objects following tendencies require no force
 Objects are corruptible (changing)
Aristotelian Universe
Celestial Realm
 Celestial Objects composed of Aether
Self luminous but does not consume
 Motion is constant, circular
 Objects are incorruptible (not changing)
Meteors and comets were phenomena of the
Earth’s atmosphere
Link
Claudius Ptolemy (150 AD)
 Accounted for
retrograde motion
within the confines of
the Geocentric Model
The Sun and Moon
Earth
Moon
The Planets
Deferent
Epicycle
Planet
Earth
Link
Retrograde Loop in
Ptolemy’s System
Ptolemaic Universe
John Milton, Paradise Lost
With Centric and Eccentric scribl'd o're,
Cycle and Epicycle, Orb in Orb
Astronomy in the Dark Ages
 Fall of Rome
 Greek knowledge went to Islam
 Alexandrian library destroyed
 Universal Illiteracy
 No mass communications
 Villages were isolated
 Thomas Aquinas
 Giordano Bruno
End of Section