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
New MoonFull Moon 1 st Quarter
Earth (12,756 km) Moon (3476 km)
Lunar Eclipses Sun Earth Moon umbra penumbra
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
Equator North Pole Eratosthenes Sun’s Rays Syene Alexandria 7° Video
Eratosthenes a 7° is about 1/50 th of a circle a Alexandria and Syene are separated by 800 km That 800 km must be 1/50 th 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 Sun January June
Why did Hipparchus fail to observe Stellar Parallax? 10 1.He only looked at bright stars. 2.He was a poor observer. 3.The stars are too far away. 4.The Earth does not orbit the Sun.
Aristotelian Universe Terrestrial Realm Earth and Water tend to sink Composition predicts motion natural tendencies Overall tendency to seek rest Objects following tendencies require no force Objects are corruptible (changing) Fire and Air tend to rise
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 Earth Planet Epicycle Deferent 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
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