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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. 10 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