Early Cosmology: Greek and Pre-Renaissance Understandings of the Universe - Prof. Alberto , Study notes of Astronomy

Download Early Cosmology: Greek and Pre-Renaissance Understandings of the Universe - Prof. Alberto and more Study notes Astronomy in PDF only on Docsity! 1/31/2008 1 Lecture 2 : Early Cosmology Before cosmology Greek astronomy/cosmology The Renaissance (part 1) THIS WEEK: Read Chapter 2 of text © Sidney Harris CA RTO ON 1/31/2008 2 I : UN-AIDED OBSERVERS Imagine a time before satellites, planes, telephones, telescopes… What would you conclude about the world using just your own senses? Earth is at rest (i.e., motionless) Earth is flat Sun, Moon, planets, stars move in the sky (from East to West) Occasional bizarre things happen (comets, meteors) 1/31/2008 5 Eratosthenes Astronomer/mathematician in Hellenistic Egypt (c.275-195 BC) Calculated circumference of Earth Measured altitude of Sun at two different points on the Earth (Alexandria & Syene): found 7° difference Multiplied (360°/7°)×800km to obtain circumference=40,000km 1/31/2008 Ry OF LECAS DRL yeh ate repens | TATROWASE (CEYLGA 1/31/2008 7 Cosmology of Eudoxus and Aristotle Fundamental “principles”: Earth is motionless Sun, Moon, planets and stars go around the Earth: geocentric model Eudoxus (408-355 B.C.) & Aristotle (384-322 B.C.) Proposed that all heavenly bodies are embedded in giant, transparent spheres that revolve around the Earth. Eudoxus needed a complex set of 27 interlocking spheres to explain observed celestial motions E.G., need to have 24-hr period =day and 365-day period=year for the Sun 1/31/2008 10 Aristotle’s celestial physics Heavens are governed by different laws from Earth Celestial bodies are composed of “ether,” a fifth element not present on Earth “Natural motions” of celestial spheres are different from terrestrial motions: circular, constant, and eternal Aristotle needed 55 spheres to explain observed motions of Sun, Moon, planets, stars Space is finite, bounded by outer sphere But the edge is unreachable: motions become circular in the ethereal domain Time is infinite (But why is such a perfect universe centered on such an imperfect Earth?) 1/31/2008 11 Aristarchus of Samos (310-230 B.C.) Using eclipse data and geometry: Measured relative sizes of Earth, Moon Curvature of Earth’s shadow on Moon during lunar eclipse ⇒ REarth=4×Rmoon Measured distance to Moon (duration of eclipse)÷(1 month)= (2REarth )÷(circumf. of Moon’s orbit) Attempted to measure distance to Sun Need to measure (using time interval ratios) the angle of Sun when Moon is exactly at 1st or 3rd quarter Then use trigonometry and known Earth-Moon distance to get Sun’s distance Meaured angle was too small, but still concluded Sun was very distant from Earth (20×Moon’s distance) and larger than the Earth (5×Earth’s diameter) Note: true distance & size 20×larger duke.usask.ca/~akkerman/ gthought/ http://www.perseus.tufts.edu/GreekScience : ou S “Relative size of Earth and ae) Moon Ss pe” s: 1/31/2008 15 Ptolemy (100-170 A.D.) Worked at observatory in Alexandria, both as observer and theorist Developed theory to accommodate detailed planetary observations: Variations in brightness over months Retrograde motions Variations in observed orbital speed Theory involved motion along small circles superposed on top of motion along large circles (think Spirograph!) Successors added additional “epicycles” to model to improve agreement with observations Very complex! No underlying law; no predictive power for new planets 1/31/2008 16 Ptolemy’s epicycles Ptolemy’s original “epicycles” Larger circle (“deferent”) not centered on the Earth Motion appeared uniform from “equant” (offset from Earth and from center of “deferent”) Dept. of Physics and Astronomy Univ. of Tennesse Needed more and more epicycles to fit observed angular motion 1/31/2008 17 Renaissance cosmology, part 1 Pre-Renaissance Copernicus Tycho http://www.physics.fsu.edu/users/Lind/AST1002/ 1/31/2008 20 Copernicus’s first work Wrote and distributed to friends the Little Commentary (1514) Axioms: 1. There is no one center in the universe. 2. The Earth's center is not the center of the universe. 3. The Earth and planets revolve around the Sun. 4. The distance from the Earth to the Sun is imperceptible compared with the distance to the stars. 5. The rotation of the Earth accounts for the apparent daily rotation of the stars. 6. The apparent annual cycle of movements of the Sun is caused by the Earth revolving round it. 7. The apparent retrograde motion of the planets is caused by the motion of the Earth from which one observes. 1/31/2008 21Dept. of Physics and Astronomy, Univ. of Tennesse Portrait from 1597 by Teothor de Bry 1/31/2008 22 Copernicus’s de Revolutionibus Full title: De Revolutionibus Orbium Coelestium Printed Nuremberg, 1543, in last year of life Spelled out, using observations and mathematics, evidence for his heliocentric model Simple and natural explanation for retrograde motion of planets Included accurate relative spacings of planetary orbits Showed that planetary speeds decrease outward from Sun Still had to include some epicycles to improve agreement with observations Book was widely read and appreciated by 16th C. astronomers some believed in Copernicus’s heliocentric physical model others considered Copernicus’s approach superior for calculating orbits, but believed in geocentric Universe No official position by Catholic church up to 1600