Lecture 4 Astronomy: Announcements Ancient Cosmology to Kepler and Galileo • Today: The Copernican Revolution • March Ch. 4; + additional material (Scientists in Timeline) Observations of the sky • Extra reading (Optional) for the interested in history of astronomy : Thomas Kuhn, “The Copernican Revolution” Astrology ? Astronomy • Next Time Pseudoscience Science • Newton puts it all together: The 3 Laws Physics The law of gravitation Horoscopes • Read March Ch 2-4 Fortune Telling . . . . . Timeline Today Middle “Modern” Asia, Egypt Greece, Rome Renaissance Ages Physics • What do we observe in the sky? Mesopotamia Al-Khawarizmi Copernicus • Sun, Moon, Stars, Planets • Ancient Observations - which are still useful! 0 -1000 1000 2000 Ptolomy Aristotle Galileo Pythagorus Newton • Ancient Cosmologies - facts or invention? Euclid Kepler • How does an esoteric topic like the motion of five tiny bright points in the sky lead to divergent • Problem of the Planets (Wanderers) world views? • The strange motion of the planets has led to • The ancient astronomers and the Renaissance two competing world views giants like Copernicus, Brahe, Kepler, and Galileo • Astronomy searches for explanations in simple laws - leads made observations and analyses that determine to new science how we think about our place in nature -- and how • Astrology treats the motion as somehow related to life on we apply “universal laws” to the universe! earth - leads to fortune telling, horoscopes, …. What are the astronomical objects What do we observe in the sky? that dominate our lives? • Sun, Moon, Stars in eternal, regular motion • Sun - appears to go around the earth once per • From a point in the Northern Hemisphere, the day in westerly direction - path changes in a regular way, repeating every year stars appear to move as shown: • Moon - appears to go around the earth slightly faster than sun - so it “laps’ the sun each 28 days – a lunar month • Stars - “millions” all appear to go around the earth together in regular paths slightly faster than the sun – eternal, unchanging! • Determines the calendar • Year -- Sun • Month -- Moon • Week -- phases of the moon • Day -- Sun 1
Lecture 4 In the Beginning . . . Example of description of the cosmos) Ancient Cosmology: Babylon, Egypt, ... Hesiod (8th Century B.C.) HEAVEN • Physics • Up and down are defined - sets the order of things up Conclusion: space not the same in all directions. • Earth is at center. OCEAN • Meaning EARTH • Each component is important to people • The explanation is purely poetic and emotional • Methods • No supporting evidence for the two conclusions above TARTARUS • No TESTABLE implications mentioned (Abyss below Hades) Classical Greece Anaximander (6th century BC) Pythagorus and followers sun (5th Century B.C.) (distances in Earth thicknesses) • Great advances in mathematics – especially geometry stars moon North pole 3 9 • Systematic Arguments for a Spherical Earth 9 Rotation 9 and other bodies - moon, sun • “Higher” Principle: A Sphere is the most How’s this differ perfect shape possible -- the most symmetric from Hesiod? EARTH Anything strange? AT CENTER • Observation: See next slides Spheres turning, Hot on Outside, Cold on Inside What observations indicate that the earth is spherical? • In a lunar eclipse, the shadow of the earth on the moon is like that of a sphere Moon SUN Earth Shadow Earth Appearance of Moon during lunar eclipse 2
Lecture 4 Classical Greece Measuring the earth 4th - 3rd Century B.C. Eratosthenes, 4th Cent. BC (Aristotle lived 384-322 B.C.) Librarian of the great library at Alexandria Long shadow Sun • Determined the radius of the earth! (Eratosthenes) • The distance to the moon and sun! (Hipparchus and Aristarchus) Short shadow DEMONSTRATION • How did they do that ??? • Shadows depend upon • North-South Location Similarly , position of stars depend upon location How Good Was the Measurement Observations that give important clues of Eratosthenes? • (Note: All the equalities given in the following are approximate!) • On a day when the sun was directly overhead Earth Moon at Syene (far southern Egypt) m • The angle at Alexandria (5000 stadia north) was 7.2 degrees, 1/50 of a full circle M (Homework) • So the circumference of the earth must be 50 x 5000 stadia = 250,000 stadia • The apparent angle of the moon gives M/m = 120 • Roughly 5% less than today’s accepted value! • The apparent angle of the sun also gives S/s = 120 around 24,000 miles, 40,000 km • (Radius = Circumference /2 π ) • How can you show that the sun is much farther than the moon? (S >> M)? How can you show that the sun is How large is the Moon? How Far? much farther than the moon? (S>>M) • (also due to Aristarchus) • In a lunar eclipse, the time the moon is in the shadow of the earth depends on the moon’s size & distance. earth observer Θ • Observation: At the moon the earth’s shadow is very nearly twice the diameter of the moon S M m Moon sun Right triangle Θ s e SUN half-moon Earth Shadow Earth • Aristarchus (250 BC) found Θ ~ 3 degrees, or diameter = 2 m about 1/100 of a full circle • So M/S = 2 π / 100 or S/M = 100/ 2 π or S is about 20M. X S M • But we still do not know M or S ! 3
Lecture 4 Aristarchus’ Calculation and results Aristarchus’ Conclusions m Moon • Diameter of Moon = 1/3 Diameter of Earth s e • Modern result: closer to 1/4 SUN • Truly an achievement in 3rd Century B. C. ! Earth Shadow Earth diameter = 2 m • Also Aristarchus found s = 20 m, so s = 7e X • So sun’s VOLUME is 7 x 7 x 7= 350 times Earth’s! S M • We already know S/s = M/m with S >> M • Not bad, but Sun is really much farther and much bigger (s=110 e). = X + M = X + M + S S M X • New observation: ~ = 2m e s s m • How many Earth’s would fit in Sun? X = 2M • Red equations • Is this little Earth the center of the Universe? = 3M m = e 2M • Finally 2m e 3 Measurement of distance to Moon Summary of the Advanced Astronomy of Classical Greece • Hipparchus (Homework) • Science of Classical Greece 5th - 3rd Centuries B.C. • Among many achievements: • Spherical Earth • Celestial Sphere of stars • Description of motion of sun, moon • Actual measurements of the sizes & distances of the earth, moon & sun • Culmination in the work of Aristotle (384 - 322 B.C.) and others ---- and finally Ptolomy (150 AD) Earth Centered Model of Exercise Sun, Moon, Stars (Ptolomy) • We now “know” that 1. The earth rotates on its axis 2. The earth revolves about the sun sun 3. The moon revolves about the earth North pole moon • How do we “know” ? • Can one prove just from observations Rotation on the earth that: • The earth revolves about the sun? EARTH But yet: stars AT CENTER • The moon revolves about the earth? What is the evidence for and against? A step backward? 4
Lecture 4 The Copernican Revolution From astronomy to gravitation Tycho Brahe, 1546-1601 • Science Proceeds in great revolutions • Actual measurements on minute details • Motion of the 5 planets Sir Isaac Newton, 1643-1727 • Observation over thousands of years • Proposal of conceptual models • Drawing conclusions that are TESTABLE Nicolaus Copernicus, 1473-1543 by experiments • Bold conclusions leading to general principles Claudius Ptolemy, 85-165 Johannes Kepler, 1571-1630 • Occurred in the renaissance • Greatly aided by the printing press and technological inventions Egypt Galileo Galilei, 1564-1642 Problem of the Planets Motion of Sun, Moon, Planets along the “Zodiac” • The model of the universe as the sun, moon, and a sphere containing the stars explains motion of “millions & millions” of stars. But fails for five • Sun moves through the constellations points of light, the wanderers: Mercury, Venus, • Observe directly by the position of the stars at Mars, Jupiter & Saturn. sunrise and sunset • The main motion is similar to the sun moving westward with the stars, but slightly slower. Relative to the stars, they move eastward along the “Zodiac”. • These are the “anomalies” that ultimately led to a revolution in our understanding of the universe. Problem of the Planets Problem of the Planets • The motion of each planet - Mercury, Venus, Mars, Jupiter & • The motion of each planet - Mercury, Venus, Mars, Saturn - follows a different path at a different speed along Jupiter & Saturn - follows a different path at a the “Zodiac” different speed along the “Zodiac” • Their speed varies and sometimes they move backward! • Their speed varies and sometimes they move backward! What is the importance for humans? 5
Recommend
More recommend
