Lecture 6 Chapter 3 The Copernican Revolution From the last lecture Question !"#$%&#$'($)"*+"$("#$&,,-$.*/#/$0#1#-0/$,-$*(/$ 1"'/#$21"'/#$,3$("#$&,,-$0#1#-0/$,-$("#$ .#4'%5#$1,/*%,-/$,3$("#$67-8$9,,-$:$;'.("<$•! If the Sun sets at 6pm, when does a full Moon rise? •! 6 pm. The side of the Moon we see if facing the Sun, so when the Sun is setting, the Moon is rising. •! Another way of looking at it: •! The moon and Sun rising times are 12 hrs apart. Sun rose at 6 am ! Moon at 6pm. chapter 2 Learning Goals 1 Describe how some ancient civilizations attempted to explain the heavens in terms of Earth-centered models of the universe. 2. Explain how the observed motions of the planets led to our modern view of a Sun-centered solar system. 3. Describe the major contributions of Galileo and Kepler to our understanding of the solar system. 4. State Kepler's laws of planetary motion. 5. Explain how astronomers have measured the true size of the solar system. 6. Explain how the law of gravitation enables us to measure the masses of astronomical bodies. chapter 2 Goal 1 Learning Goals 1 Describe how some ancient civilizations attempted to explain the heavens in terms of Earth-centered models of the universe. 2. Explain how the observed motions of the planets led to our modern view of a Sun-centered solar system. 3. Describe the major contributions of Galileo and Kepler to our understanding of the solar system. 4. State Kepler's laws of planetary motion. 5. Explain how astronomers have measured the true size of the solar system. 6. Explain how the law of gravitation enables us to measure the masses of astronomical bodies. 2.1 Ancient Astronomy • Ancient civilizations observed the skies • Many built structures to mark astronomical events Summer solstice sunrise at Stonehenge: Alignment Astronomy Egyptian pyramids. •! Use circumpolar stars (“Indestructibles”) to align the base of the pyramids due north. This was associated with eternity in the afterlife. Stonehenge. •! Main axis is aligned with the rising of the Sun on the summer solstice. Birth of Astronomy •! Need to understand day, night, seasons (hunter-gatherers, agriculture) •! !"#$%&'()"%"*+&%(,-+-.*(!(/.0".+-.(-1(23"()&%".4&' Birth of Astronomy •! !"#$%$&'()*+,)-$.'*#)-"*(&+() •! Celestial objects were considered divine, some events like eclipses were believed to be omens announcing the fate of countries and their rulers.) Predictive Astronomy: Mayans Mayans didn’t understand fractions, BUT measured the lunar cycle and solar tropical year precisely - (lunar cycle repeats every 149 new moons in 4400 days) 4400 days/149 = 29.5302 days ! less than 0.0004 difference with modern day value! Solar tropical year: 365.2422 Mayan measurement: 365.2420! The recognition and identification of events is different from understanding their causes This is not science What Is Science? “careful, disciplined, logical search for knowledge, obtained by examination of the best available evidence and always subject to correction and improvement upon discovery of better evidence” 1.! Unprejudiced 2.! Falsifiable 3.! Explain current observations and predict new ones. Pseudo-science 1.! Non falsifiable (meaning the theory CANNOT be proved right or wrong by available data) OR 2.! It doesn’t explain all observations, results are not reproducible ABOUT SCIENCE Multiple hypothesis can equally well explain observations, how do we choose? 1.! If we are lucky, predictions from one will prove it wrong, if not 2.! Use Ockham’s Razor (“keep it simple”) Ockham’s Razor “entities should no be multiplied unnecessarily” ABOUT SCIENCE For example: A.! Planets move in ellipses with the Sun in one focus because of an attractive force between the Sun and the planets B.! Planets move in ellipses with the Sun in one focus because of an attractive force between the Sun and the planets generated by powerful aliens Which one will you choose? ABOUT SCIENCE Both explain the motion of the planets, but we choose A A.! Planets move in ellipses with the Sun in one focus because of an attractive force between the Sun and the planets B.! Planets move in ellipses with the Sun in one focus because of an attractive force between the Sun and the planets generated by powerful aliens WHY? ABOUT SCIENCE Ockham’s Razor: Keep it simple! •! B is not discarded because: –! it sounds absurd –! we don’t like it or –! we don’t believe in the existence of alien beings •! B is discarded because the statement about aliens is unnecessary to describe the observe motion of the planets. •! That planets move in elliptical orbits does not provide evidence in favor or against the existence of powerful aliens. Scientific Models •! Physical model to explain the workings of nature •! Models apply known laws of nature to explain observations •! Key aspects of a scientific model !! models explain what is seen !! models predict observations accurately !! simplify your understanding of nature •! Validity of models is tested by checking how well predictions fit the best & new observations •! Scientific models are not static but evolve when new & better observations become available Scientific Models •! Physical model to explain the workings of nature •! Models apply known laws of nature to explain observations •! Key aspects of a scientific model !! models explain what is seen !! models predict observations accurately !! simplify your understanding of nature •! Validity of models is tested by checking how well predictions fit the best & new observations •! Scientific models are not static but evolve when new & better observations become available The Greeks and Astronomy had a major role in the birth of modern science Modeling the Cosmos: Early Greeks •! Greeks enjoyed philosophy which to them meant the attempt to understand all things in nature. •! Tools: Direct observation, highly developed mathematical skills (geometry & trigonometry), and logic. •! Starting point: Idea of Earth as center was simple and reinforced philosophical and religious systems that taught the unique role of humans in the universe. •! Any model of the cosmos will have to explain: –! Motion of the Sun –! Motion of the stars –! Motion of the Moon –! Motions of the 5 planets visible to the unaided eye: Mercury, Venus, Mars, Jupiter & Saturn Modeling the Cosmos: Early Greeks •! Greeks enjoyed philosophy which to them meant the attempt to understand all things in nature. •! Tools: Direct observation, highly developed mathematical skills (geometry & trigonometry), and logic. •! Starting point: Idea of Earth as center was simple and reinforced philosophical and religious systems that taught the unique role of humans in the universe. •! Any model of the cosmos will have to explain: –! Motion of the Sun –! Motion of the stars –! Motion of the Moon –! Motions of the 5 planets visible to the unaided eye: Mercury, Venus, Mars, Jupiter & Saturn 2.2 The Geocentric Universe Ancient astronomers observed: Sun Moon Stars Five planets: Mercury, Venus, Mars, Jupiter, Saturn Greek science was preserved through the dark ages by muslim astronomers Naked Eye Planets Mercury •! Closest to the Sun •! Fastest moving planet God of commerce Messenger to the Gods Venus •! “The evening star” •! Brightest object in the sky Goddess of love Earth? Sun and Moon Mars has large variations on brightness Mars •! The red planet. Mars, God of War by Diego Rodriguez de Silva y Velazques Jupiter •! Largest planet Abduction of Ganymede by Rubens Jupiter and Callisto by Rubens The rings were not known to the ancients Saturn •! Planet with the largest ring system Saturn (ca. 18th century) - Arabic illustration showing agricultural activities under direction of Saturn, God of agriculture? Planets, Gods & Days of Week •! English names for most of the days of the week come from names by Teutonic- tribes from Germany •! Tuesday : Tiw - god of war •! Wednesday: Woden - god of day & night •! Thursday: Thor - god of thunder - head god •! Friday: Fria - goddess of spring Aristotle & A Spherical Earth Painting of •! Aristotle (384-322 B.C.) showed by Aristotle by proof that the earth was spherical: Rembrandt 1! He observed that the Earth’s shadow is curved during a lunar eclipse “The shapes that the Moon itself shows are of every kind -straight, gibbous, and concave - but in eclipses the outline is always curved: and since it is the interposition of the Earth that makes the eclipse, the form of this line will be caused by the form of the Earth’s surface which is therefore spherical” Aristotle’s treatise On the Heavens Composite image of successive pictures taken during a lunar eclipse showing the Earth shadow. Aristotle & A Spherical Earth 2! Aristotle learned from travelers that the height of the Pole star above the horizon varies as you travel from North to South •! Going North: Polaris gets higher with respect to the horizon •! Going South: Polaris gets lower with respect to the horizon •! Go far enough south -- Polaris no longer visible !!Earth must be spherical! Flat Earth Spherical Earth Aristotle & Geocentric Model •! He pointed out that the motions in the sky could be explained –! by a rotating celestial sphere or –! a rotating Earth, but he rejected the idea of a moving Earth The Geocentric Paradigm Below is an image of the universe as conceived of by Aristotle and Ptolemy.! The earth is composed of four elements:! Earth, Water, Fire and Air.! It is in the center (notice the trees, mountains and water).! In this cosmology, the earth does not revolve around anything else or rotate around its own axis.! The Geocentric Paradigm BelowIt is is ansurrounded image of the universe by as tenconceived concentric of by Aristotle spheres and Ptolemy. !made of a perfectly transparent substance known as "quintessence."! The earth is composed of four elements:! Earth, Water, Fire and Air.! These spheres revolve around the earth, carrying the other celestial bodies.! It is in the center (notice the trees, mountains and water).! In this cosmology, theAs earth youdoes not can revolve see, around one anything is theelse or sphere rotate around "of its own the axis.