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Scientific Models Geometric Ideas Physical Concepts Aesthetic Notions Basic Assumptions add Analogy Metaphor A MODEL !! Intro to Solar System Scientific Models 1 Scientific Models • explain what is observed (i.e. measured) • predict future observations (accurately) Intro to Solar System Scientific Models 2 Scientific Models • explain what is observed (i.e. measured) • predict future observations (accurately) Scientific models evolve, they change with new information, they are tentative. Intro to Solar System Scientific Models 3 Scientific Models • state bases – aesthetic – geometric – physical • • • • state the assumptions Zeilik - 6/e state key observations deduce important relationship of the parts suggest predictions Intro to Solar System Scientific Models 4 If a model proves to work well (i.e. is robust), it is elevated to theory Theory is NOT a guess !! Hypothesis is an educated guess. Theory is a synthesis of a large body of information, containing hypotheses that have been verified many, many times over. Intro to Solar System Scientific Models 5 Pythagoras - 6th century B.C. • Earth has a spherical shape • stars are in a spherical shell which rotates from east to west • planets are on smaller spheres, rotating at different speeds, east to west • Earth is motionless in the center geo-centric model: Earth-centered Intro to Solar System Scientific Models 6 Plato - 400 B.C. • must be uniform motion - ideal • model must explain the observations Aristotle - 350 B.C. • tried to add in retrograde motion of planets • included some notion of physics Intro to Solar System Scientific Models 7 Aristotle’s Physics Elements earth air fire water Zeilik - 6/e Each had a natural motion towards its natural place. Intro to Solar System Scientific Models 8 The natural motion of earthly material - towards the center at rest Heavens - immutable, crystalline, and did not obey the same laws The natural motion of the Heavens - rotation Intro to Solar System Scientific Models 9 Aristotle’s explanations were natural. Earth is stationary. If it were not so, things thrown into the air would land behind you. Things do not do so. Therefore the Earth is stationary. Intro to Solar System Scientific Models 10 Two very important observations • spherical shape of the Earth • absence of stellar parallax Intro to Solar System Scientific Models 11 Two very important observations • spherical shape of the Earth • absence of stellar parallax parallax: apparent shift in position of a body due to the motion of the observer stellar: Intro to Solar System pertaining to stars Scientific Models 12 If the Earth moved through space, stellar parallax would be observed. No stellar parallax was observed. Therefore, the Earth did not move through space. Intro to Solar System Scientific Models 13 Aristarchus - 3rd Century B.C. • heliocentric model (sun-centered) • Earth rotated on its axis once a day • Earth revolved around the Sun – 1 complete revolution = 1 year Intro to Solar System Scientific Models 14 Aristarchus - 3rd Century B.C. • heliocentric model (sun-centered) • Earth rotated on its axis once a day • Earth revolved around the Sun – 1 complete revolution = 1 year Disbelief in this model was based on • Earth moving at all was unnatural • required stellar parallax, which was not observed Intro to Solar System Scientific Models 15 Hipparchus - 150 B.C. added geometric devices assumption: uniform speed, circular paths • Observation: average speed of a planet is faster through one region of the zodiac than another Intro to Solar System Scientific Models 16 Hipparchus - 150 B.C. added geometric devices assumption: uniform speed, circular paths • Observation: average speed of a planet is faster through one region of the zodiac than another Explanation: eccentric - earth is just a bit off-center of the planet’s circular orbit Intro to Solar System Scientific Models 17 • Observation: – planet moves west to east for several weeks – then it slows, reverses its motion to east to west – retrograde motion Intro to Solar System Scientific Models 18 • Observation: – planet moves west to east for several weeks – then it slows, reverses its motion to east to west – retrograde motion Explanation: planet moves on a small circle, called an epicycle center of the epicycle orbits the Earth on a path called the deferent Intro to Solar System Scientific Models 19 Ptolemy - 125 A.D. model that could predict planetary motions with 5o error • observation: variable speeds Explanation: equant, place at which the motions would appear uniform aligned the radii of the epicycles Intro to Solar System Scientific Models 20 Physics: heavenly spheres rotated due to natural motion - this drives the other motions - no forces involved Success: within 5o accuracy in use for 1400 years agreed with the Greek notions of aesthetics Intro to Solar System Scientific Models 21