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Chapter 3

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University of Toronto Mississauga
John Lester

Chapter 3 - From Observing to Understanding (Sept. 19) Lecture Notes - Look for regular behavior - seasons - Look for trends - hours of daylight increase and decrease with seasons - Look for exceptions - eclipses do not occur every month - Trust our brains - Use common sense - “not magic” The Greek “Model” - Greek astronomers (2000+ years ago) were not great observers - They applied logic to understand their observations - “Model” = idea/concept, how we imagine or picture an object in our mind - they let us get close to our subject Earth’s Shape/Size - Observational evidence on the shape/size: - lunar eclipses are always curved - merchants traveling from Greece down into Africa saw different stars - The observational evidence cannot be understood if the Earth is flat Cosmic Calculation of Earth’s Size - angle/360 degrees = distance/(pi)D earth - Therefore... D earth = distance x 360 degrees / (pi) x angle Planetary Motion - We observe the Sun, Moon, planets ... moving around us = Earth each day - We feel no motion - Conclusion: Astronomers pictured Earth as stationary with everything orbiting around - “Geocentric” Model = Earth at the centre - Wrong, but believed for about 1500 years Complication - Models also contain our assumptions based on our beliefs, not facts, without realization - Example: Greek astronomers assumed everything beyond Earth must be perfect - A circle is the perfect shape, therefore all orbits must be circular - WRONG Sept. 21 Retrograde Motion - Again - Model #2 - conceived by Claudius Ptolemy in year 150 - Geocentric + each planet is assumed to have a circular orbit around Earth - Each planet moves on a small circular orbit (“epicycle”) that move along a large circular orbit (“deferent”) - Retrograde on the “inside” of the epicycle orbit End of the Geocentric Model - Ptolemy’s model was a great success in the year 150 - From 150 onwards, more and more observations of the planets were made - In the 13th century, a team of astronomers needed 10 years to calculate an update -----> Explanations that are too complicated are probably wrong. Look for a new model. Copernican “Revolution” - Nicholas Copernicus recalled that Greek astronomers had tried a heliocentric model - Consider this idea again - let Earth orbit or revolve --> revolutionary - This much simpler explanation “won” even though it was not more accurate - Note: it was still assumed that the orbits are circular Tycho and Kepler - The heliocentric model needed work - Tycho Brahe made the best observations possible before the telescope - accurat
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