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AST101H5 Lecture Notes - Ptolemy, Nicolaus Copernicus, Ellipse

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John Lester

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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
- Models also contain our assumptions based on our beliefs, not facts, without
- 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”)
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