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Lecture 5

Astronomy 1021 Lecture Notes - Lecture 5: Nicolaus Copernicus, Axial Precession, Star Catalogue

Course Code
ASTR 1021
Robert Cockcroft

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The Ancient Greeks
- Eratosthenes: shadows, Earth’s circumference
- Aristotle: geocentrism
- Aristarchus: sizes and distances for/between Sun, Moon, Earth; suggested heliocentrism
- Hipparchus: star catalogue; brightness system; axial precession
- Ptolemy: improved Aristotle’s geocentric ideas
The Copernican Revolution
- Five main characters:
- Nicolaus Copernicus
- 1542 published “On the Revolution of the Heavenly Spheres”
- Heliocentric and not geocentric Universe
- Pros:
- Mercury and Venus close to Sun
- Changing brightness of planets
- Seasons explanation
- Cons:
- Still used circles
- Offset needed
- No more accurate than Ptolemy?
- Tycho Brahe 
- 30 years of naked-eye accuracy
- Parallax
- Nearby objects have a larger parallax than more distant objects
- Parallax can be used to measure distances
- Against core Aristotelian / Ptolemaic system
- Johannes Kepler
- Analyzed Tycho’s data
- Orbits are not circles, but ellipses
- Trust data over preconceived beliefs
- First law:
- Planets move in elliptical orbits
- Sun is at one focus of the ellipse
- No object at other focus

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- Second law:
- Sun-planet line sweeps out equal areas during equal intervals of
- Planet moves faster when closer, and slower when further
- Conservation of angular momentum (Ch 5)
- Third law:
- More distant planets orbit the Sun at slower average speeds
- Planets obey the precise mathematical relationship
- p2 = a3
- p is the planet’s orbital period in years
- a is the average distance from the Sun in AU
- Orbits will be close to circular
- Planets will either have too much speed or too little speed:
- Summary:
- First law: orbits are ellipses
- Second law: planets move faster in their  orbits when
they’re closer to  the Sun
- Third law: more distant planets move  more slowly
- Kepler more accurate than Ptolemy
- Kepler’s laws are only observations
- Doubts:
- Aristotle: if Earth moves birds, falling stones and clouds would be
left behind
- Aristotle: heavens must be perfect and unchanging
- Stellar parallax ought to be detectable if Earth orbits the Sun
- Galileo Galilei
- Galileo used the telescope to show that the heavens were not perfect
- Mountains and valleys on the Moon (not a perfect sphere)
- Sun spots on the sun
- Milky way is made of stars*
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