Origin of Modern Astronomy

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Origin of Modern Astronomy

• Earth's place in the cosmos
• The nature of planet motion
• Addressing these two key issues led to the
  Copernicus revolution and the birth of science .

2
How did the ancients make sense of what they saw
and believed?

• The heavens must be perfect
• The uniform and circle motion is perfect
• The stars appear fixed
• The Sun rises and sets daily
• The Moon shows phases
• Five planets wander with respect to the stars
  retrograde motion, but move roughly along the
  ecliptic

3
The Old Astronomy Geocentric Universe
Sphere of prime mover
Aristotle (Greek, 384-322 BC), based on ideas
proposed by Plato and Eudoxus (400-347 BC)
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"Common Sense"

• If the Earth actually spun on an axis, why didn't
  objects fly off the spinning Earth?
• If the Earth was in motion around the sun, why
  didn't it leave behind the birds flying in the
  air?

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• Two Major Problems
• "Retrograde Motion"
• Varying brightness

Ptolemy Solution add a special fix epicycle or
circle on circle.
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Ptolemic Model
Ingenious but wrong!
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The Copernican RevolutionThe Heliocentric System

• Sun is at center
• Earth orbits like any other planet, while the
  moon orbits the earth.
• Retrograde motion occurs when we lap Mars the
  other superior planets.

Nicolai Copernicus (1473-1543).
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Retrograde Motion and Varying Brightness of the
Planets
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Ptolemy vs. CopernicusWho is Right?

• Both postdict positions and retrograde motion
  about equally well.
• Copernican model seems to defy common sense
  (moving Earth).
• Copernican model is somewhat simpler, and
  sometimes models are preferred for their
  simplicity or elegance (Occams razor)
• What we really want is a test that disagrees with
  one or the other.

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The Phases of Venus

• According to the heliocentric model Venus should
  go through all phases.
• According to the geocentric model, Venus should
  always be between the Earth and Sun and always a
  crescent

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The Observed Phases of Venus

• Through a telescope, heres what we see over the
  course of several months

Copernicus was right!
12
Copernican Revolution

• Copernicus challenged the geocenter assumption,
  but not the perfect circular motion assumption.
• His idea of the heliocentric universe remained
  rather obscure for about 100 years after his
  death.
• Later work of Kepler, Galileo, and Newton would
  build on the idea and produce the revolution that
  would sweep away completely the thoughts of
  Aristotle and replace them with the modern view
  of astronomy and natural science.

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Tycho Brahe (1546-1601)

• Most accurate pre-telescopic observations of
  positions
• Charted positions of planets
• Very combativelost the bridge of his nose in a
  duel with his professor over a fine mathematical
  point.
• Constantly fought with his graduate assistant,
  then died mysteriously of mercury poisoning

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Johannes Kepler (1571-1630)

• Ran off with Brahes notes the day after he died.
• Thought Copernicus was right
• Brahe thought Earth must be stationary.
• Realized how he could use the planetary positions
  hed helped measure to learn how planets moved.

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Johannes Kepler (1571-1630)

• He used the planetary positions he and Tycho
  Brahe had accurately measured to learn the motion
  of planets.
• He shows that epicycles do not work!

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The Laws of Planetary Motion

• The orbits of the planets are ellipses, with the
  Sun at one focus of the ellipse.
• Planets move proportionally faster in their
  orbits when they are nearer the Sun.
• More distant planets take proportionally longer
  to orbit the Sun

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Calculations Using Kepler's Third Law
The semimajor axis to the third power is
proportional to the square of the revolutionary
period of a planet a(AU)3P(years)2
As an example, the length of the semimajor axis
of the Mars orbit is aP2/3(1.88)2/31.52 AU
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Galileo Galilei
Galileo used newly invented telescope to
observe the heaven and interpret his findings. He
discovered that our Moon has craters, that
Jupiter has it's own moons, that the Sun has
spots, that Venus has phases like our Moon, and
many more discoveries. These discoveries
confirmed the Copernican hypothesis that the
Earth was just another planet.
(1564-1642), Italian
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Galileo and the Concept of Inertia

• Aristotle held that objects at rest remained at
  rest unless a force acted on them, but that
  objects in motion did not remain in motion unless
  a force acted constantly on them.

Galileo concluded that an object in a state of
motion possesses an inertia'' that causes it to
remain in that state of motion unless an external
force acts on it.
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Should a heavy object fall faster than a light
one, as Aristotle believed?
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Sir Isaac Newton and the Unification of Physics
Astronomy

• Newton was by many standards the most important
  figure in the development of modern science.
• He demonstrated that the laws that governed the
  heavens were the same laws that governed motion
  on the surface of the Earth.
• Newton's Three Laws of Motion.
• Theory of Universal Gravitation

(1642-1727)
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Birth of the modern science

• All happened within a 100 year period
• Came with change in attitude of thinking during
  renaissance --- social context affects science

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Sir Isaac Newton and the Unification of Physics
Astronomy
What Really Happened with the Apple?
More importantly, if the force of gravity
reaches to the tree, might it not reach even
further? in particular, might it not reach all
the way to the orbit of the Moon?!
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Proportional and Inversely-Proportional

• Proportional When A goes up, so does B
• Inversely Proportional When A goes up, B goes
  down

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Road Trip(inverse proportionality!)

• If it takes you 2 hours to drive from Amherst to
  Boston at 60 mph, how long would it take to bike
  it, if you can bike at 15 mph?

tD/V tdriveD/Vdrive tbikeD/Vbike tbike/tdriveV
drive/Vbike tbiketdrive x Vdrive/Vbike 2 x
60/158 hours