Download Intro to Solar System

Scientific Models
Geometric Ideas
Physical Concepts
Aesthetic Notions
Basic Assumptions
add Analogy
Metaphor
A MODEL !!
Intro to Solar System
Scientific Models
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Scientific Models
• explain what is observed
(i.e. measured)
• predict future observations
(accurately)
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Scientific Models
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Scientific Models
• explain what is observed
(i.e. measured)
• predict future observations
(accurately)
Scientific models evolve, they change
with new information, they are
tentative.
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Scientific Models
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Scientific Models
• state bases
– aesthetic
– geometric
– physical
•
•
•
•
state the assumptions
Zeilik - 6/e
state key observations
deduce important relationship of the parts
suggest predictions
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Scientific Models
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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.
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Scientific Models
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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
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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
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Scientific Models
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Aristotle’s Physics
Elements
earth
air
fire
water
Zeilik - 6/e
Each had a natural motion towards its
natural place.
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Scientific Models
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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
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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.
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Scientific Models
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Two very important observations
• spherical shape of the Earth
• absence of stellar parallax
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Scientific Models
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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
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If the Earth moved through space,
stellar parallax would be observed.
No stellar parallax was observed.
Therefore, the Earth did not move
through space.
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Scientific Models
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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
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Scientific Models
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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
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Scientific Models
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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
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Scientific Models
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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
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Scientific Models
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• Observation:
– planet moves west to east for several
weeks
– then it slows, reverses its motion to east to
west
– retrograde motion
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• 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
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Scientific Models
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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
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Scientific Models
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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
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