Copernican Revolution
~1500 to ~1700
Copernicus (~1500)
Brahe
(~1570)
Kepler
(~1600)
Galileo
(~1600)
Newton
(~1670)
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The Issue: Geocentric or Heliocentric
Which model explains observations the best?
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Copernicus (~1500)
Resurrected heliocentric model
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Is solar system geocentric or heliocentric?
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Both models have:
Circular orbits (heavenly perfection)
Uniform motion (heavens cannot change)
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Both models explain observations…
Sun, Moon, Planets
Rise in east, Set in west
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Both models explain observations…
Retrograde motion
of planets
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Retrograde motion – Geocentric
Planets ride on epicycles
Sometimes appear
to move backwards
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Retrograde motion – Heliocentric
One planet overtakes
another planet.
Slower planet appears to
move backwards.
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Brahe (~1570)
Measured positions of Mars
very accurately.
Built excellent instruments.
Used data to investigate truth.
Saw a supernova! (the sky changed)
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Kepler (~1600)
Derived (from Brahe’s data)
Three mathematical laws
of planetary motion.
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Kepler’s First Law
Planets have elliptical orbits around the Sun
<
aphelion
perihelion
>
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Kepler’s Second Law
The radius vector sweeps out equal areas in equal
times.
<
>
Thirty
days
Thirty
days
Radius
vector
>
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Kepler’s Second Law means that…
A planet changes speed;
faster at perihelion, slower at aphelion
Perihelion
(faster)
Aphelion
(slower)
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Definition needed for
Kepler’s Third Law
Period = Time to complete one cycle
Ex: Earth’s period of rotation =?
Ex: Earth’s period of revolution = ?
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Kepler’s Third Law
Period2 is proportional to (Radius of orbit)3
Sometimes written as T2 α R3
Bigger orbit radius means more time to revolve
More distant planet takes more time to revolve.
More distant planet moves slower.
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Kepler’s Laws
Also applies to objects in orbit around any central mass
•Satellites around Earth
•Star around a black hole
•Planet around some other star
•Two stars orbiting each other
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Meanwhile…
Out in the streets…
Galileo (~1600)
Developed and used the telescope
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Galileo observed…
Moon craters
Heavens are not perfect
!!!Heavens are Earth-like!!!
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El Greco < 1600
Cigoli - 1622
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Frontpiece of
G.B. Riccioli's
Almagestum
Novum,
1651.
Notes:
Urania,
Models (C’s, R’s, P’s),
R’s model has Jupiter and Saturn
centered on Earth,
Solar scope.
Galileo observes…
Sunspots
Heaven is not perfect!!
Sun rotates
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Galileo observes…
Moons of
Jupiter
Earth is
not the
ONLY
center of
motion in
the
heavens
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Galileo observes…
Moons of
AND
Jupiter
Take that, Aristotle!
Hey, Aristotle.
Earth can move
and pull the
Moon along.
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Galileo observes Phases of Venus
•Geocentric model predicts:
Crescent phase only and little size change
•Heliocentric predicts:
All phases AND
gibbous is small while crescent is large
http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/PhasesofVenus.htm
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Newton (~1700)
HOW and WHY planets orbit
(and objects move)
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Newton (~1700)
Three Laws of Motion
(Read but not on test)
And
The Universal Law of Gravity
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Newton - Universal Gravity
Every mass exerts a force on every other mass.
Note the UNIVERSAL statement.
F
m
F
M
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Newton - Universal Gravity
F
m
F
M
Gravity always attracts
Bigger mass means bigger force
Bigger separation means smaller force
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Newton - Universal Gravity
F
m
F
D
M
mM
FG 2
D
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Newton - Universal Gravity
Side comments:
Predictability, Mechanistic universe
F
m
F
D
mM
FG 2
D
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M
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Newton - Universal Gravity
Side comments (not on test):
Predictability, Mechanistic universe
Chaos theory
F
Why gravity?
m1
F
D
mM
FG 2
D
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m2
Copernican Revolution - Summary
Geocentric Vs. Heliocentric
Five people, contributions, significance
Process of science
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