Transcript File

The center of the
Solar System:
Heliocentric
Model vs.
Geocentric Model
Aristotle (384-322 B.C.)
• Established the Geocentric view
– States that the Earth is the center of all
things and the Sun, moon and planets
revolve around it
– Works for the Sun and moon,
– Sucks for the planets
• Can’t explain:
– Their changing brightness and
– Retrograde motion:
» Appear to stop and move backwards in the
sky during the year
A Geocentric Solar System
Retrograde Motion of the
Planets as seen from Earth
Aristarchus of Samos (310230 B.C.)
• Greek, born 12 years after Aristotle died
• First to propose the Heliocentric model,
• Never accepted in his lifetime… century
or millennium!!
• Why: Aristotle was more influential, and
famous with more followers and writings.
Questions Aristarchus couldn’t
answer
1. Why doesn’t the earth feel like it’s
moving? Shouldn’t there be some sort of
wind or force as we move at high speed
around the Sun, like a car speeding
around a track?
• Ans.: The earth isn’t moving through
anything in space, so there is no wind
resistance acting on the planet as it
moves,
• Secondly, Earth’s gravity overpowers any
‘centrifugal force’ (inertia) we would feel
from our revolving motion
Questions Aristarchus
couldn’t answer
2. If Sun is center of the solar system’s
gravity, why don’t things fall UP, towards
the Sun, instead of down, towards the
center of Earth?
• Ans.: The effects of gravity become
exponentially weaker the farther away
you are from an object, so since we are
so close to the Earth and so far from the
Sun, we don’t feel the Sun’s gravity,
although it does contribute to the tides in
the oceans
Questions Aristarchus
couldn’t answer
3. If the vantage point of earth changes, why
don’t we see stellar parallax?
• Ans.: Even the nearest stars are so far away
that the stellar parallax we see is very small
• Similar to looking down a long straight road at
a far away mountain
• Close your right eye, and note its position,
then create parallax by opening the your right
eye and closing your left. If you do this with
your finger out in front of your face, your
finger moves compared to the background,
but the mountain is too far away and appears
to stay still
Claudius Ptolemaeus
(Ptolemy) –
around 140 A.D.
• Greek, 400 yrs. after Aristotle
• Made the best Geocentric Model
• “The Ptolemaic Model”
• Problem: very complex
• Had 80 circles to explain the sun,
moon and 5 planets known then!
Ptolemy made “epicycles” to
explain retrograde motion of
planets
Occams’ razor (Text - p.21)
• Named after English philosopher:
William of Ockam
• If two theories explain the facts and
make the same predictions, the simpler
one is better. Keep it simple!
• Good theories have no more complexity
than is absolutely necessary.
Nicholas Copernicus
(1473-1543)
• 2,000 years after Aristotle and Aristarchus
• Copernican Revolution –The change from a
belief in a Geocentric model to a
Heliocentric model
• Copernicus liked Heliocentric model
because it’s simpler:
– “…it is more pleasing to the mind”,
– It also explained some small inconsistencies in
the Geocentric theory.
A Drawing of the Heliocentric
model made by Copernicus
Nicholas Copernicus
• The only major flaw in his version of the
heliocentric model was that he used circular
orbits when we now know the planets travel in
elliptical orbits
• But that might have been because the orbit of
Mars is very close to a perfect circle around
the Sun.
• His ideas were never accepted in his lifetime
either,
• But with the observations of some later
astronomers, the world eventually did “revolve
around” to the correct understanding. 
Galileo Galilei (1564-1642)
• Italian born 21 years after Copernicus died
• A key to his success:
– He embraced cutting edge technology
and techniques of his day like
• Experimentation over logical reasoning and
• The telescope
– Invented in Holland around 1600, Galileo was
already around 36 years old
– He heard about it (never had seen one!) and built
one for himself.
Galileo’s discoveries of Jupiter’s moons
with his telescope showed that Earth was
not the center of all orbits strongly
supported a heliocentric model even though
Copernicus’ model was no more accurate than Ptolemy’s.
Galileo also observed
‘blemishes’ on the Sun
(sunspots): This
contradicted ‘perfect
spheres’ idea of
Aristotle.
Galileo’s discoveries of Venus’ phases
with his telescope showed that Venus
must orbit the Sun strongly supported a
heliocentric model
even though
Copernicus’ model was no more accurate than Ptolemy’s.
Galileo’s discoveries of Venus’ phases with his
telescope showed that Venus must orbit the Sun
strongly supported a heliocentric model
Venus can only go through phases if it orbits the Sun.
Galileo Galilei
• Some key observations he made in that
trusty telescope of his:
1. Mountains, valleys and craters on the Moon,
2. The sun has imperfections - sunspots, which
he watched rotating
3. 4 moons of Jupiter (if other planets have
moons, earth is not the center of all things)
4. Venus goes through phases like the moon,
which can only be explained by the
heliocentric theory (pic on pg.30)
Johannes Kepler (1571-1630)
• German, 7 years younger
than Galileo
• Poor eyesight
– So he based his laws on Tycho
Brahe’s observations
• Discovered 3 simple laws
that accurately described the
planets’ motion in the
heliocentric theory
Johannes Kepler
• Called: “The 3 Laws of Planetary Motion”
• Kepler’s Laws pretty much sealed the lid on
the Geocentric model’s coffin and allowed
most scientists to accept the Heliocentric
model.
– Especially because they followed the Occam’s
razor principle (simple)
• They work for every planet in our solar
system, even the ones that Kepler never
knew about!
Kepler’s
model for
the Solar
System: A
Concentric
Series of
Crystalline
Spheres
Kepler’s Three laws
 1st Law: All orbits are
Elliptical with Sun at
one focus (circular
orbits are special case of
elliptical with e = 0)
 2nd Law: Planets
sweep out equal
areas in equal time
 3rd Law: P2 = a3 (P in
yrs, a in AU)
The Copernican Revolution
9/13/2005
Kepler’s
Friend and
Boss:
Tycho Brahe
Tycho Brahe
• (1546-1601) Dutch, 25 years older than
Kepler and 18 years older than Galileo)
• Arguably the best observational
astronomer of all time
• Didn’t have the telescope yet, that was
invented a couple decades later, (around
the time he died, actually)
• So he had to essentially just use his eyes
and kept careful records and passed
them on to Kepler when he died.
Tycho Brahe
• Met Kepler when he moved to Germany (He
ticked off a bunch of the aristocracy in Denmark)
• Kepler was a great mathematician, so Brahe
(rich boy) paid him to work with his observations
and try to make sense of them with him.
• After Brahe died, Kepler spent most to the
remaining 29 years of his life carrying on Brahe’s
legacy and making sense of his observations
(resulted in the 3 laws of planetary motion).
• An amazing example of scientific collaboration
and the fulfillment of a promise from one friend to
another.
Isaac Newton
• (1642-1727) British
• Born the same year that Galileo died
• Finally explained WHY the earth orbits the
Sun.
• Before him Galileo, Brahe and Kepler
explained WHAT the solar system looked
like, but couldn’t say WHY it looked that
way.
• Newton came up with…
– The 3 laws of motion
– The law of Universal Gravitation
– and helped invent Calculus to explain WHY.
Isaac Newton
• The law of gravity says that all masses
attract each other and that the more mass
an object has the greater the attraction it
has.
– Explains why the earth and planets are pulled
towards the Sun, but why don’t they get
pulled INTO the Sun??
Isaac Newton
• The first law of motion is the law of inertia
– An object tends to maintain its current state
of motion
• In both speed and direction
– When the solar system formed, the Sun
“caught” giant masses in space, which are
now the planets.
– Those masses moving through space also
wanted to maintain their forward motion
through space because of their inertia, so
they ended up moving around the Sun
instead of stopping and sitting still in space.
Isaac Newton
– The planets continue to move because there
is no friction in space to slow them down
– If gravity was suddenly “turned off”, they
would fly off in a straight line pointing out
from their orbit because of their inertia.
• ball on a string example
• So to sum up:
–Gravity keeps
the planets
pulled inward
• towards the
Sun
–Inertia keeps
them moving
outward,
• results in a
circular path
Isaac Newton Quote
• “If I have been able to see farther than
others it is because I stood on the
shoulders of giants.”
--- Newton’s letter to Robert Hooke,
• probably referring to Galileo and Kepler
Another Sweet Quote…
• “ I know not what I appear to the
world, but to myself I seem to
have been only like a boy playing
on the sea-shore, and diverting
myself in now and then finding a
smoother pebble or a prettier seashell, whilst the great ocean of
truth lay all undiscovered before
me.”
Reviewing the Lesson:
• In-Class: textbook pg.40-41 Conceptual
Self-test #’s 1-5, 7, 9, and 10
• HW’s: pg.40, “Review and
Discussion” #1-5, 8, & 14
Lesson Review Time!
(Text p.40-41 Conceptual Self-test
#’s 1-5, 7, 9, and 10)
#1. (T/F) Aristotle was the first
to propose that all planets
revolve around the Sun.
Lesson Review Time!
(Text p.40-41 Conceptual Self-test
#’s 1-5, 7, 9, and 10)
#2. (T/F) During retrograde
motion, planets actually stop
and move backwards in
space.
Lesson Review Time!
(Text p.40-41 Conceptual Self-test
#’s 1-5, 7, 9, and 10)
#3. (T/F) The heliocentric
model of the universe holds
that the earth is the center of
the universe.
Lesson Review Time!
(Text p.40-41 Conceptual Self-test
#’s 1-5, 7, 9, and 10)
#4. (T/F) Copernicus’ theories
gained widespread
acceptance during his
lifetime.
Lesson Review Time!
(Text p.40-41 Conceptual Self-test
#’s 1-5, 7, 9, and 10)
#5. (T/F) Galileo’s observations
of the sky were made with the
naked eye
Lesson Review Time!
(Text p.40-41 Conceptual Self-test
#’s 1-5, 7, 9, and 10)
#7. (T/F) Kepler’s laws hold
true only for the 6 planets
known in his time.
Lesson Review Time!
(Text p.40-41 Conceptual Self-test
#’s 1-5, 7, 9, and 10)
#9. A major flaw in Copernicus’
model was that it still had:
A) The Sun at the center
B) Earth at the center
C) retrograde loops for the planets
D) circular orbits for the planets
Lesson Review Time!
(Text p.40-41 Conceptual Self-test
#’s 1-5, 7, 9, and 10)
#10. A accurate sketch of Mar’s orbit
around the Sun would show:
A) The Sun far off center
B) An oval twice as long as it is wide
C) A nearly perfect circle
D) Phases