Geocentric vs.
Heliocentric
A Battle for the Ages
Geocentric (Ptolemaic)
Model
• The accepted model for
•
•
•
•
1400 years
The earth is at the center
The Sun, stars, and
planets on their spheres
revolve around the earth: explains daily
movement
To account for unusual planetary motion
epicycles were introduced
Fit the Greek model of heavenly perfection –
spheres are the perfect shape, circular is the
perfect motion.
Ptolemaic
System
Mars
Jupiter
Ptolemy’s system provided
the intellectual framework
for all discussion of the
universe for nearly 1600
years!! So in a very true
sense,
this
idea
was
stupendously
successful
even though we now know
that it was incorrect.
Correction Added
Saturn
Earth & Moon
•Epicycles added to
epicycles
•Precision Timing for
Venus & Mercury
•Earth placed “off”
center.
Mercury
Sun
Venus
Off Center?!
Earth Equant
Epicycle
Deferent
The planet moves along its epicycle as the epicycle moves along the
deferent around the Earth. To make the observations as accurate as
possible, it was necessary to place the Earth slightly off center of the
orbits, but to preserve symmetry that meant that there was an equal
place (“Equant”) opposite the Earth from the center. The combined
motion of the planet and the resulting retrograde motion are shown.
The Revolution of Ideas
The Beginnings of the
Heliocentric Model
of the Universe
Heliocentric
(Copernican) System
• Sun at center (heliocentric)
• Uniform, circular motion
• No epicycles (almost)
• Moon orbited the earth, the earth orbited the
sun as another planet
• Planets and stars still on fixed spheres, stars
don’t move
• The daily motion of the stars results from the
Earth’s spin
• The annual motion of the stars results from the
Earth’s orbit
Copernicus
(1473-1543)
“De Revolutionibus Orbium Coelestium”
“On the Revolution of the
Heavenly Spheres”
1. Earth is not the center of “everything.”
2. All the Planets revolve around the Sun!
3. Stars are very much farther away than
the Sun
4. Any motion of the stars is a result of
the Earth’s rotation
5. Any movement of the Sun is due to Earth’s rotation
and revolution about the Sun.
6. Retrograde Motion was much easier to explain.
Retrograde Motion
Planet's
apparent path
around sky.
6
6
5
5
4
3
Sun
3
2
2
1
Earth
Outer
Planet
Earth overtakes slow outer planet so the
outer planet appears to slow down, move in
reverse, and then move forward again with
respect to the fixed stars.
4
1
Fixed Stars
http://upload.wikimedia.
org/wikipedia/commons/
6/65/Retrogradeorbit.gif
Changing Ideas is Not Easy.
Copernicus was no fool. He waited until his own
death before he published his new ideas.
On
March
5,
1616,
Copernicus'
work
was
banned from being taught
and discussed by the
Congregation of the Index
"until corrected." It stayed
on this list of prohibited
books and teachings until
1822!!!!
Change is Hard.
Copernicus
still
used
“perfect” circles for the
orbits.
Copernicus ended up
adding “epicycles!”
At the time his theory
was a complete disaster
at
“predicting”
the
positions of the planets
in the sky. Ptolemy’s
model “worked” much
better.
The Great
Compromise
Tycho Brahe
(1546-1601)
The Man with
the Gold Nose
Danish Nobleman
Experimentalist & Observer.
He actually made
measurements
Had his own observatory
built.
Made very careful measurements of star’s
positions
Tycho’s Observations
Tycho’s Nova! – A “new”
star appearing in the
constellation Cassiopeia.
Then diminished. (SN1572)
Heavens can Change!
A reason to doubt
Aristotle! Maybe
Ptolemy?
Tycho’s Observations
Made very precise
measurements of stellar
positions and planet
positions in the sky
Comet in 1577
is farther
away than
moon.
Still “measured” no
stellar parallax over an
entire year. Earth must
be stationary.
Tychonic Model
Tycho developed a system that
combined
the best of both worlds.
He kept the Earth in the center of
the universe, so that he could
retain Aristotelian physics and
Ptolemy’s geocentric idea.
The Moon and Sun revolved about the Earth, and
the shell of the fixed stars was centered on the
Earth.
But Mercury, Venus, Mars, Jupiter, and Saturn
revolved about the Sun.
Tychonic Model
Tycho’s conTribuTions
Lots of data! – Decades worth. He made the
most precise observations that had yet been
made by devising the best instruments available
before the invention of the telescope.
Decided by measurement and observation,
NOT BY DOGMA
His observations of planetary motion,
particularly that of Mars, provided the crucial
data for later astronomers
Tycho No More
History or Myth?
How did Tycho die?
A Model Reveals Itself
Geocentric vs. Heliocentric
Reaction of Contemporaries
Martin Luther (1483-1546): [Copernicus] “is a fool who
wishes to reverse the entire scheme of astronomy; but
sacred scripture tells us that Joshua commanded the
Earth to stand still, not the Sun.”
Conservative who wanted to reform Church by returning it to
a simpler, less corrupt time.
Agitator for the abolishment of indulgences (payment to
lessen time of deceased in purgatory).
A founder of the Protestant branch of Christianity.
Response of Catholic Church to threat posed by the
Reformation Movement:
At first, tolerance of dissent and liberal treatment of new
ideas.
Later, institution of the Counter-Reformation and the
Inquisition.
Giordano Bruno (1548-1600): burned at the stake for
advocating that stars are suns in their own right, and that
there is a plurality of worlds like the Earth.
Galileo (1564-1642):
Ah! Here is the person to decide the debate.
Galileo Galilei
(1564 – 1642)
"Eppur Si Muove“
"(And, yet it moves!")
1610 “Siderius Nuncius” (The Starry
Messenger)
“Spots” on the Sun!
The Moon has mountains, craters,
rocky surface with imperfections!
The “planet” Jupiter is not a
pinpoint star – but a disc in the sky!
WITH MOONS!
Venus has “PHASES” like the
MOON
Galileo Galilei (1564 – 1642)
“Revealing great, unusual and
remarkable spectacles, opening
these to the consideration of every
man, and especially of
philosophers and astronomers….”
SUNSPOTS
Galileo Galilei (1564 – 1642)
The Moon Has Mountains and Valleys
Galileo Galilei (1564 – 1642)
Phases of Venus
Galileo observed that Venus
showed phases entirely like
those of the moon from full to
crescent, which it must do if the
Copernican theory was correct.
According to the Ptolemaic
theory Venus would have to be
a perpetual crescent.
Galileo Galilei (1564 – 1642)
Jupiter Has Moons
Galilean Moons – 4 Largest “moons” of Jupiter
Io, Europa, Callisto, & Ganymede
Galileo Galilei (1564 – 1642)
1632: "Dialogo Dei Massimi Sistemi" (In Italian! Not Latin! For the common
people!) He published his masterpiece, Dialogue on the Two Chief World
Systems, in which he had two people, one representing the view of Ptolemy and
other the view of Copernicus, present their arguments before an intelligent layman.
Galileo of course gave the Copernican the brilliant best of the battle.
The Pope was persuaded that
Simplico, the character who
upheld the views of Ptolemy in
the book, was a deliberate and
insulting caricature of himself.
The book was all the more
damaging to those who felt
themselves insulted, because it
was written in vigorous Italian
for the general public (and not
merely for the Latin-learned
scholars) and was quickly
translated into other languages
-- including Chinese!
Galileo Galilei (1564 – 1642)
Scientific Martyr
Teaches Heliocentric Ideas
Claims “proof” for Earth’s motion.
Trial by Church (threatened with
instruments of torture), forced to recant
views (1633).
Confined to house arrest. Formulates
“new science” of mechanics.
Loses his sight (blinded) by Sun
observations
Most influential combination of
experimentalist and theorist world has
ever seen.
Often credited rightly with having
Still, there are problems to solve….
"...a sickly child, with thin limbs and a large,
pasty face surrounded by dark curly hair. He
was born with defective eyesight-myopia plus
anocular polyopy (multiple vision). His stomach
and gall bladder gave constant trouble; he
suffered from boils, rashes, and possibly from
piles, for he tells us that he could never sit still
for any length of time..."
Johannes Kepler’s Work
Advancing science, more than
ethics, Kepler simply stole the
observations upon the sudden
and ironic death of Tycho
Math skills - Kelper's skills
were extraordinary. He could
not reconcile Tycho's very
careful observations with the
Models of Ptolemy, Copernicus,
nor Tycho!!!
An Idea! Orbits are not
Circles!
His incredible effort took 29 years!! He
waged a "war" on understanding the orbit
of Mars.
Heliocentric Model fits the observations
best
from
a
simple
view
point
(Copernician) But *not* circular orbits.
Ellipses
Kepler’s Three Empirical
Laws of Planetary Motion
1. The orbital paths of the planets are
elliptical, with the Sun at one focus
Kepler’s Three Empirical
Laws of Planetary Motion
2. An imaginary line connecting the Sun to
any planet sweeps out equal areas in
equal time..
Area 1
Area 2
Kepler’s Three Empirical
Laws of Planetary Motion
3. The square of a planet’s orbital period
is proportional to the cube of its semimajor axis.
P2
(years)
= A3
(astronomical units)
1 Astronomical Unit = The Earth-Sun Distance
(98 million miles)
Kepler’s Third Law
Object
a (AU)
P (year)
a**3
P**2
Mercury
0.387
0.241
0.058
0.058
Venus
0.723
0.615
0.378
0.378
Earth
1.00
1.00
1.00
1.00
Mars
1.52
1.88
3.51
3.53
Jupiter
5.20
11.9
141.
142.
Saturn
9.54
29.5
868.
870.
Uranus
19.2
84.0
7,080.
7,060.
Neptune
30.1
165.
27,300.
27,200.
Pluto
39.5
248.
61,600.
61,500.
But!!…..
Kepler did not know
WHY
planet’s move as they do.
Isaac Newton (1642-1727)
Born in England on
Christmas day.
Bubonic Plague 1665?
While home for 2 years with
nothing to do he made his most
profound
discoveries
and
proposed his most startling
theories.
Time Marches On
In 1684 – was in a discussion with Edmund
Halley, when Halley remarked “But why do
planet’s move the way they do?”
Newton astounded everyone with the
answers ( from his work 20 years earlier!).
“Philosophiae Naturalis
Principia Mathematica”
Newton’s Three Laws of Motion
The laws explained not only why planets
move as they do, but why objects in
general move as they do.
Newton’s First Law
 Objects at rest stay at rest unless a “net
force” is applied.
 Objects in uniform straight line motion
stay in straight line motion unless a “net
force” is applied.
• Note: Newton’s First Law Does NOT say…
• This law does not say that every moving
object has a “force” acting on it!
• This law does not say an object at rest
has “no force(s)” acting on it!
Newton’s Second Law
The acceleration of an object is directly
proportional to the net force acting on the
object, is in the direction of the net force,
and is inversely proportional to the mass of
the object.
a = Fnet/m
OR
Fnet = ma
Newton’s Third Law
Whenever one object exerts a force
on a second object, the second
object exerts an equal and opposite
force on the first.
“To every action there is an equal and opposite re-action.”
Law of Universal
Gravitation
Matter attracts
matter
GM1M2
F
2
R
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Geocentric vs. Heliocentric