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Chapter 2
The Copernican Revolution
Units of Chapter 2
2.1 Ancient Astronomy
2.2 The Geocentric Universe
2.3 The Heliocentric Model of the Solar System
The Foundations of the Copernican Revolution
2.4 The Birth of Modern Astronomy
Units of Chapter 2, continued
2.5 The Laws of Planetary Motion
Some Properties of Planetary Orbits
2.6 The Dimensions of the Solar System
2.7 Newton’s Laws
The Moon is Falling!
2.8 Newtonian Mechanics
Weighing the Sun
2.1 Ancient Astronomy
• Ancient civilizations observed the skies
• Many built structures to mark astronomical
events
Summer solstice
sunrise at
Stonehenge:
2.1 Ancient Astronomy
Spokes of the Big Horn Medicine Wheel are
aligned with rising and setting of Sun and
other stars
2.1 Ancient Astronomy
This temple at
Caracol, in Mexico,
has many windows
that are aligned with
astronomical events
2.2 The Geocentric Universe
Ancient astronomers
observed:
Sun
Moon
Stars
Five planets: Mercury,
Venus, Mars, Jupiter,
Saturn
2.2 The Geocentric Universe
Sun, Moon, and stars all have simple
movements in the sky
Planets:
• Move with respect to
fixed stars
• Change in brightness
• Change speed
• Undergo retrograde
motion
2.2 The Geocentric Universe
• Inferior planets: Mercury, Venus
• Superior planets: Mars, Jupiter, Saturn
Now know:
Inferior planets have
orbits closer to Sun
than Earth’s
Superior planets’
orbits are farther
away
2.2 The Geocentric Universe
Early observations:
• Inferior planets never too far from Sun
• Superior planets not tied to Sun; exhibit
retrograde motion
• Superior planets brightest at opposition
• Inferior planets brightest near inferior
conjunction
2.2 The Geocentric Universe
Earliest models had Earth at center of solar
system
Needed lots of
complications to
accurately track
planetary motions
The Geocentric Theory
• The earth is
located at the
center of the
universe and all
the planets
revolve around
the earth.
The Geocentric Theory
• The Geocentric theory was believed by
the Catholic church especially because the
church taught that God put earth as the
center of the universe which made earth
special and powerful.
• The idea of the Earth actually moving was
widely felt as a foolish suggestion
because, as they saw it, if the Earth was
moving they would be able to feel it.
Aristotle (384-322 B.C.)
• Developed an early model
•
based on the concept of
uniform circular motion. He
placed the earth at the center
of the universe and all of the
planets, sun and stars around
it.
When Aristotle lived, if a
person could “reason” out why
something happened, then you
didn’t need to do any
experiments to see what would
happen.
• In the realm of change, the
natural motion of earthy
materials was to seek the center
of the universe.
• This is why Aristotle placed the
earth at the center of the
cosmos. This is also his
explanation for why objects fall
when dropped. A dropped
object is just following its
natural tendency to seek the
center of the universe.
• Aristotle reasoned that if the earth rotated
about its axis, we should fly off into
space. Since we don't, the earth must be
stationary.
• It would be almost 1900 years before
Galileo introduced the concepts of gravity
and inertia that explain why these effects
are not observed even though the earth
does move.
Greek Contributions
The Ancient Greek philosophers refined
astronomy, changing it from merely
observational science into a full-blown
theoretical science.
Greek Contributions
The ancient astronomers used astronomy to
track time and cycles, for agricultural
purposes, as well as adding astrology to their
sophisticated observations.
Rather than generating astronomical theories,
they were more concerned with what was
happening and when, but without addressing
the why, other than vague attributions to Gods
and astrology.
Ptolemy (140 A.D.)
• Ptolemy advanced
the geocentric
theory in a form
that prevailed for
1400 years.
• He added
mathematics to
support the theory
Ptolemy’s Problem
• Many supporters of the
geocentric theory had one
piece of evidence they
couldn’t explain – the
movement of MARS.
• Let’s take a break and
investigate this movement.
Mars’ Motion
http://csep10.phys.utk.edu/astr161/lect/retrograde/retrograde.html
How did Ptolemy Explain this
Problem?
• Ptolemy used geometric models to predict
the positions of the sun, moon, and
planets, using combinations of circular
motion known as epicycles.
• An epicycle is an orbit within an orbit
• Having set up this model, Ptolemy then
went on to describe the mathematics
which he needed in the rest of the work.
Ptolemy’s
Model –
Epicycles
Included
2.3 The Heliocentric Model of the
Solar System
Sun is at center of solar system. Only Moon
orbits around Earth; planets orbit around Sun.
This figure
shows
retrograde
motion of
Mars.
The Heliocentric Theory
• The Sun is the center of our solar system
Copernicus (1473-1543)
• Polish astronomer who
advanced the theory that the
Earth and other planets
revolve around the Sun. This
was highly controversial at
the time.
• The Ptolemaic model had
been widely accepted in
Europe for 1000 years when
Copernicus proposed his
model.
Nicolaus Copernicus
• In 1543,Copernicus
published On the
Revolutions of the
Heavenly Spheres
which argued that the
sun, rather than the
Earth, stood at the
center of the universe
and that the planets
revolved around the
sun
Nicolaus Copernicus
• Copernicus’s idea harmonized much
better with observed data than did
Ptolemy’s, but it was not warmly received
• If Copernicus was right, then the Earth
was just another planet and human beings
were not the center of the universe
• At the time, this had serious religious
ramifications
• The Church became so
angry – the Geocentric
theory made human
beings seem closer to God
and since earth was in the
center that meant
humans were more
special. The heliocentric
theory changed that
perspective completely,
making humans lose that
position in the universe.
2.4 The Birth of Modern Astronomy
Telescope invented around
1600
Galileo built his own, made
observations:
• Moon has mountains and
valleys
• Sun has sunspots, and
rotates
• Jupiter has moons (shown):
• Venus has phases
Discovery 2-1: The Foundations of
the Copernican Revolution
1. Earth is not at the center of everything.
2. Center of earth is the center of moon’s orbit.
3. All planets revolve around the Sun.
4. The stars are very much farther away than the
Sun.
5. The apparent movement of the stars around the
Earth is due to the Earth’s rotation.
6. The apparent movement of the Sun around the
Earth is due to the Earth’s rotation.
7. Retrograde motion of planets is due to Earth’s
motion around the Sun.
2.4 The Birth of Modern Astronomy
Phases of
Venus cannot
be explained by
geocentric
model
2.5 The Laws of Planetary Motion
Kepler’s laws were
derived using
observations made by
Tycho Brahe
Tycho Brahe
Tycho Brahe
Tycho Brahe created extensive and detailed
observations of the positions of the planets, all
with his naked eye!
At Uraniborg - part observatory, part palace Brahe's quadrant that allowed him to measure
the positions of planets and stars was 2
metres high and let him measure to an
accuracy of 1/360th of a degree - the size of a
baseball seen from a mile away.
Tycho Brahe
Later Brahe realised that the building wasn't
stable enough, and he built a new part of his
observatory underground, with an armillary
sphere even larger than the quadrant - a huge
structure of metal rings that let him measure
the precise positions of objects in any part of
the sky. (A quadrant would only allow
measurements of things that were due south.)
Tycho Brahe
In 1572, Tycho Brahe observes a brilliant
supernova (now called SN 1572). At the time,
Brahe believed it to be a comet.
This supernova proves that it is traveling
beyond Earth's atmosphere and therefore
provides the first evidence that the heavens
can change.
Brahe was the man!
How Kepler Got His Start…
Brahe's precise measurements laid the
foundation for a new understanding of the motion
of the planets.
German astronomer Johannes Kepler contacted
Brahe at the end of the sixteenth century in an
effort to obtain copies of the Danish astronomer's
research. Brahe countered with a suggestion that
Kepler could work as his assistant, helping him to
compile his data.
How Kepler Got His Start…
However, Brahe proved more tightfisted
than Kepler had anticipated and refused to
share his measurements of the planets and
their orbits.
Instead, he suggested Kepler work on
solving the Mars dilemma that plagued
astronomers.
How Kepler Got His Start…
Kepler, using Brahe's detailed observations,
realized that the planets moved around the sun
not in circles but in stretched out circles known
as ellipses. However, the problem took him
almost a decade to solve, and Kepler didn't
publish it until well after Brahe's death.
Although Brahe's family intended to reap as
much financial gain as possible from Brahe's
observations, Kepler, by his own admission, lessthan-ethically acquired them after Tycho died.
Johannes Kepler (1571-1630)
• Johannes Kepler
used mathematics to
demonstrate that
planetary orbits are
elliptical, not circular
as in the Ptolemaic
theory
Kepler’s First Law
• The orbits of the planets are ellipses, with the sun
at one focus of the ellipse.
Kepler’s Second Law
• The line joining the planet to the sun sweeps
out equal areas in equal times as the planet
travels around the ellipse.
Kepler’s Third Law
• The ratio of the squares of the revolutionary
periods for two planets is equal to the ratio of
the cubes of their semimajor axes:
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.
2.5 The Laws of Planetary Motion
1. Planetary orbits are ellipses, Sun at one focus
2.5 The Laws of Planetary Motion
2. Imaginary line connecting Sun and planet
sweeps out equal areas in equal times
2.5 The Laws of Planetary Motion
3. Square of period of planet’s orbital motion
is proportional to cube of semimajor axis
More Precisely 2-1: Some
Properties of Planetary Orbits
Semimajor axis and eccentricity of orbit
completely describe it
Perihelion: closest approach to Sun
Aphelion: farthest distance from Sun
2.6 The Dimensions of the Solar System
Astronomical unit: mean distance from
Earth to Sun
First measured during transits of Mercury
and Venus, using triangulation
2.6 The Dimensions of the Solar System
Now measured using radar:
Ratio of mean
radius of Venus’s
orbit to that of
Earth very well
known
Galileo (1564-1642)
• An Italian scientist, Galileo
was renowned for his
contributions to physics,
astronomy, and scientific
philosophy. He is regarded
as the chief founder of
modern science.
• Galileo was condemned by
the Catholic Church for his
view of the cosmos based
on the theory of
Copernicus.
Galileo Galilei’s Telescope
• Galileo used the
telescope to
observe spots on
the sun and moon
– Discredited the
Ptolemaic notion that
the heavenly bodies
are smooth,
immaculate,
unchanging and
perfectly spherical
Galileo’s drawing of the
moon showing craters
Galileo Galilei
• Other achievements:
– Noticed four of the moons
that orbit Jupiter
– Observed previously
unknown distant stars
– Meant universe is much
larger than previously
suspected
– Showed that the velocity of
falling bodies depends not
on their weight but on the
height from which they fall
Galileo’s telescope
However, Galileo was unable to clearly resolve Saturn’s rings!
Galileo’s Books
• Galileo published his discoveries and support
for the Copernican model in two books
published in 1616 and 1632.
• Galileo was unusual for the time because he
wrote in Italian rather than Latin like most
scholars.
• Galileo also took great pains to make his books
interesting often writing them in the form of
dialogues rather than dry, boring dissertations.
• After his first book, "Starry Messenger", was
published he was warned by the Church not to
publicly support Copernicism again.
Galileo Galilei
• In 1632, he published
his Dialogue
Concerning the Two
Chief World Systems
which compares the
Copernican and
Ptolemaic systems
– Found guilty of heresy
by the Spanish
Inquisition and spent
the rest of his life under
house arrest
Galileo’s Dialogue
Trial Before the Inquisition
• Galileo abided by this edict until 1632 when
he published "A Dialogue on the Two Chief
World Systems". This book's outright support
for the Copernican model and its ridiculing of
the Ptolemaic model earned Galileo a trial
before the Inquisition.
• Galileo was accused of heresy and sentenced
to house arrest for life. However, he got off
easily compared to fellow Italian Giordano
Bruno who was burned at the stake in 1600
for teaching Copernican ideas.
• In 1992, the Roman
Catholic Church finally
repealed the ruling of
the Inquisition against
Galileo. The Church
gave a pardon to Galileo
and admitted that the
heliocentric theory was
correct. This pardon
came 350 years after
Galileo's death.