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Chapter 4
The origin of modern astronomy
Copernicus, Kepler, and Galileo
challenged the geocentric model
Copernicus (1473-1543):
•
•
Published heliocentric model (1543)
Used model to determine arrangement
of solar system
• Much simpler than geocentric!!
• Assumed circular orbits
But . . .
•
Heliocentric model was no more
accurate than geocentric model in
predicting planetary positions
Tycho Brahe (1546-1601)
•
Believed earth was at center
•
Compiled very accurate
measurements of star and planet
positions.
•
could not detect stellar parallax
•
Concluded earth is at center, but
planets orbit the sun
•
Hired Kepler, who used Tycho’s
observations to discover the truth
about planetary motion.
• Kepler tried to explain Tycho’s
observations with circular orbits
• But Mars’ orbit was not circular!
• Kepler found that it is elliptical.
Johannes Kepler
(1571-1630)
Ellipse
a
Kepler’s three laws of planetary motion
Kepler’s First Law: The orbit of each planet around
the Sun is an ellipse with the Sun at one focus.
a
Perihelion = a(1-e)
Aphelion = a(1+e)
Kepler’s Second Law: As a planet orbits the Sun, it
sweeps out equal areas in equal times.
a planet travels faster when it is closer to the Sun and
slower when it is farther from the Sun.
Kepler’s Third Law
Kepler’s Third Law
More distant planets orbit the Sun at slower speeds
3
a

1
2
p
p = orbital period in years
a = avg. distance from Sun in AU

1 AU = astronomical unit = 150 million km
= average earth-sun distance
Thought Question:
An asteroid orbits the Sun at an average distance
a = 4 AU. How long does it take to orbit the Sun?
A.
B.
C.
D.
4 years
8 years
16 years
64 years
Hint: Remember that p2 = a3
An asteroid orbits the Sun at an average distance
a = 4 AU. How long does it take to orbit the Sun?
A.
B.
C.
D.
4 years
8 years
16 years
64 years
We need to find p so that p2 = a3
Since a = 4, a3 = 43 = 64
Therefore p = 8, p2 = 82 = 64
Galileo Galilei (1564-1642)
Before Galileo, it was “obvious” that
1. Earth could not be moving because
objects in air would be left behind.
2. Non-circular orbits are not “perfect”
as heavens should be.
3. If Earth were really orbiting Sun,
we’d detect stellar parallax.
Parallax
If you can’t see it, is it because it is too small, or does not exist?
Do all moving objects eventually slow down
on their own?
• Galileo showed that objects will stay in
motion unless another object acts on them
(Newton’s first law of motion).
What about “heavenly perfection”?
Tycho’s observations of comets and a supernova
already challenged this idea.
Using his telescope, Galileo saw:
What about the lack of stellar parallax?
• Tycho thought he knew the distances to stars, so lack
of parallax meant Earth stands still.
• Galileo showed stars must be much farther than
Tycho thought — used his telescope to show that the
Milky Way is countless individual stars.
•He could still not measure parallax!
Galileo discovered four
large moons orbiting
Jupiter
Venus has phases similar to the Moon!
Galileo’s observations of phases of Venus proved that it
orbits the Sun and not Earth.
in 1633, the Catholic Church
ordered Galileo to recant his
claim that Earth orbits the
Sun
His book on the subject was
removed from the Church’s
index of banned books in
1824
Galileo Galilei
Galileo was formally
vindicated by the Church in
1992
• How did Copernicus, Tycho and Kepler
challenge the Earth-centered idea?
– Copernicus created a sun-centered model
– Tycho provided the data needed to improve this
model
– Kepler found a model that fit Tycho’s data
• Kepler’s three laws of planetary motion
– 1. The orbit of each planet is an ellipse with the Sun
at one focus
– 2. As a planet moves around its orbit it sweeps our
equal areas in equal times
– 3. More distant planets orbit the Sun at slower
average speeds: p2 = a3
• What was Galileo’s role in solidifying the
Copernican revolution?
– His experiments and observations overcame the
remaining objections to the Sun-centered solar system
3.4 The Nature of Science
• How can we distinguish science from
non-science?
• What is a scientific theory?
• Cognitive bias
• Defining science can be surprisingly difficult.
• Science from the Latin scientia, meaning “knowledge.”
The idealized scientific method
•
Based on proposing and
testing hypotheses
•
hypothesis = educated guess
But science rarely proceeds in this idealized
way… For example:
• Sometimes we start by “just looking” then
coming up with possible explanations.
• Sometimes we follow our intuition rather
than a particular line of evidence.
Hallmarks of Science: #1
Modern science seeks explanations for
observed phenomena that rely solely on
natural causes.
(A scientific model cannot include divine intervention or
other supernatural explanations)
Hallmarks of Science: #2
Start with the simplest possible model:
make predictions, tests and refinements.
(Simplicity = “Occam’s razor”)
Hallmarks of Science: #3
A scientific model must make testable
predictions about natural phenomena that
would force us to revise or abandon the
model if the predictions do not agree with
observations.
“No other explanation” does NOT prove
anything!
What is a scientific theory?
• “Theory” has a different meaning in science than
in everyday life.
• In science, a Theory is NOT just a good idea!
• A scientific Theory must:
—Explain a wide variety of observations with a few
simple principles, AND
—Must be supported by a large, compelling body of
evidence.
—Must NOT have failed any crucial test of its validity.
Thought Question:
Which of the following is true of Darwin’s Theory of
Evolution?
A.
B.
C.
D.
Scientific opinion is about evenly split as to whether
evolution really explains the diversity of life on Earth.
Scientific opinion runs about 90% in favor of the theory
of evolution and about 10% opposed.
After more than 100 years of testing, Darwin’s theory
has successfully met every scientific challenge to its
validity.
In a court of law, there would still be “reasonable doubt”
about whether evolution is true: Judicial proof is much
harder to achieve than scientific proof.
A. Scientific opinion is about evenly split as to
whether evolution really happened.
B. Scientific opinion runs about 90% in favor of the
theory of evolution and about 10% opposed.
C. After more than 100 years of testing, Darwin’s
theory has successfully met every scientific
challenge to its validity.
D. There is no longer any doubt that the theory of
evolution is absolutely true.
Correlation does not prove
causation
• Events may occur at the same time, but how
are they related, if at all?
• The hypothesis: Smoking causes lung
cancer
• More accurate: Smoking dramatically
increases one’s chances of developing lung
cancer
Anecdotes vs. Studies
• Anecdotal evidence: “My grandmother
smoked a pack a day for 60 years, she lived
to be 75.”
• Scientific study: Monitor the health of 1000
smokers and non-smokers for 20 years.
Compare rates of lung cancer, longevity,
etc.
Power of suggestion
Have you heard claims that vaccines can
sometimes cause autism in children? Which of
the following is true:
1. Studies show that there is a slight risk of
developing autism due to vaccines but the
benefit outweighs the risk
2. Studies have shown that there is no cause-andeffect relationship between vaccines and autism
3. Studies into the proposed links between autism
and vaccines have not yet been conducted
What have we learned?
• How can we distinguish science from non-science?
– Science seeks explanations that rely solely on natural
causes
– progresses through development and testing of models of
nature
– models must make testable predictions
• A scientific theory
– A model that explains a wide variety of observations in
terms of a few general principles
– Has survived repeated and varied testing
– Subject to modification as new data and observations are
acquired
3.5 Astrology
Our goals for learning:
• How is astrology different from astronomy?
• Does astrology have any scientific validity?
How is astrology different from
astronomy?
• Astronomy is a science focused on learning about
how stars, planets, and other celestial objects
work.
• Astrology is a search for hidden influences on
human lives based on the positions of planets and
stars in the sky.
Does astrology have any scientific
validity?
• Scientific tests have
shown that astrological
predictions are no more
accurate than we should
expect from pure
chance.
What have we learned?
• How is astrology different from astronomy?
– Astronomy is the scientific study of the universe
and the celestial objects within it.
– Astrology assumes that the positions of celestial
objects influence human events.
• Does astrology have any scientific validity?
– Scientific tests show that the predictions of
astrology are no more accurate than pure chance.