The Solar System
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The Solar System
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The solar system
Pluto
Neptune
Uranus
Saturn
Jupiter
Mars
Earth
Venus
Mercury
Sun
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Sun
Planets
Asteroids
Comets
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Historical figures in the Copernican Revolution
Ptolemy – the geocentric model, that the Earth is at
rest at the center of the Universe.
Copernicus – published the heliocentric model.
Galileo – his observations by telescope verified the
heliocentric model.
Kepler – deduced empirical laws of planetary motion
from Tycho’s observations of planetary positions.
Newton – developed the full theory of planetary orbits.
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The Copernican Revolution
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Nicolaus Copernicus
The Earth moves, in two ways.
• It rotates on an axis (period = 1 day).
• It revolves around the sun (period = 1
year).
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Where is this?
Inscription: By reforming astronomy he initiated
modern science.
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The heliocentric model
The reasons for seasons – the Earth travels around the sun,
and its axis of rotation is tilted by 23.5 degrees to the plane
of the orbit. In July, the northern hemisphere is getting more
sunlight than in January. ISP 209 - 3A
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HEAVEN
HELL
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The Copernican Model
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Galileo Galilei
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Galileo’s discoveries in Astronomy
Galileo did not invent the telescope, but when he heard
about the invention he figured out how to build
telescopes. He had the best telescopes of his time. He
was the first person to use telescopes to look at the
heavenly bodies. He was first, so he made the
discoveries.
 Craters and mountains on the moon
 Moons of Jupiter
 Phases of Venus
 The Milky Way consists of innumerable stars.
 Sunspots move across the sun’s face.
All of these favored the Copernican (heliocentric)
model.
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Galileo’s sketch of the
moon as seen from his
telescope
A photograph of
the moon
“What do you think of the foremost philosophers of this university?
In spite of my oft repeated efforts and invitations, they have
refused, with the obstinacy of a glutted adder, to look at the
planets
or Moon or my telescope.”
(letter to Kepler)
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Galileo made many specific discoveries in
astronomy, through observations by telescope.
The general implications…
● The planets are objects like the Earth – masses –
rather than some kind of special heavenly objects. Or,
to put it another way, the Earth is just another planet.
● The planets, including Earth, travel around the sun.
● The Universe is a lot bigger than we can see by the
naked eye.
These ideas could be dangerous.
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The Trial of Galileo
His Church decided that the heliocentric model was a heresy.
But Galileo continued to argue in favor of the theory, and
published his reasoning. The Inquisition in Rome accused him
of heresy, and required him to answer.
Found guilty of “suspicion of heresy,” Galileo recanted
publicly. [A story, probably not true, is that he muttered
“Eppur si muove” – “And yet, it does move.”]
He was put under arrest and sentenced to prison. But because
of his fame and age (70 years old at that time) he was allowed
to serve under house arrest in Florence, for the remaining
years of his life.
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Is there a conflict between Science and Religion?
There is no major conflict today, but there have been
conflicts in the past, e.g. the trial of Galileo. Another
example is the conflict between Darwin’s theory of
evolution and the literal interpretation of the Book of
Genesis ().
 That latter conflict survives today in
the controversy about teaching
Creationism in public schools.
Pope John Paul II ordered a review of the trial
of Galileo. Did the Inquisition make mistakes?
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Their work was finished in 1992.
On October 31st, Cardinal Poupard delivered the final report during a papal
audience. The fifth paragraph reads:
“Certain theologians, Galileo’s contemporaries, being heirs of a unitary concept of
the world universally accepted until the dawn of the seventeenth century, failed to
grasp the profound, non-literal meaning of the Scriptures when they described the
physical structure of the created universe. This led them unduly to transpose a
question of factual observation into the realm of faith.
It is in that historical and cultural framework, far removed from our own times, that
Galileo’s judges, unable to dissociate faith from an age-old cosmology, believed
quite wrongly that the adoption of the Copernican revolution, in fact not yet
definitively proven, was such as to undermine Catholic tradition, and that it was
their duty to forbid its being taught. This subjective error of judgment, so clear to us
today, led them to a disciplinary measure from which Galileo had much to suffer.
These mistakes must be frankly recognized, as you, Holy Father, have requested.”
(L’Osservatore Romano, November 1, 1992)
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Science and Religion
Modern science has discovered that the Earth and
solar system have changed dramatically in time,
over a period of billions of years since its creation.
•Age of Earth ~ 4.5 Gy
•Geological processes
•Ice ages
•Biological evolution
This scientific understanding conflicts with a strict
literal interpretation of the Book of Genesis.
But science does not know (maybe cannot know)
how the universe was created.
Each person must decide what to believe, but may
not impose his beliefs on others.
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Johannes Kepler (1571 – 1630)
… discovered three empirical laws of
planetary motion in the heliocentric solar
system
1. Each planet moves on an elliptical orbit.
2. The radial vector sweeps out equal areas in equal
times.
3. The square of the period is proportional to the cube
of the radius.
(needed for the CAPA)
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How did Kepler determine the planetary orbits?
Mars
Compare the heliocentric model to
naked-eye astronomy
The inner planet is Earth;
the outer one is Mars. Plot
their positions every
month. Mars lags behind
the Earth so its
appearance with respect
to the Zodiac is shifting.
Earth
The most complete data had
been collected over a period
of many years by Kepler’s
predecessor, Tycho Brahe of
Denmark.
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Astrology…
a persistent
superstition
It’s all nonsense, of course.
The motions of the planets
are prescribed by natural
laws of motion and gravity.
How can their motion predict
the future?
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Ellipse Geometry
To draw an ellipse: Take a string. Tack down the two ends. Put a
pencil in the string and pull the string taut. Move the pencil
around keeping the string taut.
An ellipse is the locus of points
for which the sum of the
distances to two fixed points is
fixed.
The two fixed points are called the
focal points of the ellipse.
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Parameters of an elliptical orbit (a,e)
► Semi-major axis = a =
one half the largest
diameter
► Eccentricity = e = ratio of
the distance between the
focal points to the major
diameter
For example, this ellipse
has a = 1 and e = 0.5.
► Perihelion and aphelion
Perihelion = r2 = 0.5
Aphelion = r1 = 1.5
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Example.
Determine a, e, r1, r2.
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Isaac Newton
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The observed solar system at the time of Newton
Sun
Mercury
Venus
Earth
Mars
Jupiter
Saturn
(all except Earth are named
after Roman gods, because
astrology was practiced in
ancient Rome)
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Three outer planets discovered later…
Uranus (1781, Wm Herschel)
Neptune (1846 Adams; LeVerrier)
Pluto (1930, Tombaugh)
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Isaac Newton
Newton solved the premier scientific problem of
his time --- to explain the motion of the planets.
To explain the motion of the planets, Newton
developed three ideas:
1. The laws of motion
2. The theory of universal gravitation
3. Calculus, a new branch of mathematics
F=
F
a=
m
Gm1m2
r2
“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,
perhaps referring to Galileo and Kepler
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Newton’s Theory of Universal Gravitation
Newton and the Apple
Newton asked good questions  the key to his success.
Observing Earth’s gravity
acting on an apple, and seeing
the moon, Newton asked
whether the Earth’s gravity
extends as far as the moon.
(The apple never fell on his head,
but sometimes a stupid person
will say that, trying to be funny.)
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Quiz Question
Who discovered that the orbit
of a planet is an ellipse?
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