The Copernican Revolution

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The Copernican Revolution

• and the Development of Modern Astronomy

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In 1543, Nicolaus Copernicus published the
book De Revolutionibus Orbium Clestium, On
the Revolutions of the Celestial Spheres in
which he proposed a new model of the Solar
System, in which the Sun is at the center
(Heliocentric).
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The new model kept some archaic ideas (perfect
circles for the orbits, and even some epicycles),
but it was revolutionary because it displaced the
Earth from the center of the Solar System, and
thus the Universe.
Its great advantage lay in its ability to explain
simply the two basic observational properties of
the Solar System.
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• The Retrograde
• Motion of the
• Superior Planets

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2) The Behavior of the Inferior Planets
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Recall how awkward these explanations were with
the Ptolemaic System
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What was the single greatest advantage of the
Copernican System over the Ptolemaic system
It did away with epicycles
It reproduced the motions of the planets
without having to use complicated and artificial
devices
It was heliocentric instead of geocentric
It made Venus the closest planet to the Earth
instead of Mercury
It was able to predict positions of the planets
more accurately than the Ptolemaic system
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Before we continue the story of the ultimate
triumph of the Copernican system over the
Ptolemaic system, let us digress, and consider
some terminology associated with the Copernican
system.
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• Opposition
• Conjunction
• Quadrature

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Synodic Period The time between two
successive identical configurations.
Sidereal Period The true orbital period of the
planet
Let P Sidereal Period of the Planet
E Sidereal Period of the Earth (about 365.25
days) S Synodic Period of the Planet
Inferior Planet
Superior Planet
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Example Venus
P ? (Sidereal Period) E 365.25 days S 584
days (Synodic Period)
P 1/0.004450 225 days
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You try Mars E 365.25 days S 780 days
(synodic period of Mars) P ? (sidereal period
of Mars)
P 687 days
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The Copernican System was not immediately
accepted by astronomers, and especially by the
Church because

• Displacing the Earth from the center was counter
• to Aristotelian philosophy and Church
  doctrine.

• Though simpler than the Ptolemaic system the
• Copernican system did not predict the
  positions
• of the planets any more accurately.

• Contemporary astronomers had their whole careers
• invested in the Ptolemaic system.

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Copernicus was aware of these difficulties, and
did not publish his book until the last year of
his life. Defense of the Copernican system thus
fell to other astronomers.
Some important figures in the Copernican
Debate Copernicus 1473 1543 Galileo
Galilei 1564 1642 Tycho Brahe 1546
1601 Johannes Kepler 1571 1630 Issac Newton
1642 - 1727
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Galileo
Galileo was the first astronomer to use the
teles-cope (invented in Holland in 1609) for
astro- nomical obser-vations.
His discoveries ultimately disproved the
Ptolemaic system, and helped establish the
Copernican system.
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The Discoveries of Galileo
1) Craters on the Moon
- cast doubt on the philosophy of Aristotle
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2) Discovery of Sunspots
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3) Discovery of the Moons of Jupiter
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• Discovery that Venus goes through a complete
• set of phases.

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The discovery Venus goes through a complete set
of phases is consistent with the Copernican
system, but is impossible under the Ptolemaic
system.
Copernican System
Ptolemaic System
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Why were believers in the Ptolemaic system
reluctant to believe that Jupiter had moons?
Because they had not discovered them first
themselves
Because astronomical bodies being in motion
seemed to contradict Aristotles ideas of the
unchanging heavens
Because this meant there was another center
in the solar system instead of the Earth being
the only center
Because the Ptolemaic system did not predict
the phases of these moons.

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Galileos efforts to prove the Copernican system
got him in trouble with the Church. His book
Siderius Nuncius (1610) raised controversy, and
he was warned by the Church not to teach the
Copernican system as the truth. After the
publication of his second major book, Dialogue
Concerning the Two Chief World Systems (1632), he
was placed under house arrest.
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Tycho Brahe
Tycho Brahe was a Danish nobleman who was
convinced that both the Copernican and Ptolemaic
models were incorrect.
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Tycho Brahe thought that both systems were in
error because

• In the year 1572, a supernova
• appeared, and in 1577 a comet,
• disproving the idea that the
• heavens were immutable and
• thus casting doubt on the
• Ptolemaic system.

• Tycho Brahe could not observe the phenomenon of
• stellar parallax, and thus he did not
  believe that the
• Earth was in motion suggesting to him that
  the
• Copernican system was wrong.

In addition, neither theory gave very good
predictions of planetary positions.
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He came up with his own system, called the
Tychonic System. He decided the only way to
prove the correctness of any system was to make a
large number of very accurate observations of
the positions of the planets over a number of
years. For that purpose, he established a major
observatory called Uraniburg.
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How did the Tychonic system differ from the
Copernican and Ptolemaic system?
It was heliocentric, but all the planets
orbited around the earth
It was geocentric, but all the planets orbited
around the sun
It was Venus-centric
It completely did away with epicycles and
introduced The idea of elliptical orbits

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Tychos observatory was established before the
invention of the telescope, so all his
observations were naked-eye.
However, he was able to compile the most
accurate and extensive collection of planetary
observations at that time.
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Tycho Brahe had an assistant, Johannes
Kepler. On his deathbed, Tycho willed his
observations to Kepler, and instructed him to use
them to prove the Tychonic system. Kepler used
them to prove the Copernican system, and to
discover elliptical orbits.
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Over a period of 25 years, Johannes Kepler used
Tychos observations to deduce his Three Laws of
Planetary Motion.
Keplers First Law The orbits of the Planets
are ellipses with the sun at one focus.
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Over a period of 25 years, Johannes Kepler used
Tychos observations to deduce his Three Laws of
Planetary Motion.
Keplers First Law The orbits of the Planets
are ellipses with the sun at one focus.
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The semi-major axis (a) is half the major axis
The eccentricity (e) runs from 0 (circle) to 1
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Perihelion
Aphelion
Major axis 2a
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The semi-major axis, a, is
the distance between the sun and the planet at
perihelion
the distance between the sun and the planet at
aphelion
the average distance between the sun and planet
the length of the major axis of the elliptical
orbit

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Keplers Second Law A line from the planet to
the sun sweeps over equal areas in equal times.
This implies that the planet travels faster in
its orbit near Perihelion than at Aphelion.
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Keplers Third Law The Orbital Period of a
planet squared is proportional to the length of
its semi-major axis cubed.
If we use the units of Years for the Period and
Astronomical Units (A.U.) for the semi-major
axis, the equation becomes
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Keplers Third Law An Example Mars has an
orbital period P 1.881 years. Its
semi-major axis a 1.524 A.U.
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Keplers Third Law Another Example
The orbit of a comet has a semi-major axis (a)
of 9 A.U. What is its orbital period?
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Try this one yourself
Venus has an orbital period of 0.615 years. What
is The semi-major axis of its orbit?
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Yet another example!
A comet is observed with a perihelion distance of
1 A.U. and an aphelion distance of 49 A.U.. What
is the period in years?
Perihelion
Aphelion
1 A.U.
49 A.U.
The major axis 2a 50A.U. a 25A.U.
Thus, P2a3 253 15625