Week Three: The Solar System

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Week Three The Solar System
This lecture notes corresponds to Chapter 4 of
the textbook
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• Next time you look at the sky, imagine how
  prehistoric families felt as they huddled around
  the safety of their fires and looked up at the
  stars.

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• Astronomy had its beginnings in simple human
  curiosity about the lights in the sky.
• As early civilizations developed, great
  philosophers struggled to understand the
  movements of the sun, moon, and planets.
• Later mathematical astronomers made precise
  measurements and computed detailed models
  attempting to describe the motions of the
  heavenly bodies.

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• It took hard work and years of effort, but the
  passions of astronomy gripped some of the
  greatest minds in history and drove them to try
  to understand the sky.

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Classical Astronomy

• The great philosophers of ancient Greece wrote
  about many different subjects, including what
  they saw in the sky.
• Those writings became the foundation upon which
  later astronomers built modern astronomy.

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The Aristotelian Universe

• Plato and Aristotle, the two greatest
  philosophers of ancient Greece, both influenced
  the history of astronomy.
• Plato (427?347 BC) wrote about moral
  responsibility, ethics, the nature of reality,
  and the ideals of civil government.
• His student Aristotle (384322 BC) wrote on
  almost every area of knowledge and is probably
  the most famous philosopher in history.
• These two philosophers established the first
  widely accepted ideas about the structure of the
  universe.

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The Aristotelian Universe

• Ancient philosophers and astronomers accepted as
  first principles that Earth was located at the
  center of the universe and that the heavens
  moved in uniform circular motion.

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The Aristotelian Universe

• The observed motion of the planetsdid not fit
  the theory very well.
• The retrograde loops the planets made were very
  difficult to explain using geocentrism and
  uniform circular motion.

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The Aristotelian Universe

• Claudius Ptolemy attempted to explain the motion
  of the planets mathematicallyby devising a small
  circle rotating along the edge of a larger circle
  that enclosed a slightly off-center Earth.

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The Aristotelian Universe

• He even allowed the speed of the planets to vary
  slightly as they circled Earth.
• In these ways, he weakened the principles of
  geocentrism and uniform circular motion.

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The Aristotelian Universe

• When Plato, Aristotle, and Ptolemy said Earth was
  the center of the universe, they were thinking of
  a very small universe.
• They didnt know how far away stars and galaxies
  are.

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The Aristotelian Universe

• Ptolemy lived roughly five centuries after
  Aristotle in the Greek colony in Egypt.
• Although he believed in the Aristotelian
  universe, he was interested in a different
  problemthe motion of the planets.
• He was a brilliant mathematician, and he used his
  talents to create a mathematical description of
  the motions he saw in the heavens.

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The Aristotelian Universe

• Aristotles universeas embodied in Ptolemys
  mathematical modeldominated ancient astronomy.
• However, it was wrong.
• The universe is not geocentric.
• The planets dont follow circles at uniform
  speeds.

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The Aristotelian Universe

• In the middle of the 13th century, a team of
  astronomers supported by King Alfonso X of
  Castile studied the Almagest (great book) for 10
  years.
• Although they did not revise the theory very
  much, they simplified the calculation of the
  positions of the planets using the Ptolemaic
  system and published the result as The Alfonsine
  Tablesthe last great attempt to make the
  Ptolemaic system of practical use.

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Copernicus

• You would not have expected Nicolaus Copernicus
  to trigger a revolution in astronomy and science.
• He was born in 1473 to a merchant family in
  Poland.
• Orphaned at the age of 10, he was raised by his
  uncle, an important bishop, who sent him to the
  University of Cracow and then to the best
  universities in Italy.
• He studied law and medicine and pursued a
  lifelong career as an important administrator in
  the Church.
• Nevertheless, he had a passion for astronomy.

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The Copernican Model

• If you had sat beside Copernicus in his astronomy
  classes, you would have studied the Ptolemaic
  universe.
• The central location of Earth was widely
  accepted, and everyone knew that the heavens
  moved by the combination of uniform circular
  motions.

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The Copernican Model

• For most scholars, questioning these principles
  was not an option.
• Over the course of centuries, Aristotles
  proposed geometry of the heavens had become
  linked with the teachings of the Church.
• According to the Aristotelian universe, the most
  perfect region was in the heavens and the most
  imperfect at Earths center.
• This classical geocentric universe matched the
  commonly held Christian geometry of heaven and
  hell, and anyone who criticized the Ptolemaic
  model was questioning Aristotles geometry and
  indirectly challenging belief in heaven and hell.

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The Copernican Model

• His close connection with the Church
  notwithstanding, Copernicus began to consider an
  alternative to the Ptolemaic universe, probably
  while he was still at university.
• Sometime before 1514, he wrote an essay that
  discussed his hypothesis that the sun, not Earth,
  was the center of the universe.
• That is, he proposed that the universe was
  heliocentricor sun-centered.
• To explain the daily and annual cycles of the
  sky, he proposed that Earth rotated on its axis
  and revolved around the sun.

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The Copernican Model

• He distributed this commentary in handwritten
  form, without a title, and in some cases
  anonymouslyto friends and astronomical
  correspondents.
• He may have been cautious out of modesty, out of
  respect for the Church, or out of fear that his
  revolutionary ideas would be attacked unfairly.
• After all, the place of the Earth was a
  controversial theological subject.

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The Copernican Model

• Although his essay discusses every major aspect
  of his later work, it did not include
  observations and calculations to add support.
• His ideas needed further work.
• He began gathering observations and making
  detailed calculations in order to publish a book
  that would demonstrate the truth of his
  revolutionary ideas.

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De Revolutionibus

• Copernicus worked on his book De Revolutionibus
  Orbium Coelestium (The Revolutions of the
  Celestial Spheres) over a period of many years
  and was essentially finished by about 1529.
• Yet, he hesitated to publish it, even though
  other astronomers knew of his theories.
• Church officials concerned about the reform of
  the calendar sought his advice and looked forward
  to the publication of his book.

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De Revolutionibus

• Also, Earths place in astronomical theory was
  linked to the geometry of heaven and hell.
• So, moving Earth from its central place was a
  controversial and perhaps heretical idea.

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De Revolutionibus

• Another reason Copernicus may have hesitated was
  that his work was incomplete.
• His model could not accurately predict planetary
  positions, and he continued to refine it.

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De Revolutionibus

• Finally, in 1540, he allowed the visiting
  astronomer Joachim Rheticus (15141576) to
  publish an account of the Copernican universe in
  Rheticuss book Prima Narratio (First Narrative).
  
• In 1542, Copernicus sent the manuscript for De
  Revolutionibus off to be printed.
• He died in the spring of 1543before the printing
  was completed.

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De Revolutionibus

• The most important idea in the book was the
  placing the sun at the center of the universe.
• That single innovation had an astonishing
  consequencethe retrograde motion of the planets
  was immediately explained in a straightforward
  way without the large epicycles that Ptolemy had
  used.

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De Revolutionibus

• In the Copernican system, Earth moves faster
  along its orbit than the planets that lie farther
  from the sun.
• Consequently, Earth periodically overtakes and
  passes these planets.

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De Revolutionibus

• The

• Although Mars moves steadily along its orbit, as
  seen from Earth, it appears to slow to a stop and
  move westward (retrograde) as Earth passes it.
• As the planetary orbits do not lie in precisely
  the same plane, a planet does not resume its
  eastward motion in precisely the same path it
  followed earlier.
• Consequently, it describes a loop whose shape
  depends on the angle between the orbital planes.

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De Revolutionibus

• Copernicus could explain retrograde motion
  without epicycles.
• That was impressive.
• The Copernican system was elegant and simple
  compared with the whirling epicycles and
  off-center equants of the Ptolemaic system.

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De Revolutionibus

• However, De Revolutionibus failed in one critical
  way.
• The Copernican model could not predict the
  positions of the planets any more accurately than
  the Ptolemaic system could.
• To understand why it failed this critical test,
  you must understand Copernicus and his world.

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De Revolutionibus

• In devising his model, Copernicus returned to a
  strong belief in uniform circular motion.
• Although he did not need epicycles to explain
  retrograde motion, he discovered that the sun,
  moon, and planets suffered other smaller
  variations in their motions that he could not
  explain with uniform circular motion centered on
  the sun.

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De Revolutionibus

• Today, astronomers recognize that those
  variations are typical of objects following
  elliptical orbits.
• Copernicus, though, held firmly with uniform
  circular motion.
• So, he had to introduce small epicycles to
  reproduce these minor variations in the motions
  of the sun, moon, and planets.

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De Revolutionibus

• The Copernican model is inaccurate.
• It includes uniform circular motion.
• Thus, it does not precisely describe the motions
  of the planets.

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De Revolutionibus

• However, the Copernican hypothesis that the
  universe is heliocentric was correctgiven how
  little astronomers of the time knew of other
  stars and galaxies.
• The planets circle the sun, not Earth.
• So, the universe that Copernicus knew was
  heliocentric.

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De Revolutionibus

• The most astonishing consequence of the
  Copernican hypothesis was not what it said about
  the sun but what it said about Earth.
• By placing the sun at the center, Copernicus made
  Earth move along an orbit like the other planets.
  
• By making Earth a planet, Copernicus
  revolutionized humanitys view of its place in
  the universe and triggered a controversy that
  would eventually bring the astronomer Galileo
  Galilei before the Inquisitiona controversy over
  the nature of scientific truth that continues
  even today.

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De Revolutionibus

• How the Copernican hypothesis was gradually
  recognized as correct has been called the
  Copernican revolution.
• It was not just the adoption of a new idea but a
  total change in the way astronomers think about
  the place of the Earth.

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Quiz

• What is at the center of the Universe in the
  theories of Aristotle and that of Copernicus?
• What do you think is the reason that the
  Copernicus theory does not predict the motion of
  the planets accurately?

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Planetary Motion

• The puzzle of planetary movement was solved
  during the century following the death of
  Copernicusthrough the work of two men.
• One compiled the observations and one made the
  analysis.

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Tycho Brahe

• Tycho Brahe, born on December 14, 1546, was not a
  churchman like Copernicus, but rather a nobleman
  from an important family educated at the finest
  universities.
• He was well known for his vanity and his lordly
  manners and, by all accounts, was a proud and
  haughty nobleman.

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Tycho Brahe

• His disposition was not improved by a dueling
  injury from his university days.
• His nose was badly disfigured and, for the rest
  of his life, he wore false noses made of gold and
  silver and stuck on with wax.

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Tycho Brahe

• Tycho was interested in astronomy even during his
  days at university.
• He expressed amazement that neither The Alfonsine
  Tables nor The Prutenic Tables properly described
  the motions of the planets along the ecliptic.

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Tycho Brahe

• Not long after his university days, in 1572, a
  new star (now called Tychos supernova)
  appeared in the sky.
• Tycho rushed to measure its position.
• He was surprised to find that it displayed no
  parallax.
• To understand the significance of this
  observation, you should note that Tycho believed
  that the heavens rotated westward around Earth.

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Tycho Brahe

• The new star was a change in the heavens.
• Therefore, according to classical astronomy, it
  had to lie below the sphere of the moon.
• In that case, Tycho reasoned, the new star should
  show parallax.
• That is, it would appear slightly too far east as
  it rose and slightly too far west as it set.