Around 4000 B.C.

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• Around 4000(?) B.C.
• The oldest astronomical observation ever recorded
  (Egypt and Central America)
  
• Around 3000 B.C.
• The first written materials on astronomy (Egypt,
  China, Mesopotamia and Central America)

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• 2697 B.C.
• The oldest preserved relation on the Sun eclipse
  (China)
  
• Around 2000 B.C.
• The first solar-lunar calendars in Egypt and
  Mesopotamia
  
• Stonehenge Sanctuary (England)
• Constellations first drawn up by the ancient
  astronomers

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• VI century B.C.
• Pythagoras and Thales of Miletus speculate that
  the Earth is a sphere.
  
• Around 330 B.C.
• Aristotle's On Heavens

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• Aristotle was perhaps the greatest name in an age
  of great men. He was born in Stagira, in Greece
  and is often known as "the Stagirite". Aristotle
  studied under Plato. He founded a school in
  Athens. Aristotle's astronomy was based on the
  Theory of Eudoxus who tried to explain the
  motions of the planets by having them move in a
  system of 27 concentric spheres with the Earth in
  the center. Aristotle said that there were 55
  concentric spheres. His theory was held for 2,000
  years.

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• Around 280 B.C.
• Aristrachus of Samos suggests that the Earth
  revolves about the Sun (heeliocentric concept of
  the Universe). He also provides the first
  estimations on Earth-Sun distance
• Around 240 B.C.
• Eratosthenes of Cyrene (now Shahhat, Libya)
  measures the circumference of the earth with
  extraordinary accuracy by determining
  astronomically the difference in latitude between
  the cities of Syene (now Aswan) and Alexandria,
  Egypt

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• 45 B.C.
• The introduction of Julian calendar ( purely
  solar calendar) to the Roman Empire upon the
  advice of the Greek astronomer Sosigenes-Caesar
  
• Around A.D. 140
• Ptolemy suggests the geocentric theory of the
  Universe in his famous work Mathematike Syntaxis
  widely recognized from its Arabic translation as
  Almagest

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                      PtolemyPublic Domain
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• Not just an astronomer, Ptolemy was very
  important in the history of geography and
  cartography. He was well aware that the Earth is
  a sphere. His is the first known projection of
  the sphere onto a plane. His work, "Geography"
  remained the principal work on the subject until
  the time of Columbus. It was amazingly accurate
  for the time, but had Asia extending much too far
  east. This may have been a deciding factor in
  Columbus's decision to sail west for the Indies.

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• This system came to be called the Ptolemaic
  System and was the center of astronomical beliefs
  for nearly a millennium and a half. It predicted
  the positions of the planets accurately enough
  for naked-eye observations.

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• 1054
• Chinese astronomers observe supernova in Taurus
  (now this supernova remnants are known as the
  Crab Nebula (M1)
  

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• The Ptolemaic System was the accepted wisdom
  until the Polish scholar Copernicus proposed a
  heliocentric view in 1543.

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• 1543
• Copernicus publishes De Revolutionibus Orbium
  Coelestium in which he provides mathematical
  evidence for the heliocentric theory of the
  Universe.

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• Around 1514 he distributed a little book, not
  printed but hand written, to a few of his friends
  who knew that he was the author even though no
  author is named on the title page. This book,
  usually called the Little Commentary, set out
  Copernicus's theory of a universe with the sun at
  its center. What are the axioms?

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• There is no one center in the universe.
• The Earth's center is not the center of the
  universe.
  
• The center of the universe is near the sun.
• The distance from the Earth to the sun is
  imperceptible compared with the distance to the
  stars.
  
• The rotation of the Earth accounts for the
  apparent daily rotation of the stars.
  
• The apparent annual cycle of movements of the sun
  is caused by the Earth revolving round it.
  
• The apparent retrograde motion of the planets is
  caused by the motion of the Earth from which one
  observes.

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• The most remarkable of the axioms is 7, for
  although earlier scholars had claimed that the
  Earth moved, some claiming that it revolved round
  the sun, nobody before Copernicus appears to have
  correctly explained the retrograde motion of the
  outer planets.

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• The Copernican system by banishing the idea that
  the Earth was the center of the Solar System,
  immediately led to a simple explanation of both
  the varying brightness of the planets and
  retrograde motion
• The planets in such a system naturally vary in
  brightness because they are not always the same
  distance from the Earth.
  
• The retrograde motion could be explained in terms
  of geometry and a faster motion for planets with
  smaller orbits.

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• 1572
• Tycho Brahe discoveres a supernova in the
  constellation Cassiopeia (now this supernova
  remenents are known as Cassiopeiae A).
  
• 1576
• Tycho Brahe founds the observatory at Uraniborg.
  
  

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• He allegedly challenged a fellow student to a
  duel with swords in a dispute over who was the
  better mathematician. Brahe's nose was partially
  cut off, and he was said to wear a gold and
  silver replacement upon which he would
  continually rub oil.

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• As an astronomer, Tycho worked to combine what he
  saw as the benefits of the Copernican system with
  the philosophical benefits of the Ptolemaic
  system into his own model of the universe, the
  Tychonic system.

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• He is credited with the most accurate
  astronomical observations of his time, and the
  data was used by his assistant Kepler to derive
  the laws of planetary motion.

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• 1582 (October 15)
• The Pope Gregory XIII introduces the Gregorian
  calendar. The one we use today!
  
• 1600 (February 17)
• Giordano Bruno after some eight years of
  imprisonment is charged with blasphemy, immoral
  conduct, and heresy for challenging the official
  church doctrine on the origin and structure of
  the universe and is burned at the stake.

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• Galileo Galilei was born on February 15, 1564 in
  Pisa, Italy. Galileo pioneered "experimental
  scientific method" and was the first to use a
  refracting telescope to make important
  astronomical discoveries.

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• In 1609 Galileo learned of the invention of the
  telescope in Holland. From the barest description
  he constructed a vastly superior model. Galileo
  made a series of profound discoveries using his
  new telescope, including the moons of the planet
  Jupiter and the phases of the planet Venus
  (similar to those of Earth's moon).

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• As a professor of astronomy at University of
  Pisa, Galileo was required to teach the accepted
  theory of his time that the sun and all the
  planets revolved around the Earth. Later at
  University of Padua he was exposed to a new
  theory, proposed by Nicolaus Copernicus, that the
  Earth and all the other planets revolved around
  the sun. Galileo's observations with his new
  telescope convinced him of the truth of
  Copernicus's sun-centered or heliocentric theory.

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• Galileo's support for the heliocentric theory got
  him into trouble with the Roman Catholic Church.
  In 1633 the Inquisition convicted him of heresy
  and forced him to recant (publicly withdraw) his
  support of Copernicus. They sentenced him to life
  imprisonment, but because of his advanced age
  allowed him serve his term under house arrest at
  his villa outside of Florence, Italy.

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• 1609
• Galileo first uses the telescope for astronomical
  purposes (discovers four Jovian moons, the Moon
  craters, and the Milky Way).
  
• The first two of Kepler's Laws of Planetary
  Motions are announced.

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• 1632
• Galileo publishes his Dialogue on the Two Chief
  World Systems - the discussion of Ptolemaic and
  Copernican hypotheses in relation to the physics
  of tides ( the original version including title -
  originally Dialogue on the Tides -was licensed
  and altered by the Roman Catholic censors in
  Rome.
• 1633
• Galileo is forced by the Inquistion to recant his
  theories.

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• 1666
• Cassini observes the polar caps on Mars.
• 1668
• Newton builds the first reflecting telescope
  (Newtonian). Born the same year Galileo died,
  1642.

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• 1675
• Foundation of Greenwich Observatory. Greenwich
  Mean Time.
  
• Romer measures the velocity of light.
• Cassini discovers the main division in Saturn's
  rings.

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• 1687
• Sir Issac Newton publishes his revolutionary
  Philosophiae Naturalis Principia Mathematica
  establishing the theory of universal gravition
  which is considered to be a turning point in the
  history of science.
• 1705
• Halley predicts the return of Halley's Comet in
  1758.

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• 1839-40
• The first application of photography to astronomy
  (Draper takes the first photograph of the Moon.)
  
  
• 1846
• Johann Gottfried Galle discovers the planet
  Neptune based on its position calculated by the
  French astronomer Joseph Leverrier.

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• 1905
• Einstein proposes the basis of the special Theory
  of Relativity, first described in his paper On
  the Electrodynamics of Moving Bodies. The theory
  is based on two principles
• the principle of relativity, that physical laws
  are the same in all inertial reference systems.
  
• the principle of the invariance of the speed of
  light, that the speed of light in a vacuum is a
  universal constant.
  

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• 1923
• Hubble proves that the galaxies lie beyond the
  Milky Way.
  

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