The Motions of the Planets

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The Motions of the Planets
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Ancient people observed that

• The sun appeared to move from east to west across
  the sky during the day.
• The stars did the same thing at night, staying in
  the same position relative to each other.
• The Greeks noticed that five objects seemed to
  wander among the stars. They called these
  objects planets.

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The Romans gave these planets names

• Mercury
• Venus
• Mars
• Jupiter
• Saturn

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A Greek astronomer Ptolemy (A.D. 100-165 )
theorized that
Objects such as the sun, planets and moon,
travel in orbits around the Earth. These
orbits are perfect circles.
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• An orbit is the path that one object takes when
  moving around another object in space.
• Ptolemys theory is known as the geocentric or
  Earth-at-the-center of the universe theory.
• Ptolemy felt that the orbits must be circular
  because the circle is the most perfect of all
  forms.

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About 1400 years later, Polish astronomer
Copernicus (1543) challenged Ptolemys theory.
He thought that objects orbit around the sun.
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Copernicus also said

• Each orbit is a perfect circle.
• All planets revolve in the same direction.
• Each planet takes a different amount of time to
  complete one orbit.

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His model is also known as the heliocentric or
sun-at-the-center model of the solar system.
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Italian astronomer Galileo Galilei, who lived
about 100 years after Copernicus, thought that
the heliocentric model was correct.
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Galileo constructed a telescope to look at
objects in the sky. He discovered that

• Four moons revolve around Jupiter. So not
  everything revolves around Earth.

• Venus goes through phases similar to our moon.
  This would not be what we would see if Earth were
  at the center.

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In the late 1500s Danish astronomer Tycho Brahe
made very careful observations of the positions
of the planets
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Johannes Kepler (1609) also supported the
heliocentric theory of Copernicus.
But
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He determined that the orbit of the planet is an
ellipse
not a circle
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Today, modern astronomers believe that

• Each planet travels in a counterclockwise
  elliptical orbit around the sun.
• The greater the distance from the sun, the longer
  a planet takes to complete one orbit.