UNIT NINE: Matter and Motion in the Universe

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UNIT NINE Matter and Motionin the Universe

• Chapter 26 The Solar System
• Chapter 27 Stars
• Chapter 28 Exploring the Universe

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Chapter Twenty-Six The Solar System

• 26.1 Motion and the Solar System
• 26.2 Motion and Astronomical Cycles
• 26.3 Objects in the Solar System

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Section 26.1 Learning Goals

• Explain the significance of gravity in
  maintaining the solar system.
• Distinguish between Sun-centered and
  Earth-centered models of the solar system.
• Explain the current model of the solar system.

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Investigation 26A
Phases of the Moon

• Key Question
• What causes the lunar cycle?

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26.1 Motion and the solar system

• Ancient astronomers used a landmark, such as a
  building or tree, to mark the point where the Sun
  rose or set each day.
• The position of the sunset and sunrise changes
  over time.

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26.1 Observing patterns in the sky

• The Moon appears to change its shape and the time
  and position at which it rises and sets.

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26.1 Observing patterns in the sky

• The rising and setting positions of the stars do
  not appear to change along the horizon over short
  periods of time.
• However, the time that stars rise or set each
  night gradually changes during a year.

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26.1 Observing patterns in the sky

• A constellation is a group of stars that, when
  seen from Earth, form a pattern.

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26.1 The Earth-centered model

• Ancient observers noticed that five bright
  objects seemed to wander among the stars at
  night.
• They called these objects planets, from the Greek
  word meaning wandering star, and named them
  Mercury, Venus, Mars, Jupiter, and Saturn.

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26.1 The Earth-centered model

• In 140 AD, the Greek astronomer Ptolemy developed
  a model that explained the apparent path of the
  planets.
• He hypothesized that each planet moved on a
  circle, which, in turn, moved on a larger circle
  around Earth.

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26.1 The sun-centered model

• While the Ptolemaic model could predict the
  positions of the planets, Nicholas Copernicus
  found that its predictions became less and less
  accurate over the centuries.
• In Copernicus model, the Sun was at the center
  of the solar system and the planets orbited in
  circles around the Sun.

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26.1 The sun-centered model

• The phases of Venus, discovered by Galileo in the
  1600s, were part of the evidence that eventually
  overturned Ptolemys model.
• Using a telescope he built himself, Galileo made
  two discoveries.

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26.1 The sun-centered model

• First, he argued that the phases of Venus could
  not be explained if Earth were at the center of
  the planets.
• Second, Galileo saw that there were four moons
  orbiting Jupiter.

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26.1 Gravitational force

• Newtons law of universal gravitation explains
  how the strength of the force depends on the mass
  of the objects and the distance between them.

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26.1 Gravitational force

• Gravitational force is the force of attraction
  between all objects.
• All objects that have mass attract each other.

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26.1 Orbits

• An orbit is a regular, repeating path that an
  object in space follows around another object.
• An object in orbit is called a satellite.

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26.1 Orbits

• In 1600, German mathematician Johannes Kepler
  determined that the orbits of the planets were
  not perfect circles but slightly elliptical.

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26.1 Orbits

• Isaac Newton explained that an orbit results from
  the balance between inertia (the forward motion
  of an object in space), and gravitational.
• Without the pull of gravity, a planet would
  travel off into space in a straight line.

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26.1 Current model of the solar system

• Today, we define the solar system as the sun and
  all objects that are gravitationally bound to the
  sun.
• The solar system is roughly divided into the
  inner planets (Mercury, Venus, Earth, and Mars)
  and the outer planets (Jupiter, Saturn, Uranus,
  and Neptune)
• The dwarf planet Pluto is the oldest known member
  of a smaller group of frozen worlds orbiting
  beyond Neptune.

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26.1 Comparing size and distance

• The Sun is by far the largest object in the solar
  system.
• One astronomical unit (AU) is equal to 150
  million km, or the distance from Earth to the Sun.

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